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The value of exhaled nitric oxide to identify asthma in smoking patients with asthma-like symptoms
Respiratory Medicine (2012) 106, 794e801
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/rmed
The value of exhaled nitric oxide to identify asthma in smoking patients with asthma-like symptoms Andrei Malinovschi a,*, Vibeke Backer b, Henrik Harving c, Celeste Porsbjerg b a
Department of Medical Sciences, Clinical Physiology, Uppsala University, Uppsala, Sweden Respiratory Research Unit, Bispebjerg Hospital, University Hospital of Copenhagen, Denmark c Department of Respiratory Diseases, Aalborg Hospital, University of Aarhus, Denmark b
Received 1 December 2011; accepted 17 February 2012 Available online 8 March 2012
KEYWORDS Exhaled nitric oxide; Smoking habits; Asthma; Diagnostic value; Cut-offs
Summary Background: The fraction of nitric oxide in exhaled air (FeNO) is used in asthma diagnosis and management. Smoking reduces FeNO and 20e35% of asthmatics are smoking. However no guidelines exist on the diagnostic value of FeNO in smokers. Therefore we assessed the value of FeNO to diagnose asthma in a population of subjects with asthma-like symptoms and different smoking habits. Methods: Measurements of FeNO, lung function, bronchial responsiveness and allergy testing were performed in 282 subjects (108 never-, 62 ex- and 112 current smokers) aged 14e44 years, with symptoms suggestive of asthma. These subjects were a subset of subjects reporting respiratory symptoms (n Z 686) in a random population sample (n Z 10,400). Results: A diagnosis of asthma was given to 96 of the 282 subjects. Subjects with asthma had higher FeNO levels than subjects with non-specific asthma symptoms in all three smoking strata (p < 0.001), with a percentual increase of FeNO by 76% in never-, 71% in ex- and 60% in current smokers. The area under the ROC-curve was similar in never-, ex- and current smokers (0.72 vs. 0.74 vs. 0.70). The cut-offs were approximately 30% lower for either 90% specificity (22 vs. 31 ppb) or 90% sensitivity (7 vs. 10 ppb) in current vs. never-smokers. Conclusions: FeNO could differentiate asthmatic subjects from non-asthmatic subjects with asthma-like symptoms equally well in both never- and current smokers within a random population sample. The FeNO cut-off levels needed in order to achieve high sensitivity or specificity were lower in current smokers. ª 2012 Elsevier Ltd. All rights reserved.
* Corresponding author. Department of Medical Sciences, Clinical Physiology, Uppsala University, Akademiska sjukhuset Ing 35, 1tr, 75185 Uppsala, Sweden. Tel.: þ46 18 6115856; fax: þ46 18 554079. E-mail address: [email protected] (A. Malinovschi). 0954-6111/$ - see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.rmed.2012.02.009
Smoking and the diagnostic value of exhaled NO
Background The fraction of nitric oxide in exhaled air (FeNO) is a marker of steroid-sensitive airway inflammation, used in monitoring the treatment of patients with asthma.1 However, the diagnostic value of exhaled NO for asthma is still debated and is more modest in the general populations2 than in subjects with symptoms highly suggestive of asthma.3,4 Different optimal FeNO cut-offs for diagnosing asthma have been reported in different studies.5 The general recommendations, also reiterated by the recently published ATS Clinical Practice Guideline, are that a FeNOvalue below 25 ppb, measured at a exhaled flow-rate of 50 mL/s, suggests that there is no steroid-sensitive inflammation present, while a value greater than 50 ppb is likely to be associated with steroid-sensitive inflammation.1,6,7 These cut-offs have been described for adult nonsmoking subjects and although it is well acknowledged that smoking reduces FeNO both in healthy and asthmatic subjects,8e12 no clear guidelines exist for the use of exhaled NO in smoking asthmatics1,6,7,13, indicating a need for more knowledge in this field. Smoking asthmatics are a category of patients representing up to 20e35% of the adult asthma patients in Northern Europe.14,15 Smoking induces a decrease of exhaled NO levels in both healthy8,9 and asthmatic subjects.10,11 In epidemiological studies, this decrease in FeNO has been reported to be in the range 30e60% when compared with FeNO-values of never-smoking subjects.16e18 Several mechanisms, such as an inhibited bronchial epithelial iNOS-related production of NO19,20 or an increased catabolism of NO,21 have been postulated to explain this decrease. The clinical use of exhaled NO in smoking asthmatic subjects has been questioned, as a more neutrophil- and steroid-insensitive inflammation has been described in smoking asthmatics.22 Contradictory results are reported regarding the presence of increased levels of exhaled NO in asthmatic smokers compared with healthy smokers.23e25 Furthermore, the effects of previous smoking on exhaled NO are controversial with a slight decrease16 or no decrease17 in FeNO. No study has addressed the value of exhaled NO in diagnosing asthma in ex-smoking subjects. A problem with the previous studies2,23e25 looking at the diagnostic value of FeNO in current smokers is that the reference group consisted of healthy smokers. However, in clinical practice the need is in differentiating subjects with asthma-like symptoms that have asthma diagnosed from the ones that do not have asthma. Therefore, the aim of the present study was to assess the value of exhaled NO to diagnose asthma in a population of subjects with asthmalike symptoms, with special emphasis on the value of exhaled NO to diagnose asthma in subjects with different smoking habits.
Methods Screening population A random population sample of 10,400 subjects, aged 14e44 years, and living in Denmark was drawn from the civil registration list,26 with the aim of examining asthma
795 treatment in a Western society. All subjects were mailed a letter with a validated self-administered asthma and rhinitis screening questionnaire including 20 questions adopted from the ACAAI screening program.27 Of the 47% that responded, 2250 had at least one respiratory and/or nasal symptom. 686 of these subjects recorded two or more respiratory symptoms, accepted the study invitation and were examined.
Examination All participants had their lung function measured (Vitalograph), and a specific IgE test was performed, followed by a bronchial provocation test with methacholine and a reversibility test with inhaled beta2-agonist, as previously described elsewhere.26 Halfway through the study it became possible to measure exhaled NO, after which these measurements were performed on 320 of 329 (97%) of the participants. Exhaled NO (FeNO) was measured using an online technique at a exhalation flow-rate of 50 mL/s, based on electrochemical sensor (NIOX Mino, Aerocrine AB, Solna, Sweden), according to ATS guidelines.28 The measurements of exhaled NO were performed at approximately 1 h after the start of examination and before any lung function tests.
Exclusion and inclusion criteria Subjects with at least two symptoms suggestive of asthma (listed in the Online Supplement Table S1) were eligible for the present study and therefore 33 subjects with no symptoms when examined at the clinic or only one asthmatic symptom were excluded. Subjects fulfilling the spirometric criteria for COPD were excluded (n Z 5), leaving a total of 282 subjects included in the study.
Smoking history All participants filled in a questionnaire regarding smoking habits. Subjects were classified as never-smokers, exsmokers (subjects who ever have smoked or tried smoking) and current smokers (subjects currently smoking). Lifetime tobacco exposure was calculated for ex- and current smokers as pack-years (tobacco consumption [cigarettes/ day]/20 * duration of smoking [years]).
Asthma diagnosis All subjects were interviewed by a respiratory specialist (70% of the cases the same specialist), who made a diagnosis of asthma based on the presence of symptoms of asthma (i.e. shortness of breath, chest tightness, cough, exercise induced dyspnoea, night-time awakenings, or respiratory symptoms induced by allergen contact) in combination with at least one of the following: 1) Airway hyper-responsiveness (AHR) to inhaled methacholine < 8.0 mmol. 2) At least 250 ml increase in FEV1 after bronchodilator.29 3) Daily use of systemic steroid, inhaled steroid, or inhaled beta2-agonist.
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4) Asthma symptoms during but not outside the pollen season, eventually supported by allergic rhinitis, although no objective signs of asthma outside season were found.
ROC-curves was defined as corresponding to the maximum value of Youden’s index (sensitivity þ specificity - 1).31
Ethics Diagnosis of rhinitis Patients with rhinitis were defined as subjects with symptoms of rhinitis such as runny nose, stuffy nose, sneeze, itchy nose, mouth, ear or eyes, red and runny eyes, reduced sense of smell, or symptoms due to allergen exposure.
The study was approved by the Ethics Committee of Aalborg, Denmark. All participants received information in both oral and written forms and gave their consent in writing before enrolment.
Results
Statistical analysis STATA version 10.1 (Stata Corp, College Station, TX, USA) was used for all analyses. The distribution of FeNO levels is skewed to the right and therefore log-transformation was used in order to meet normality assumptions. Similarly, methacholine doseresponse slope (DRS) and blood eosinophils were logtransformed as they are known to have a similar distribution as FeNO. Consequently, FeNO, DRS and blood eosinophils were presented as geometrical mean (95% CI) in descriptive statistics. Percentual increase of FeNO in asthmatic subjects vs. non-asthmatic subjects was obtained from linear regression models before and after adjusting for other known determinants of FeNO (sex, height, age, airway calibre (FEV1(%pred)), use of inhaled corticosteroids and pollen season30) after backtransformation of regression coefficients, as previously described elsewhere.23 The optimal cut-off for the
Population characteristics with regard to asthma diagnosis The subjects who underwent FeNO measurements did not differ from the other participants undergoing clinical examination in terms of any of the variables presented in Table 1 (all p-values > 0.05, data not shown), apart from the percentage of asthma patients, which was smaller then found in the group that did not have FeNO measured (33% vs 46%, p Z 0.01). The characteristics of the subjects included in the present study are shown, with regard to final diagnosis, in Table 1. Subjects who were diagnosed with asthma were more often IgE-sensitised, had rhinitis, and used inhaled corticosteroids or bronchodilators more frequently, compared with subjects with symptoms that were concluded to be non-specific. Furthermore, asthmatic subjects had a higher degree of
Table 1 Subject characteristics expressed as mean (standard deviation(SD)), N(%) or geometrical mean (GM) with 95% confidence interval (95% CI).
Age (mean (SD)) Female gender (N(%)) IgE-sensitisation (N(%)) Blood eosinophils (GM(95%CI)) Smoking history Never (N(%)) Ex (N(%)) Current (N(%)) Rhinitis (N(%)) FEV1% pred (mean (SD)) FVC% pred (mean (SD)) FEV1/FVC (mean (SD)) AHR (N(%)) DRS (GM(95%CI)) SABA-use (N(%)) ICS-use (N(%)) Asthma symptomsb
Non-specific asthma symptoms (n Z 186)
Asthma (n Z 96)
p-value
32.7 112 40 0.17
(8.9) (60.2%) (22.0%) (0.16, 0.19)
32.7 57 50 0.21
(8.7) (59.4%) (54.4%) (0.19, 0.24)
1.00 0.89 <0.001 0.002
63 43 80 86 97.0 96.8 0.82 4 0.65 3 1 4
(33.9%) (23.1%) (43.0%) (46.2%) (10.8) (11.5) (0.07) (2.2%)a (0.59, 0.71)a (1.6%) (0.5%) (3, 6)
45 19 32 63 94.4 97.2 0.80 35 1.65 46 24 6
(46.9%) (19.8%) (33.3%) (65.6%) (15.2) (12.8) (0.09) (36.5%)a (1.30,2.08)a (47.9%) (25.2%) (4, 8)
0.10
0.002 0.11 0.82 0.01 <0.001 <0.001 <0.001 <0.001 0.02
Abbreviations: FEV1 e forced expiratory volume in 1 s, FVC eforced vital capacity, AHR e airway hyper-responsiveness (defined as a PD20 methacholine < 8 mmol), DRS e dose-response slope to methacholine provocation, SABA - short-acting beta2-agonist, ICS - inhaled corticosteroids. a Two subjects in the non-specific asthma symptoms group and seven subjects in the asthma group did not perform methacholine provocation test. b Expressed as median (interquantile range).
Smoking and the diagnostic value of exhaled NO bronchial responsiveness to methacholine, higher blood eosinophils and had a lower FEV1/FVC-ratio (Table 1). The pack-year consumption in the currently smoking group (n Z 112) was 10 (0, 30) (median, range) and 14 subjects had a pack-year consumption < 1. The ex-smoking group (n Z 62) was characterised by a pack-year consumption of 0 (0, 7.5) and 50 subjects had a pack-year consumption < 1. Atopy was less common in subjects who were current smokers than never- or ex-smokers (22% vs. 46% and 48%, respectively).
Exhaled NO and smoking habits The FeNO-values were significantly lower in the currently smoking group (11.9 ppb (10.5, 13.5)) than in the neversmoking group (17.6 ppb (15.2, 20.2), p < 0.001) and the ex-smoking group (18.0 ppb (15.6, 20.8), p < 0.001). No significant differences between never- and ex-smoking subjects were found in regard to FeNO.
Using FeNO to differentiate asthma from nonspecific asthma symptoms Subjects with asthma had higher levels of exhaled NO than subjects with non-specific asthma symptoms (21.8 ppb (18.6, 25.5) vs. 12.5 ppb (11.5, 13.6), p < 0.001). The area under the curve (AUC) for the ROC of FeNO for asthma diagnosis was 0.72 (0.66, 0.79). The value was similar after excluding the subjects currently using inhaled corticosteroids (AUC 0.73 (0.65, 0.80), n Z 256). Subjects with asthma had higher FeNO levels than subjects with non-specific asthma symptoms in all three smoking strata (p < 0.001, Table 2) with a percentual increase of FeNO of 76% in never-, 71% in ex- and 60% in current smokers (Fig. 1A). Even after adjustments for potential confounders as sex, height, age, airway calibre (FEV1(%pred)), use of inhaled corticosteroids and pollen
797 season, increased exhaled NO was significantly related to presence of asthma (Fig. 1B). ROC-curves for the value of exhaled NO to detect asthma in never-, ex- and current smokers are presented in Fig. 2. The areas under the ROC-curves, optimal cut-offs and cut-offs for 90% sensitivity and 90% specificity for FeNO levels to diagnose asthma are presented in Table 3. No significant differences between the AUC-values for the ROC-curves of FeNO were found between the three smoking strata (p > 0.05). The AUC for the ROC-curve for current smokers, after excluding smokers with a pack-year history less than 1 pack-year, was unchanged (0.70 (0.57, 0.82), n Z 98) as were the cut-offs for 90% sensitivity (6 ppb), 90% specificity (22 ppb) and optimal cut-off (17 ppb). Excluding subjects on inhaled corticosteroids, the AUC values were 0.76 (0.65, 0.86) for never-smokers (n Z 96), 0.74 (0.55, 0.93) for ex-smokers (n Z 55) and 0.68 (0.56, 0.81) for current smokers (n Z 105). No significant differences between the AUC-values for the ROC-curves in the three smoking strata were found (p > 0.05). The cut-offs for 90% sensitivity were 11 ppb (spec 33.9%), 11 ppb (spec 23.3%) and 6 ppb (spec 20%) in never-, ex- and current smokers. The cut-offs for 90% specificity were 31 ppb (sens 41.2%), 26 ppb (sens 41.7%) and 22 ppb (sens 36%) in never-, ex- and current smokers. In Table 4, the sensitivity and specificity at different FeNO cut-off levels are given. Positive predictive values and negative predictive values for these arbitrary cut-offs are presented in Online Supplement Table S2.
Exhaled NO and its relationship to atopy, bronchial responsiveness and blood eosinophils in the different smoking strata There was a positive association between bronchial responsiveness and exhaled nitric oxide among never-
Figure 1 Percentual increase (mean (95%CI)) of FeNO in asthmatics vs. subjects with non-specific asthma symptoms in relation to smoking history before (panel A) and after adjustments for sex, height, age, airway calibre (FEV1(%pred)), use of inhaled corticosteroids, pollen season (panel B).
798 Table 2
A. Malinovschi et al. Levels of FeNO (GM(95%CI)) in subjects with non-specific asthma symptoms or asthma in different smoking strata.
Never-smokers Ex-smokers Current smokers
Non-specific asthma symptoms (n Z 186)
Asthma (n Z 96)
p-value
13.9 (12.0, 16.0) (n Z 63) 15.3 (13.5, 17.4) (n Z 43) 10.4 (9.1, 11.9) (n Z 80)
24.4 (19.0, 31.3) (n Z 45) 26.1 (18.9, 36.1) (n Z 19) 16.7 (12.8, 21.7) (n Z 32)
<0.001 <0.001 <0.001
(r Z 0.39, p < 0.001), ex- (r Z 0.40, p Z 0.002) and current smokers (r Z 0.36, p < 0.001). A positive association was found between exhaled NO levels and presence of atopy in all three smoking strata (p < 0.001) as well as between FeNO levels and blood eosinophil levels in never- (r Z 0.45, p < 0.001) and current smokers (r Z 0.31, p Z 0.001).
Discussion The main finding of the present study was that exhaled nitric oxide could differentiate asthmatic subjects from non-asthmatic subjects equally well in both the never- and current smoker strata within a population with asthma-like symptoms. The FeNO cut-off levels needed in order to achieve high sensitivity or specificity for asthma diagnosis were thus lower in current smokers. However, the percentual asthma-associated increase of FeNO was of a similar size in currently smoking and in never- or exsmoking subjects. The main strength of this study is that the present selection of the subjects corresponds to a primary care setting where subjects would seek for their asthma-like respiratory symptoms. The relatively even distribution of this population with regard to smoking history and the majority of the assessments being performed by the same nursing staff are other strengths of the study. We acknowledge however the low life-time tobacco consumption in this group of young adults, especially in the exsmoking group, that might have an impact on generalisation of present results. A weakness of this type of study, where patients are asked to participate via questionnaire, is that subjects with milder symptoms might not participate. However, this would not have affected the clinical importance of the present results, as these patients would probably not seek medical care for their symptoms anyway. In this study performed in 2005e2006, FeNO measurements
were commenced halfway through the study and as all participants were invited simultaneously to participate, it is plausible to believe that asthma patients would respond faster, and hence be among the first to be tested. We cannot precisely know how this difference in the proportion of asthmatics tested affects the estimates of sensitivity and specificity, but it is unlikely that the effect is different for smokers vs non-smokers. Exhaled NO discriminated between asthmatics and nonasthmatics in all the three smoking strata. That exhaled NO can differentiate asthmatics from non-asthmatics in never-smokers is well-known.1,32 The discriminatory value in currently smoking individuals was however questioned in previous studies17,23,33 and mechanistically explained by a predominant neutrophilic22 or paucigranulocytic34 inflammation of the airways in smoking asthmatics, which is not accompanied by increased FeNO. This is however not supported by a recent study that demonstrated a reduction of bronchial production of NO in response to oral steroids in smoking asthmatics, giving support for presence of eosinophil, steroid-sensitive inflammation in the airways of smoking asthmatic subjects.35 Moreover, similar levels of sputum eosinophils, as well as similar predictive value of FeNO for detecting sputum eosinophilia were found in smoking and non-smoking asthmatics.36 Similarly, in the present material we could show similar relations between FeNO and blood eosinophils in both current and neversmoking strata. Rouhos et al.25 and Matsunaga et al.37 reported higher levels of exhaled NO in asthmatic smokers than in healthy smoking subjects, results in line with the present study. One major difference between these studies and our study is the fact that the controls in our study were also subjects with asthma-like symptoms and not healthy subjects without any lower respiratory symptoms, which probably explains the lower discriminative value of the FeNO in the present study compared with in Matsunaga et al.37 This study design difference makes the results of the present study more relevant in the clinical setting.
Figure 2 ROC-curve for the diagnostic value of exhaled NO to predict asthma in a population of never- (left panel), ex- (middle panel) and currently smoking (right panel) subjects with asthma like-symptoms.
Smoking and the diagnostic value of exhaled NO
799
Table 3 Description of AUC and three chosen cut-offs (optimal, 90% sensitivity and 90% specificity) for ROC-curves presented in Fig. 2.
Number of asthma cases AUC Optimal cut-off (sensitivity, specificity) (PPV, NPV at this cut-off) Cut-off for 90% sensitivity (specificity at this cut-off) (PPV, NPV at this cut-off) Cut-off for 90% specificity (sensitivity at this cut-off) (PPV, NPV at this cut-off)
Never-smokers (n Z 108)
Ex-smokers (n Z 62)
Current smokers (n Z 112)
45 0.72 (0.62, 0.82) 15 ppb (77.8%, 63.5%) (60%, 80%) 10 ppb 26.9% 47%, 81% 31 ppb 35.6% 72%, 66%
19 0.74 (0.60, 0.89) 22 ppb (63.2%, 86.1%) (67%, 84%) 11 ppb 23.3% 36%, 91% 26 ppb 42.1% 65%, 78%
32 0.70 (0.59, 0.82) 17 ppb (56.3%, 82.5%) (57%, 82%) 7 ppb 15.1% 32%, 89% 22 ppb 37.5% 61%, 78%
Abbreviations: AUC e area under the curve, ROC e receiver operating characteristic, PPV e positive predictive value, NPV e negative predictive value.
Exhaled NO showed similar values in ex-smoking and in never-smoking subjects. It might have been expected that this group would show a discrepancy between symptoms and inflammation as an improvement of symptoms with continuing oxidant and protease burden has been reported by Louhelainen et al.38 On the other hand, the epithelial changes seen in smoking asthmatics are not found in exsmoking asthmatics, suggesting that at least these changes can be reversed by smoking cessation. Regarding exhaled NO, the effects of previous smoking have previously been considered transitory,39 and the effects of exsmoking in populational studies has been contradictory, with a decrease of about 10% in some studies16,40 and no clear effect in other studies.17 In the present population of young adults, the large majority of ex-smokers had a very low life-time tobacco consumption, which may also explain why no significant differences in the levels of exhaled NO were found. An important finding of the present study was that FeNO cut-off levels needed in order to achieve either high specificity or sensitivity were lower in current than neversmokers. We proposed a cut-off of 22 ppb in current smokers vs. 31 ppb in never-smokers for 90% specificity. These levels correspond to an approximately 30% lower cutoff in current smokers, results in line with the proposed best cut-offs by Matsunaga et al.37 or Schleich et al.36 The magnitude of decrease of FeNO cut-off levels in current vs.
Table 4
Sensitivity and specificity for different arbitrary FeNO cut-offs in the range 10e50 ppb in different smoking strata.
FeNO cut-off
10 15 20 25 30 35 50
never-smokers corresponds to reported smoking-induced decrease of FeNO (30e60%) in population-based materials.16e18 Different optimal FeNO cut-offs to diagnose asthma have been reported in the literature, as summarized by Majid et al.5 Defining the optimal cut-off by maximising the sum of sensitivity and specificity resulted in a higher cut-off for current smokers than for never-smokers in the present material, as opposed to an expected lower cut-off, which was the case for the cut-offs for 90% sensitivity or specificity. This underlines the weakness of presenting only optimal cut-offs. An approach of the type rule-in/rule-out has been suggested, with a FeNO-value < 25 ppb in order to exclude the presence of steroid-sensitive asthmatic inflammation and value > 50 ppb to signal the presence of steroid-sensitive asthmatic inflammation.1,6,7 Looking at the present material, it must be underlined that the high sensitivity (90%) necessary to rule out was obtained only at very low levels of FeNO, under 10 ppb. These cut-offs were also associated with a good negative predictive value of around 80% in never-smokers and 90% in ex- and current smokers. A value of FeNO > 25 ppb in never- and exsmokers was associated with values of specificity around 90% in all three groups. However, in the current-smoking group this cut-off was too high and resulted in a lower sensitivity compared with never- and ex-smokers. The achieved positive predictive values were more modest in
Never-smokers (n Z 108)
Ex-smokers (n Z 62)
Current smokers (n Z 112)
Sensitivity
Specificity
Sensitivity
Specificity
Sensitivity
Specificity
91% 78% 51% 40% 35% 27% 20%
27% 64% 82% 89% 90% 94% 95%
95% 80% 68% 42% 42% 37% 16%
16% 47% 74% 91% 95% 98% 98%
78% 56% 44% 25% 19% 16% 9%
48% 73% 88% 93% 94% 96% 98%
800 the current-smoking group and the best PPV-values were 77% in never-smokers and 89% in ex-smokers, both for a cutoff of 35 ppb. In conclusion, the present study offers support for the use of exhaled NO for diagnosing asthma in subjects with asthma-like symptoms even if they are ex-smokers or current smokers, as the diagnostic value of exhaled NO for asthma was similar in all three examined smoking strata. For both smokers and non-smokers, there was a clear separation in FeNO-values between asthmatics and nonasthmatics. The different cut-off values shown in this article as necessary in order to achieve higher sensitivity or specificity should offer decision support for the clinician dealing with the interpretation of the test in patients with asthma-like symptoms who have previously smoked or are still smoking. These FeNO cut-off values should be further validated in other studies, and in particular, there is a need for studies documenting these cut-offs in relation to measures of eosinophilic airway inflammation or response to steroid treatment, in order to evaluate their ability to rule in or rule out the presence of steroid-sensitive airway inflammation in smokers.
Funding The present data collection has been funded with an unrestricted grant from AstraZeneca. Andrei Malinovschi was funded by Bror Hjerpsteds Stiftelse and Uppsala University Hospital.
Conflict of interest None of the authors have any conflicts of interest to declare.
Authors’ contributions VB, HH and CP designed the original study. AM and CP proposed the tested hypothesis and performed the statistical analysis and data interpretation. The first draft of the manuscript was prepared by AM. All authors contributed to and approved the final version of the manuscript.
Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.rmed.2012. 02.009.
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A. Malinovschi et al. 3. Malmberg LP, Pelkonen AS, Haahtela T, Turpeinen M. Exhaled nitric oxide rather than lung function distinguishes preschool children with probable asthma. Thorax 2003;58(6):494e9. 4. Smith AD, Cowan JO, Filsell S, et al. Diagnosing asthma: comparisons between exhaled nitric oxide measurements and conventional tests. Am J Respir Crit Care Med 2004;169(4): 473e8. 5. Majid H, Kao C. Utility of exhaled nitric oxide in the diagnosis and management of asthma. Curr Opin Pulm Med 2010;16(1): 42e7. 6. Barnes PJ, Dweik RA, Gelb AF, et al. Exhaled nitric oxide in pulmonary diseases: a comprehensive review. Chest 2010; 138(3):682e92. 7. Dweik RA, Boggs PB, Erzurum SC, et al. An Official ATS clinical practice guideline: interpretation of exhaled nitric oxide levels (FENO) for clinical Applications. Am J Respir Crit Care Med 2011;184(5):602e15. 8. Kharitonov SA, Robbins RA, Yates D, Keatings V, Barnes PJ. Acute and chronic effects of cigarette smoking on exhaled nitric oxide. Am J Respir Crit Care Med 1995;152(2):609e12. 9. Persson MG, Zetterstrom O, Agrenius V, Ihre E, Gustafsson LE. Single-breath nitric oxide measurements in asthmatic patients and smokers. Lancet 1994;343(8890):146e7. 10. Horvath I, Donnelly LE, Kiss A, Balint B, Kharitonov SA, Barnes PJ. Exhaled nitric oxide and hydrogen peroxide concentrations in asthmatic smokers. Respiration 2004;71(5):463e8. 11. Verleden GM, Dupont LJ, Verpeut AC, Demedts MG. The effect of cigarette smoking on exhaled nitric oxide in mild steroidnaive asthmatics. Chest 1999;116(1):59e64. 12. Nadif R, Matran R, Maccario J, et al. Passive and active smoking and exhaled nitric oxide levels according to asthma and atopy in adults. Ann Allergy Asthma Immunol 2010;104(5):385e93. 13. Gibson PG. Using fractional exhaled nitric oxide to guide asthma therapy: design and methodological issues for ASthma TReatment ALgorithm studies. Clin Exp Allergy 2009;39(4): 478e90. 14. James AL, Palmer LJ, Kicic E, et al. Decline in lung function in the Busselton Health Study: the effects of asthma and cigarette smoking. Am J Respir Crit Care Med 2005;171(2):109e14. 15. Siroux V, Pin I, Oryszczyn MP, Le Moual N, Kauffmann F. Relationships of active smoking to asthma and asthma severity in the EGEA study. Epidemiological study on the Genetics and Environment of Asthma. Eur Respir J 2000;15(3):470e7. 16. Malinovschi A, Janson C, Holmkvist T, Norback D, Merilainen P, Hogman M. Effect of smoking on exhaled nitric oxide and flowindependent nitric oxide exchange parameters. Eur Respir J 2006;28(2):339e45. 17. Olin AC, Rosengren A, Thelle DS, Lissner L, Bake B, Toren K. Height, age, and atopy are associated with fraction of exhaled nitric oxide in a large adult general population sample. Chest 2006;130(5):1319e25. 18. Taylor DR, Mandhane P, Greene JM, et al. Factors affecting exhaled nitric oxide measurements: the effect of sex. Respir Res 2007;8:82. 19. Hoyt JC, Robbins RA, Habib M, et al. Cigarette smoke decreases inducible nitric oxide synthase in lung epithelial cells. Exp Lung Res 2003;29(1):17e28. 20. Higman DJ, Strachan AM, Buttery L, et al. Smoking impairs the activity of endothelial nitric oxide synthase in saphenous vein. Arterioscler Thromb Vasc Biol 1996;16(4):546e52. 21. Corradi M, Pesci A, Casana R, et al. Nitrate in exhaled breath condensate of patients with different airway diseases. Nitric Oxide 2003;8(1):26e30. 22. Thomson NC, Chaudhuri R, Livingston E. Asthma and cigarette smoking. Eur Respir J 2004;24(5):822e33. 23. Malinovschi A, Janson C, Hogman M, et al. Both allergic and nonallergic asthma are associated with increased FE(NO) levels, but only in never-smokers. Allergy 2009;64(1):55e61.
Smoking and the diagnostic value of exhaled NO 24. Michils A, Louis R, Peche R, Baldassarre S, Van Muylem A. Exhaled nitric oxide as a marker of asthma control in smoking patients. Eur Respir J 2009;33(6):1295e301. 25. Rouhos A, Ekroos H, Karjalainen J, Sarna S, Haahtela T, Sovijarvi AR. Smoking attenuates increase in exhaled nitric oxide in atopic but not in nonatopic young adults with asthma. Int Arch Allergy Immunol 2010;152(3):226e32. 26. Backer V, Nolte H, Pedersen L, Dam N, Harving H. Unawareness and undertreatment of asthma: follow-up in a different geographic area in Denmark. Allergy 2009;64(8):1179e84. 27. Winder JA, Nash K, Brunn JW. Validation of a life quality (LQ) test for asthma. Ann Allergy Asthma Immunol 2000;85(6 Pt 1):467e72. 28. ATS/ERS recommendations for standardized procedures for the online and offline measurement of exhaled lower respiratory nitric oxide and nasal nitric oxide. Am J Respir Crit Care Med 2005;171(8):912e30. 29. Ringbaek T, Seersholm N, Viskum K. Standardised mortality rates in females and males with COPD and asthma. Eur Respir J 2005;25(5):891e5. 30. Alving K, Malinovschi A. Basic aspects of exhaled nitric oxide. Eur Respir Monogr 2010;49:1e31. 31. Youden WJ. Index for rating diagnostic tests. Cancer 1950;3(1): 32e5. 32. Alving K, Weitzberg E, Lundberg JM. Increased amount of nitric oxide in exhaled air of asthmatics. Eur Respir J 1993;6(9): 1368e70. 33. McSharry CP, McKay IC, Chaudhuri R, Livingston E, Fraser I, Thomson NC. Short and long-term effects of cigarette smoking
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independently influence exhaled nitric oxide concentration in asthma. J Allergy Clin Immunol 2005;116(1):88e93. Hillas G, Kostikas K, Mantzouranis K, et al. Exhaled nitric oxide and exhaled breath condensate pH as predictors of sputum cell counts in optimally treated asthmatic smokers. Respirology 2011;16(5):811e8. Spears M, Weir CJ, Smith AD, et al. Bronchial nitric oxide flux (J’aw) is sensitive to oral corticosteroids in smokers with asthma. Respir Med 2011;105(12):1823e30. Schleich FN, Seidel L, Sele J, et al. Exhaled nitric oxide thresholds associated with a sputum eosinophil count >/Z3% in a cohort of unselected patients with asthma. Thorax 2010; 65(12):1039e44. Matsunaga K, Hirano T, Akamatsu K, et al. Exhaled nitric oxide Cutoff values for asthma diagnosis according to rhinitis and smoking Status in Japanese subjects. Allergol Int 2011;60(3): 331e7. Louhelainen N, Stark H, Mazur W, Rytila P, Djukanovic R, Kinnula VL. Elevation of sputum matrix metalloproteinase-9 persists up to 6 months after smoking cessation: a research study. BMC Pulmonary Medicine 2010;10:13. Hogman M, Holmkvist T, Walinder R, et al. Increased nitric oxide elimination from the airways after smoking cessation. Clin Sci (Lond) 2002;103(1):15e9. Travers J, Marsh S, Aldington S, et al. Reference ranges for exhaled nitric oxide derived from a random community survey of adults. Am J Respir Crit Care Med 2007;176(3): 238e42.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/rmed
The value of exhaled nitric oxide to identify asthma in smoking patients with asthma-like symptoms Andrei Malinovschi a,*, Vibeke Backer b, Henrik Harving c, Celeste Porsbjerg b a
Department of Medical Sciences, Clinical Physiology, Uppsala University, Uppsala, Sweden Respiratory Research Unit, Bispebjerg Hospital, University Hospital of Copenhagen, Denmark c Department of Respiratory Diseases, Aalborg Hospital, University of Aarhus, Denmark b
Received 1 December 2011; accepted 17 February 2012 Available online 8 March 2012
KEYWORDS Exhaled nitric oxide; Smoking habits; Asthma; Diagnostic value; Cut-offs
Summary Background: The fraction of nitric oxide in exhaled air (FeNO) is used in asthma diagnosis and management. Smoking reduces FeNO and 20e35% of asthmatics are smoking. However no guidelines exist on the diagnostic value of FeNO in smokers. Therefore we assessed the value of FeNO to diagnose asthma in a population of subjects with asthma-like symptoms and different smoking habits. Methods: Measurements of FeNO, lung function, bronchial responsiveness and allergy testing were performed in 282 subjects (108 never-, 62 ex- and 112 current smokers) aged 14e44 years, with symptoms suggestive of asthma. These subjects were a subset of subjects reporting respiratory symptoms (n Z 686) in a random population sample (n Z 10,400). Results: A diagnosis of asthma was given to 96 of the 282 subjects. Subjects with asthma had higher FeNO levels than subjects with non-specific asthma symptoms in all three smoking strata (p < 0.001), with a percentual increase of FeNO by 76% in never-, 71% in ex- and 60% in current smokers. The area under the ROC-curve was similar in never-, ex- and current smokers (0.72 vs. 0.74 vs. 0.70). The cut-offs were approximately 30% lower for either 90% specificity (22 vs. 31 ppb) or 90% sensitivity (7 vs. 10 ppb) in current vs. never-smokers. Conclusions: FeNO could differentiate asthmatic subjects from non-asthmatic subjects with asthma-like symptoms equally well in both never- and current smokers within a random population sample. The FeNO cut-off levels needed in order to achieve high sensitivity or specificity were lower in current smokers. ª 2012 Elsevier Ltd. All rights reserved.
* Corresponding author. Department of Medical Sciences, Clinical Physiology, Uppsala University, Akademiska sjukhuset Ing 35, 1tr, 75185 Uppsala, Sweden. Tel.: þ46 18 6115856; fax: þ46 18 554079. E-mail address: [email protected] (A. Malinovschi). 0954-6111/$ - see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.rmed.2012.02.009
Smoking and the diagnostic value of exhaled NO
Background The fraction of nitric oxide in exhaled air (FeNO) is a marker of steroid-sensitive airway inflammation, used in monitoring the treatment of patients with asthma.1 However, the diagnostic value of exhaled NO for asthma is still debated and is more modest in the general populations2 than in subjects with symptoms highly suggestive of asthma.3,4 Different optimal FeNO cut-offs for diagnosing asthma have been reported in different studies.5 The general recommendations, also reiterated by the recently published ATS Clinical Practice Guideline, are that a FeNOvalue below 25 ppb, measured at a exhaled flow-rate of 50 mL/s, suggests that there is no steroid-sensitive inflammation present, while a value greater than 50 ppb is likely to be associated with steroid-sensitive inflammation.1,6,7 These cut-offs have been described for adult nonsmoking subjects and although it is well acknowledged that smoking reduces FeNO both in healthy and asthmatic subjects,8e12 no clear guidelines exist for the use of exhaled NO in smoking asthmatics1,6,7,13, indicating a need for more knowledge in this field. Smoking asthmatics are a category of patients representing up to 20e35% of the adult asthma patients in Northern Europe.14,15 Smoking induces a decrease of exhaled NO levels in both healthy8,9 and asthmatic subjects.10,11 In epidemiological studies, this decrease in FeNO has been reported to be in the range 30e60% when compared with FeNO-values of never-smoking subjects.16e18 Several mechanisms, such as an inhibited bronchial epithelial iNOS-related production of NO19,20 or an increased catabolism of NO,21 have been postulated to explain this decrease. The clinical use of exhaled NO in smoking asthmatic subjects has been questioned, as a more neutrophil- and steroid-insensitive inflammation has been described in smoking asthmatics.22 Contradictory results are reported regarding the presence of increased levels of exhaled NO in asthmatic smokers compared with healthy smokers.23e25 Furthermore, the effects of previous smoking on exhaled NO are controversial with a slight decrease16 or no decrease17 in FeNO. No study has addressed the value of exhaled NO in diagnosing asthma in ex-smoking subjects. A problem with the previous studies2,23e25 looking at the diagnostic value of FeNO in current smokers is that the reference group consisted of healthy smokers. However, in clinical practice the need is in differentiating subjects with asthma-like symptoms that have asthma diagnosed from the ones that do not have asthma. Therefore, the aim of the present study was to assess the value of exhaled NO to diagnose asthma in a population of subjects with asthmalike symptoms, with special emphasis on the value of exhaled NO to diagnose asthma in subjects with different smoking habits.
Methods Screening population A random population sample of 10,400 subjects, aged 14e44 years, and living in Denmark was drawn from the civil registration list,26 with the aim of examining asthma
795 treatment in a Western society. All subjects were mailed a letter with a validated self-administered asthma and rhinitis screening questionnaire including 20 questions adopted from the ACAAI screening program.27 Of the 47% that responded, 2250 had at least one respiratory and/or nasal symptom. 686 of these subjects recorded two or more respiratory symptoms, accepted the study invitation and were examined.
Examination All participants had their lung function measured (Vitalograph), and a specific IgE test was performed, followed by a bronchial provocation test with methacholine and a reversibility test with inhaled beta2-agonist, as previously described elsewhere.26 Halfway through the study it became possible to measure exhaled NO, after which these measurements were performed on 320 of 329 (97%) of the participants. Exhaled NO (FeNO) was measured using an online technique at a exhalation flow-rate of 50 mL/s, based on electrochemical sensor (NIOX Mino, Aerocrine AB, Solna, Sweden), according to ATS guidelines.28 The measurements of exhaled NO were performed at approximately 1 h after the start of examination and before any lung function tests.
Exclusion and inclusion criteria Subjects with at least two symptoms suggestive of asthma (listed in the Online Supplement Table S1) were eligible for the present study and therefore 33 subjects with no symptoms when examined at the clinic or only one asthmatic symptom were excluded. Subjects fulfilling the spirometric criteria for COPD were excluded (n Z 5), leaving a total of 282 subjects included in the study.
Smoking history All participants filled in a questionnaire regarding smoking habits. Subjects were classified as never-smokers, exsmokers (subjects who ever have smoked or tried smoking) and current smokers (subjects currently smoking). Lifetime tobacco exposure was calculated for ex- and current smokers as pack-years (tobacco consumption [cigarettes/ day]/20 * duration of smoking [years]).
Asthma diagnosis All subjects were interviewed by a respiratory specialist (70% of the cases the same specialist), who made a diagnosis of asthma based on the presence of symptoms of asthma (i.e. shortness of breath, chest tightness, cough, exercise induced dyspnoea, night-time awakenings, or respiratory symptoms induced by allergen contact) in combination with at least one of the following: 1) Airway hyper-responsiveness (AHR) to inhaled methacholine < 8.0 mmol. 2) At least 250 ml increase in FEV1 after bronchodilator.29 3) Daily use of systemic steroid, inhaled steroid, or inhaled beta2-agonist.
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4) Asthma symptoms during but not outside the pollen season, eventually supported by allergic rhinitis, although no objective signs of asthma outside season were found.
ROC-curves was defined as corresponding to the maximum value of Youden’s index (sensitivity þ specificity - 1).31
Ethics Diagnosis of rhinitis Patients with rhinitis were defined as subjects with symptoms of rhinitis such as runny nose, stuffy nose, sneeze, itchy nose, mouth, ear or eyes, red and runny eyes, reduced sense of smell, or symptoms due to allergen exposure.
The study was approved by the Ethics Committee of Aalborg, Denmark. All participants received information in both oral and written forms and gave their consent in writing before enrolment.
Results
Statistical analysis STATA version 10.1 (Stata Corp, College Station, TX, USA) was used for all analyses. The distribution of FeNO levels is skewed to the right and therefore log-transformation was used in order to meet normality assumptions. Similarly, methacholine doseresponse slope (DRS) and blood eosinophils were logtransformed as they are known to have a similar distribution as FeNO. Consequently, FeNO, DRS and blood eosinophils were presented as geometrical mean (95% CI) in descriptive statistics. Percentual increase of FeNO in asthmatic subjects vs. non-asthmatic subjects was obtained from linear regression models before and after adjusting for other known determinants of FeNO (sex, height, age, airway calibre (FEV1(%pred)), use of inhaled corticosteroids and pollen season30) after backtransformation of regression coefficients, as previously described elsewhere.23 The optimal cut-off for the
Population characteristics with regard to asthma diagnosis The subjects who underwent FeNO measurements did not differ from the other participants undergoing clinical examination in terms of any of the variables presented in Table 1 (all p-values > 0.05, data not shown), apart from the percentage of asthma patients, which was smaller then found in the group that did not have FeNO measured (33% vs 46%, p Z 0.01). The characteristics of the subjects included in the present study are shown, with regard to final diagnosis, in Table 1. Subjects who were diagnosed with asthma were more often IgE-sensitised, had rhinitis, and used inhaled corticosteroids or bronchodilators more frequently, compared with subjects with symptoms that were concluded to be non-specific. Furthermore, asthmatic subjects had a higher degree of
Table 1 Subject characteristics expressed as mean (standard deviation(SD)), N(%) or geometrical mean (GM) with 95% confidence interval (95% CI).
Age (mean (SD)) Female gender (N(%)) IgE-sensitisation (N(%)) Blood eosinophils (GM(95%CI)) Smoking history Never (N(%)) Ex (N(%)) Current (N(%)) Rhinitis (N(%)) FEV1% pred (mean (SD)) FVC% pred (mean (SD)) FEV1/FVC (mean (SD)) AHR (N(%)) DRS (GM(95%CI)) SABA-use (N(%)) ICS-use (N(%)) Asthma symptomsb
Non-specific asthma symptoms (n Z 186)
Asthma (n Z 96)
p-value
32.7 112 40 0.17
(8.9) (60.2%) (22.0%) (0.16, 0.19)
32.7 57 50 0.21
(8.7) (59.4%) (54.4%) (0.19, 0.24)
1.00 0.89 <0.001 0.002
63 43 80 86 97.0 96.8 0.82 4 0.65 3 1 4
(33.9%) (23.1%) (43.0%) (46.2%) (10.8) (11.5) (0.07) (2.2%)a (0.59, 0.71)a (1.6%) (0.5%) (3, 6)
45 19 32 63 94.4 97.2 0.80 35 1.65 46 24 6
(46.9%) (19.8%) (33.3%) (65.6%) (15.2) (12.8) (0.09) (36.5%)a (1.30,2.08)a (47.9%) (25.2%) (4, 8)
0.10
0.002 0.11 0.82 0.01 <0.001 <0.001 <0.001 <0.001 0.02
Abbreviations: FEV1 e forced expiratory volume in 1 s, FVC eforced vital capacity, AHR e airway hyper-responsiveness (defined as a PD20 methacholine < 8 mmol), DRS e dose-response slope to methacholine provocation, SABA - short-acting beta2-agonist, ICS - inhaled corticosteroids. a Two subjects in the non-specific asthma symptoms group and seven subjects in the asthma group did not perform methacholine provocation test. b Expressed as median (interquantile range).
Smoking and the diagnostic value of exhaled NO bronchial responsiveness to methacholine, higher blood eosinophils and had a lower FEV1/FVC-ratio (Table 1). The pack-year consumption in the currently smoking group (n Z 112) was 10 (0, 30) (median, range) and 14 subjects had a pack-year consumption < 1. The ex-smoking group (n Z 62) was characterised by a pack-year consumption of 0 (0, 7.5) and 50 subjects had a pack-year consumption < 1. Atopy was less common in subjects who were current smokers than never- or ex-smokers (22% vs. 46% and 48%, respectively).
Exhaled NO and smoking habits The FeNO-values were significantly lower in the currently smoking group (11.9 ppb (10.5, 13.5)) than in the neversmoking group (17.6 ppb (15.2, 20.2), p < 0.001) and the ex-smoking group (18.0 ppb (15.6, 20.8), p < 0.001). No significant differences between never- and ex-smoking subjects were found in regard to FeNO.
Using FeNO to differentiate asthma from nonspecific asthma symptoms Subjects with asthma had higher levels of exhaled NO than subjects with non-specific asthma symptoms (21.8 ppb (18.6, 25.5) vs. 12.5 ppb (11.5, 13.6), p < 0.001). The area under the curve (AUC) for the ROC of FeNO for asthma diagnosis was 0.72 (0.66, 0.79). The value was similar after excluding the subjects currently using inhaled corticosteroids (AUC 0.73 (0.65, 0.80), n Z 256). Subjects with asthma had higher FeNO levels than subjects with non-specific asthma symptoms in all three smoking strata (p < 0.001, Table 2) with a percentual increase of FeNO of 76% in never-, 71% in ex- and 60% in current smokers (Fig. 1A). Even after adjustments for potential confounders as sex, height, age, airway calibre (FEV1(%pred)), use of inhaled corticosteroids and pollen
797 season, increased exhaled NO was significantly related to presence of asthma (Fig. 1B). ROC-curves for the value of exhaled NO to detect asthma in never-, ex- and current smokers are presented in Fig. 2. The areas under the ROC-curves, optimal cut-offs and cut-offs for 90% sensitivity and 90% specificity for FeNO levels to diagnose asthma are presented in Table 3. No significant differences between the AUC-values for the ROC-curves of FeNO were found between the three smoking strata (p > 0.05). The AUC for the ROC-curve for current smokers, after excluding smokers with a pack-year history less than 1 pack-year, was unchanged (0.70 (0.57, 0.82), n Z 98) as were the cut-offs for 90% sensitivity (6 ppb), 90% specificity (22 ppb) and optimal cut-off (17 ppb). Excluding subjects on inhaled corticosteroids, the AUC values were 0.76 (0.65, 0.86) for never-smokers (n Z 96), 0.74 (0.55, 0.93) for ex-smokers (n Z 55) and 0.68 (0.56, 0.81) for current smokers (n Z 105). No significant differences between the AUC-values for the ROC-curves in the three smoking strata were found (p > 0.05). The cut-offs for 90% sensitivity were 11 ppb (spec 33.9%), 11 ppb (spec 23.3%) and 6 ppb (spec 20%) in never-, ex- and current smokers. The cut-offs for 90% specificity were 31 ppb (sens 41.2%), 26 ppb (sens 41.7%) and 22 ppb (sens 36%) in never-, ex- and current smokers. In Table 4, the sensitivity and specificity at different FeNO cut-off levels are given. Positive predictive values and negative predictive values for these arbitrary cut-offs are presented in Online Supplement Table S2.
Exhaled NO and its relationship to atopy, bronchial responsiveness and blood eosinophils in the different smoking strata There was a positive association between bronchial responsiveness and exhaled nitric oxide among never-
Figure 1 Percentual increase (mean (95%CI)) of FeNO in asthmatics vs. subjects with non-specific asthma symptoms in relation to smoking history before (panel A) and after adjustments for sex, height, age, airway calibre (FEV1(%pred)), use of inhaled corticosteroids, pollen season (panel B).
798 Table 2
A. Malinovschi et al. Levels of FeNO (GM(95%CI)) in subjects with non-specific asthma symptoms or asthma in different smoking strata.
Never-smokers Ex-smokers Current smokers
Non-specific asthma symptoms (n Z 186)
Asthma (n Z 96)
p-value
13.9 (12.0, 16.0) (n Z 63) 15.3 (13.5, 17.4) (n Z 43) 10.4 (9.1, 11.9) (n Z 80)
24.4 (19.0, 31.3) (n Z 45) 26.1 (18.9, 36.1) (n Z 19) 16.7 (12.8, 21.7) (n Z 32)
<0.001 <0.001 <0.001
(r Z 0.39, p < 0.001), ex- (r Z 0.40, p Z 0.002) and current smokers (r Z 0.36, p < 0.001). A positive association was found between exhaled NO levels and presence of atopy in all three smoking strata (p < 0.001) as well as between FeNO levels and blood eosinophil levels in never- (r Z 0.45, p < 0.001) and current smokers (r Z 0.31, p Z 0.001).
Discussion The main finding of the present study was that exhaled nitric oxide could differentiate asthmatic subjects from non-asthmatic subjects equally well in both the never- and current smoker strata within a population with asthma-like symptoms. The FeNO cut-off levels needed in order to achieve high sensitivity or specificity for asthma diagnosis were thus lower in current smokers. However, the percentual asthma-associated increase of FeNO was of a similar size in currently smoking and in never- or exsmoking subjects. The main strength of this study is that the present selection of the subjects corresponds to a primary care setting where subjects would seek for their asthma-like respiratory symptoms. The relatively even distribution of this population with regard to smoking history and the majority of the assessments being performed by the same nursing staff are other strengths of the study. We acknowledge however the low life-time tobacco consumption in this group of young adults, especially in the exsmoking group, that might have an impact on generalisation of present results. A weakness of this type of study, where patients are asked to participate via questionnaire, is that subjects with milder symptoms might not participate. However, this would not have affected the clinical importance of the present results, as these patients would probably not seek medical care for their symptoms anyway. In this study performed in 2005e2006, FeNO measurements
were commenced halfway through the study and as all participants were invited simultaneously to participate, it is plausible to believe that asthma patients would respond faster, and hence be among the first to be tested. We cannot precisely know how this difference in the proportion of asthmatics tested affects the estimates of sensitivity and specificity, but it is unlikely that the effect is different for smokers vs non-smokers. Exhaled NO discriminated between asthmatics and nonasthmatics in all the three smoking strata. That exhaled NO can differentiate asthmatics from non-asthmatics in never-smokers is well-known.1,32 The discriminatory value in currently smoking individuals was however questioned in previous studies17,23,33 and mechanistically explained by a predominant neutrophilic22 or paucigranulocytic34 inflammation of the airways in smoking asthmatics, which is not accompanied by increased FeNO. This is however not supported by a recent study that demonstrated a reduction of bronchial production of NO in response to oral steroids in smoking asthmatics, giving support for presence of eosinophil, steroid-sensitive inflammation in the airways of smoking asthmatic subjects.35 Moreover, similar levels of sputum eosinophils, as well as similar predictive value of FeNO for detecting sputum eosinophilia were found in smoking and non-smoking asthmatics.36 Similarly, in the present material we could show similar relations between FeNO and blood eosinophils in both current and neversmoking strata. Rouhos et al.25 and Matsunaga et al.37 reported higher levels of exhaled NO in asthmatic smokers than in healthy smoking subjects, results in line with the present study. One major difference between these studies and our study is the fact that the controls in our study were also subjects with asthma-like symptoms and not healthy subjects without any lower respiratory symptoms, which probably explains the lower discriminative value of the FeNO in the present study compared with in Matsunaga et al.37 This study design difference makes the results of the present study more relevant in the clinical setting.
Figure 2 ROC-curve for the diagnostic value of exhaled NO to predict asthma in a population of never- (left panel), ex- (middle panel) and currently smoking (right panel) subjects with asthma like-symptoms.
Smoking and the diagnostic value of exhaled NO
799
Table 3 Description of AUC and three chosen cut-offs (optimal, 90% sensitivity and 90% specificity) for ROC-curves presented in Fig. 2.
Number of asthma cases AUC Optimal cut-off (sensitivity, specificity) (PPV, NPV at this cut-off) Cut-off for 90% sensitivity (specificity at this cut-off) (PPV, NPV at this cut-off) Cut-off for 90% specificity (sensitivity at this cut-off) (PPV, NPV at this cut-off)
Never-smokers (n Z 108)
Ex-smokers (n Z 62)
Current smokers (n Z 112)
45 0.72 (0.62, 0.82) 15 ppb (77.8%, 63.5%) (60%, 80%) 10 ppb 26.9% 47%, 81% 31 ppb 35.6% 72%, 66%
19 0.74 (0.60, 0.89) 22 ppb (63.2%, 86.1%) (67%, 84%) 11 ppb 23.3% 36%, 91% 26 ppb 42.1% 65%, 78%
32 0.70 (0.59, 0.82) 17 ppb (56.3%, 82.5%) (57%, 82%) 7 ppb 15.1% 32%, 89% 22 ppb 37.5% 61%, 78%
Abbreviations: AUC e area under the curve, ROC e receiver operating characteristic, PPV e positive predictive value, NPV e negative predictive value.
Exhaled NO showed similar values in ex-smoking and in never-smoking subjects. It might have been expected that this group would show a discrepancy between symptoms and inflammation as an improvement of symptoms with continuing oxidant and protease burden has been reported by Louhelainen et al.38 On the other hand, the epithelial changes seen in smoking asthmatics are not found in exsmoking asthmatics, suggesting that at least these changes can be reversed by smoking cessation. Regarding exhaled NO, the effects of previous smoking have previously been considered transitory,39 and the effects of exsmoking in populational studies has been contradictory, with a decrease of about 10% in some studies16,40 and no clear effect in other studies.17 In the present population of young adults, the large majority of ex-smokers had a very low life-time tobacco consumption, which may also explain why no significant differences in the levels of exhaled NO were found. An important finding of the present study was that FeNO cut-off levels needed in order to achieve either high specificity or sensitivity were lower in current than neversmokers. We proposed a cut-off of 22 ppb in current smokers vs. 31 ppb in never-smokers for 90% specificity. These levels correspond to an approximately 30% lower cutoff in current smokers, results in line with the proposed best cut-offs by Matsunaga et al.37 or Schleich et al.36 The magnitude of decrease of FeNO cut-off levels in current vs.
Table 4
Sensitivity and specificity for different arbitrary FeNO cut-offs in the range 10e50 ppb in different smoking strata.
FeNO cut-off
10 15 20 25 30 35 50
never-smokers corresponds to reported smoking-induced decrease of FeNO (30e60%) in population-based materials.16e18 Different optimal FeNO cut-offs to diagnose asthma have been reported in the literature, as summarized by Majid et al.5 Defining the optimal cut-off by maximising the sum of sensitivity and specificity resulted in a higher cut-off for current smokers than for never-smokers in the present material, as opposed to an expected lower cut-off, which was the case for the cut-offs for 90% sensitivity or specificity. This underlines the weakness of presenting only optimal cut-offs. An approach of the type rule-in/rule-out has been suggested, with a FeNO-value < 25 ppb in order to exclude the presence of steroid-sensitive asthmatic inflammation and value > 50 ppb to signal the presence of steroid-sensitive asthmatic inflammation.1,6,7 Looking at the present material, it must be underlined that the high sensitivity (90%) necessary to rule out was obtained only at very low levels of FeNO, under 10 ppb. These cut-offs were also associated with a good negative predictive value of around 80% in never-smokers and 90% in ex- and current smokers. A value of FeNO > 25 ppb in never- and exsmokers was associated with values of specificity around 90% in all three groups. However, in the current-smoking group this cut-off was too high and resulted in a lower sensitivity compared with never- and ex-smokers. The achieved positive predictive values were more modest in
Never-smokers (n Z 108)
Ex-smokers (n Z 62)
Current smokers (n Z 112)
Sensitivity
Specificity
Sensitivity
Specificity
Sensitivity
Specificity
91% 78% 51% 40% 35% 27% 20%
27% 64% 82% 89% 90% 94% 95%
95% 80% 68% 42% 42% 37% 16%
16% 47% 74% 91% 95% 98% 98%
78% 56% 44% 25% 19% 16% 9%
48% 73% 88% 93% 94% 96% 98%
800 the current-smoking group and the best PPV-values were 77% in never-smokers and 89% in ex-smokers, both for a cutoff of 35 ppb. In conclusion, the present study offers support for the use of exhaled NO for diagnosing asthma in subjects with asthma-like symptoms even if they are ex-smokers or current smokers, as the diagnostic value of exhaled NO for asthma was similar in all three examined smoking strata. For both smokers and non-smokers, there was a clear separation in FeNO-values between asthmatics and nonasthmatics. The different cut-off values shown in this article as necessary in order to achieve higher sensitivity or specificity should offer decision support for the clinician dealing with the interpretation of the test in patients with asthma-like symptoms who have previously smoked or are still smoking. These FeNO cut-off values should be further validated in other studies, and in particular, there is a need for studies documenting these cut-offs in relation to measures of eosinophilic airway inflammation or response to steroid treatment, in order to evaluate their ability to rule in or rule out the presence of steroid-sensitive airway inflammation in smokers.
Funding The present data collection has been funded with an unrestricted grant from AstraZeneca. Andrei Malinovschi was funded by Bror Hjerpsteds Stiftelse and Uppsala University Hospital.
Conflict of interest None of the authors have any conflicts of interest to declare.
Authors’ contributions VB, HH and CP designed the original study. AM and CP proposed the tested hypothesis and performed the statistical analysis and data interpretation. The first draft of the manuscript was prepared by AM. All authors contributed to and approved the final version of the manuscript.
Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.rmed.2012. 02.009.
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