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Use of likelihood ratios improves clinical interpretation of IgG and IgA anti-DGP antibody testing for celiac disease in adults and children
Clinical Biochemistry 44 (2011) 248–250
Contents lists available at ScienceDirect
Clinical Biochemistry j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c l i n b i o c h e m
Case Report
Use of likelihood ratios improves clinical interpretation of IgG and IgA anti-DGP antibody testing for celiac disease in adults and children Pieter Vermeersch a, Thomas Richter d, Almuthe C. Hauer e, Martin Stern f, Holm H. Uhlig g, Klaus-Peter Zimmer h, Martin W. Laass i, Ilse Hoffman b, Martin Hiele c, Thomas Mothes j, Xavier Bossuyt a,⁎ a
Laboratory Medicine, Immunology, University Hospitals Leuven, Catholic University of Leuven, Belgium Department of Pediatrics, University Hospitals Leuven, Catholic University of Leuven, Belgium Department of Gastroenterology, University Hospitals Leuven, Catholic University of Leuven, Belgium d Municipal Children's Hospital “Sankt Georg”, Leipzig, Germany e Children's University Hospital Graz, Austria f Children's University Hospital, Tübingen, Germany g Children's University Hospital Leipzig, Germany h Children's University Hospital, Giessen, Germany i Children's University Hospital, Dresden, Germany j Institute of Laboratory Medicine, Clinical Chemistry and Molecular Diagnostics, University Hospital, Leipzig, Germany b c
a r t i c l e
i n f o
Article history: Received 13 August 2010 Received in revised form 20 September 2010 Accepted 26 September 2010 Available online 12 October 2010 Keywords: Celiac disease Diagnosis Deamidated gliadin peptide IgG IgA Sensitivity and specificity
a b s t r a c t Objective: To evaluate whether taking into account anti-deamidated gliadin peptide (DGP) antibody concentrations improves clinical interpretation. Design and methods: We calculated likelihood ratios (LR) using data from two previously published studies for assays from EUROIMMUN and INOVA. Results: LRs markedly increased with increasing IgG anti-DGP concentrations. LRs also increased with increasing IgA anti-DGP concentrations, although they were lower than for IgG anti-DGP. Conclusions: Use of LRs for different test result intervals improves clinical interpretation. © 2010 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.
Introduction Celiac disease (CD) is an autoimmune disorder characterized by a heightened immunological responsiveness to ingested gluten (from wheat, barley or rye) in genetically susceptible individuals. Selective deamidation of gliadin (which is rich in glutamine residues) by tTG to deamidated gliadin peptides (DGP) is thought to play an important role in the pathogenesis of celiac disease [1]. The definitive diagnosis of celiac disease requires a small intestinal biopsy examination. Detection of IgA autoantibodies against tissue transglutaminase (tTG) or endomysium is often used as first-line test to identify individuals who require a duodenal biopsy. Detection of IgG and IgA antibodies against deamidated gliadin peptides (anti-DGP) is a new tool for the diagnosis of CD. In 2001, Aleanzi et al. showed that deamidation of
Abbreviations: CD, celiac disease; DGP, deamidated gliadin peptide; LR, likelihood ratio; tTG, tissue transglutaminase. ⁎ Corresponding author. Laboratory Medicine, Immunology, University Hospitals Leuven, Herestraat 49, B-3000 Leuven, Belgium. Fax: + 32 16 347042. E-mail address: [email protected] (X. Bossuyt).
gliadin increases the binding of anti-gliadin antibodies in serum of CD patients, but not of non-CD patients [2]. Recent studies have shown that the performance of IgG anti-DGP antibodies is comparable to IgA anti-tTG [3–5] and at least as good as that of IgA anti-DGP [5–8]. Traditionally, a single cut-off is used for the clinical interpretation of serologic tests and all values above or below the cut-off value are given the same interpretation. This will result in a significant loss of diagnostic accuracy if the likelihood for CD increases with increasing antibody concentration. It has previously been shown that IgG and IgA anti-DGP titers are higher in CD patients compared to non-CD patients [8] and that the positive predictive value increases when the cut-off for IgG and IgA anti-DGP is increased [9]. We have previously demonstrated that the likelihood for CD increases with increasing IgA anti-tTG concentrations [10], but this has not yet been investigated for anti-DGP antibodies. The additional information provided by the antibody concentration can be taken into account by using likelihood ratios for different test result intervals. The likelihood ratio (LR) is the ratio of the probability that a patient with the disease has a particular test result over the probability that a patient without the disease has the same
0009-9120/$ – see front matter © 2010 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.clinbiochem.2010.09.026
P. Vermeersch et al. / Clinical Biochemistry 44 (2011) 248–250
We calculated LRs for test result intervals for the IgG anti-DGP assays from EUROIMMUN, (Lübeck, Germany; IgG Anti-Gliadin GAF-3X) and INOVA (San Diego, USA; Quanta Lite Gliadin IgG II) in 229 CD patients and 900 disease control patients. Serologic data were derived from two previously published studies by two independent groups [3,4]. The study by Prause et al. included only pediatric patients, while the study by Vermeersch et al. included both adults and children. The CD group consisted of 57 adult patients (aged 18 years or older) and 172 pediatric patients and the disease control group consisted of 519 adult patients and 381 children. All CD patients and disease control patients aged 3 months or older underwent an intestinal biopsy (on a normal glutencontaining diet). The LRs for the IgA anti-DGP assays from the same manufacturers were calculated in 229 CD patients and 505 disease controls. Only 346 of the 519 adult disease control patients and none of the 222 disease control children from the study by Vermeersch et al. were tested. Results The LR for CD depends on the IgG and IgA anti-DGP concentration The overall sensitivity, specificity and LR of the IgG anti-DGP assays was 82.5%, 98.9% and 74.3 for the EUROIMMUN test and 87.8%, 98.8%, and 71.9 for the INOVA test. For IgA anti-DGP, the overall sensitivity, specificity, and LR was, respectively, 81.7%, 95.4%, and 17.9 for the EUROIMMUN assay and 84.7%, 94.3%, and 14.8 for the INOVA assay. The LRs for the different test result intervals are shown in Table 1. The two independent studies gave similar results. For the two IgG anti-DGP assays, the LR for CD markedly increased with increasing IgG anti-DGP concentrations: from 12.9 for results between 25 and 50 U/
Table 1 Overview of the LRs as a function of antibody concentration for the different IgG and IgA anti-DGP assays. Assay
Cut-off
Interval
LR (95% confidence interval)
EUROIMMUN IgG anti-DGP
25 U/mL
INOVA IgG anti-DGP
20 U/mL
b 10 U/mL 10 to b 25 U/mL 25 to 50 U/mL N 50 to 100 U/mL N 100 U/mL b 10 U/mL 10 to b 20 U/mL 20 to 30 U/mL N 30 to 50 U/mL N 50 U/mL b 10 U/mL 10 to b 25 U/mL 25 to 50 U/mL N 50 to 100 U/mL N 100 U/mL b 10 U/mL 10 to b 20 U/mL 20 to 30 U/mL N 30 to 50 U/mL N 50 U/mL
0.09 2.0 12.9 60.9 531 0.11 0.48 10.5 29.9 +∞ 0.19 1.7 5.5 8.5 32.0 0.14 0.86 2.2 4.4 59.9
EUROIMMUN IgA anti-DGP
INOVA IgA anti-DGP
25 U/mL
20 U/mL
(0.06 to 0.14) (1.2 to 3.3) (5.6 to 29.7) (14.7 to 253) (74.6 to 3774) (0.07 to 0.16) (1.19 to 1.20) (4.1 to26.5) (11.9 to75.0) (0.13 to 0.29) (1.1 to 2.8) (2.2 to 14.0) (3.8 to 19.2) (17.2 to 59.5) (0.08 to 0.23) (0.55 to 1.33) (1.1 to 4.4) (1.9 to 10.2) (26.9 to 133)
Use of LRs improves clinical interpretation The posttest probabilities for the different test result intervals for the IgG anti-DGP assays as a function of pretest probabilities can be calculated through application of Bayes' Theorem. The graphical representations of the two IgG anti-DGP assays are shown in Fig. 1. The graphical representations of the two IgA anti-DGP assays are available upon request. The use of LRs for different test result intervals can improve the clinical interpretation of serologic testing for CD. We illustrate this for the EUROIMMUN IgG anti-DGP assay. For a pretest probability of 2.4% (estimated prevalence of CD in patients who underwent IgA anti-tTG testing at the University Hospitals Leuven), the posttest probability of CD will be 0.2%, 4.6%, 24.1%, 59.9%, and 92.9% for a test result of, respectively, b10 U/mL, 10–b25 U/mL, 25–50 U/mL, N50–100 U/mL, and N100 U/mL. For a pretest probability of 7.3% (prevalence of CD in patients who underwent IgA anti-tTG testing and an intestinal biopsy), the probabilities will be, respectively, 0.7%, 13.6%, 50.4%, 82.7%, and 97.6%.
EUROIMMUN IgG anti-DGP
1
Posttest probability
Materials and methods
mL up to 531 for results N100 U/mL for EUROIMMUN and from 10.5 for results between 20 and 30 U/mL (“weak positive” according to the manufacturer's instructions) to + ∞ for results N50 U/mL for INOVA. For the two IgA anti-DGP assays, the LR also increased with increasing antibody concentration, although the LRs were lower than for IgG anti-DGP (Table 1). For EUROIMMUN, a test result between 10 and 25 U/mL had a LR of 2.0 for IgG and 1.7 for IgA. For INOVA, a test result between 10 and 20 U/mL had a LR of 0.48 for IgG and 0.86 for IgA. This means that such test results indicate only small differences in pretest to posttest probabilities. Values b10 U/mL had a LR of 0.09 (INOVA) and 0.11 (EUROIMMUN) for IgG antibodies and 0.19 (EUROIMMUN) and 0.14 (INOVA) for IgA antibodies. These results demonstrate that the LR for CD depends on the IgG and IgA anti-DGP concentration.
0.8 >100 50-100 25-<50 10-<25 <10
0.6 0.4 0.2 0 0
0.2
0.4
0.6
0.8
1
Pretest probability INOVA IgG anti-DGP 1
Posttest probability
test result. A LR of 10, for example, means that a patient with the disease is 10 times more likely to have a certain test result than a patient without the disease. In the present paper, we demonstrate that the use of likelihood ratios (LR) for different IgG and IgA anti-DGP test result intervals improves the clinical interpretation of serologic testing for CD. In contrast to the positive and negative predictive value, LRs are not dependent on the prevalence of the disease and allow to calculate the posttest probability for any given pretest probability through application of Bayes' Theorem [11].
249
0.8 >50 30-50 20-<30 10-<20 <10
0.6 0.4 0.2 0
0
0.2
0.4
0.6
0.8
1
Pretest probability Fig. 1. Posttest probability for celiac disease as a function of the pretest probability and IgG anti-DGP concentration.
250
P. Vermeersch et al. / Clinical Biochemistry 44 (2011) 248–250
Discussion In the present study, we demonstrate that the LR for CD increases with increasing IgG and IgA anti-DGP antibody concentrations and that interpreting anti-DGP testing as negative/positive only results in a significant loss of clinically relevant information. The increase in LRs as a function of the antibody concentration was lower for IgA antiDGP than for IgG anti-DGP antibodies, confirming the better diagnostic performance of IgG anti-DGP antibodies for the diagnosis of celiac disease [5–8]. The accuracy of a diagnostic test is traditionally reported in terms of sensitivity and specificity in scientific papers and systematic reviews. The use of sensitivity and specificity does, however, result in the loss of diagnostic accuracy information if the likelihood for CD disease increases with increasing antibody concentration. The use of likelihood ratios for different test result intervals allows to take into account this additional information. Calculating the posttest probability using Bayes' Theorem is impractical and difficult for clinicians, but a graphical representation of the posttest probability as a function of the pretest probability can overcome this problem [11]. In the present paper, we provide a graphical representation for the IgG and IgA anti-DGP assays for different test result intervals. This graphical representation could be provided to clinicians together with the test results in order to help the clinician to better interpret a given test result. While such an approach is ill suited for printed laboratory reports, the advances in information technology today allow laboratories to provide this graphical representation “one mouse click away” to clinicians when results are checked electronically. Moreover, a graphical representation of the pretest probability as a function of the posttest probability is the most efficient way to convey diagnostic accuracy information to clinicians [11]. Using sensitivity and specificity, doctors tend to overestimate the posttest probability of disease, particularly when the pretest probability is low. In conclusion, interpreting anti-DGP results using LRs for different test result intervals improves the clinical interpretation for the diagnosis of CD in children and adults. A graphical representation of the posttest probability as a function of the pretest probability improves clinical interpretation. Conflict of interest Part of this the study was supported by a local grant from the University Hospital, Leipzig, Germany.
Mothesand H. Uhlig described sequences used as antigens for detection of antibodies against deamidated gliadin peptides (Osman et al. 2000, Schwertz et al. 2004) (patented as T. Mothes, A.A. Osman, H.H. Uhlig, T. Günnel, A. Dietl: Peptide und Verfahren zur Diagnostik von Zöliakie und Dermatitis herpetiformis) and T. Mothes belongs to inventors of the patent “Verfahren und Immunabsorbentien zur spezifischen Detektion und Absorption Zöliakie- und Dermatitis herpetiformis assoziierter Antikörper” (EUROIMMUN AG, Lübeck, Germany) submitted 2008. The other authors report no conflicts of interest. Acknowledgments We thank EUROIMMUN (Lübeck, Germany) and INOVA (San Diego, CA) for the generous donation of assays. References [1] Aleanzi M, Demonte AM, Esper C, Garcilazo S, Waggener M. Celiac disease: antibody recognition against native and selectively deamidated gliadin peptides. Clin Chem 2001;47:2023–8. [2] Vermeersch P, Geboes K, Marien G, Hoffman I, Hiele M, Bossuyt X. Diagnostic performance of IgG anti-deamidated gliadin peptide antibody assays is comparable to IgA anti-tTG in celiac disease. Clin Chim Acta 2010;411:931–5. [3] Prause C, Richter T, Koletzko S, Uhlig HH, Hauer AC, Stern M, et al. New developments in serodiagnosis of childhood celiac disease: assay of antibodies against deamidated gliadin. Ann N Y Acad Sci 2009;1173:28–35. [4] Prause C, Ritter M, Probst C, Daehnrich C, Schlumberger W, Komorowski L, et al. Antibodies against deamidated gliadin as new and accurate biomarkers of childhood coeliac disease. J Pediatr Gastroenterol Nutr 2009;49:52–8. [5] Volta U, Granito A, Fiorini E, Parisi C, Piscaglia M, Pappas G, et al. Usefulness of antibodies to deamidated gliadin peptides in celiac disease diagnosis and followup. Dig Dis Sci 2008;53:1582–8. [6] Niveloni S, Sugai E, Cabanne A, Vazquez H, Argonz J, Smecuol E, et al. Antibodies against synthetic deamidated gliadin peptides as predictors of celiac disease: prospective assessment in an adult population with a high pretest probability of disease. Clin Chem 2007;53:2186–92. [7] Sugai E, Vazquez H, Nachman F, Moreno ML, Mazure R, Smecuol E, et al. Accuracy of testing for antibodies to synthetic gliadin-related peptides in celiac disease. Clin Gastroenterol Hepatol 2006;4:1112–7. [8] Sugai E, Moreno ML, Hwang HJ, Cabanne A, Crivelli A, Nachman F, et al. Celiac disease serology in patients with different pretest probabilities: is biopsy avoidable? World J Gastroenterol 2010;7:3144–52. [9] Vermeersch P, Coenen D, Geboes K, Marie G, Hiele M, Bossuyt X. Use of likelihood ratios improves clinical interpretation of IgA anti-tTG antibody testing for celiac disease. Clin Chim Acta 2010;411:13–7. [10] Bossuyt X. Clinical performance characteristics of a laboratory test. A practical approach in the autoimmune laboratory. Autoimmun Rev 2009;8:543–8. [11] Vermeersch P, Bossuyt X. Comparative analysis of different approaches to report diagnostic accuracy. Arch Intern Med 2010;170:734–5.
Contents lists available at ScienceDirect
Clinical Biochemistry j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c l i n b i o c h e m
Case Report
Use of likelihood ratios improves clinical interpretation of IgG and IgA anti-DGP antibody testing for celiac disease in adults and children Pieter Vermeersch a, Thomas Richter d, Almuthe C. Hauer e, Martin Stern f, Holm H. Uhlig g, Klaus-Peter Zimmer h, Martin W. Laass i, Ilse Hoffman b, Martin Hiele c, Thomas Mothes j, Xavier Bossuyt a,⁎ a
Laboratory Medicine, Immunology, University Hospitals Leuven, Catholic University of Leuven, Belgium Department of Pediatrics, University Hospitals Leuven, Catholic University of Leuven, Belgium Department of Gastroenterology, University Hospitals Leuven, Catholic University of Leuven, Belgium d Municipal Children's Hospital “Sankt Georg”, Leipzig, Germany e Children's University Hospital Graz, Austria f Children's University Hospital, Tübingen, Germany g Children's University Hospital Leipzig, Germany h Children's University Hospital, Giessen, Germany i Children's University Hospital, Dresden, Germany j Institute of Laboratory Medicine, Clinical Chemistry and Molecular Diagnostics, University Hospital, Leipzig, Germany b c
a r t i c l e
i n f o
Article history: Received 13 August 2010 Received in revised form 20 September 2010 Accepted 26 September 2010 Available online 12 October 2010 Keywords: Celiac disease Diagnosis Deamidated gliadin peptide IgG IgA Sensitivity and specificity
a b s t r a c t Objective: To evaluate whether taking into account anti-deamidated gliadin peptide (DGP) antibody concentrations improves clinical interpretation. Design and methods: We calculated likelihood ratios (LR) using data from two previously published studies for assays from EUROIMMUN and INOVA. Results: LRs markedly increased with increasing IgG anti-DGP concentrations. LRs also increased with increasing IgA anti-DGP concentrations, although they were lower than for IgG anti-DGP. Conclusions: Use of LRs for different test result intervals improves clinical interpretation. © 2010 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.
Introduction Celiac disease (CD) is an autoimmune disorder characterized by a heightened immunological responsiveness to ingested gluten (from wheat, barley or rye) in genetically susceptible individuals. Selective deamidation of gliadin (which is rich in glutamine residues) by tTG to deamidated gliadin peptides (DGP) is thought to play an important role in the pathogenesis of celiac disease [1]. The definitive diagnosis of celiac disease requires a small intestinal biopsy examination. Detection of IgA autoantibodies against tissue transglutaminase (tTG) or endomysium is often used as first-line test to identify individuals who require a duodenal biopsy. Detection of IgG and IgA antibodies against deamidated gliadin peptides (anti-DGP) is a new tool for the diagnosis of CD. In 2001, Aleanzi et al. showed that deamidation of
Abbreviations: CD, celiac disease; DGP, deamidated gliadin peptide; LR, likelihood ratio; tTG, tissue transglutaminase. ⁎ Corresponding author. Laboratory Medicine, Immunology, University Hospitals Leuven, Herestraat 49, B-3000 Leuven, Belgium. Fax: + 32 16 347042. E-mail address: [email protected] (X. Bossuyt).
gliadin increases the binding of anti-gliadin antibodies in serum of CD patients, but not of non-CD patients [2]. Recent studies have shown that the performance of IgG anti-DGP antibodies is comparable to IgA anti-tTG [3–5] and at least as good as that of IgA anti-DGP [5–8]. Traditionally, a single cut-off is used for the clinical interpretation of serologic tests and all values above or below the cut-off value are given the same interpretation. This will result in a significant loss of diagnostic accuracy if the likelihood for CD increases with increasing antibody concentration. It has previously been shown that IgG and IgA anti-DGP titers are higher in CD patients compared to non-CD patients [8] and that the positive predictive value increases when the cut-off for IgG and IgA anti-DGP is increased [9]. We have previously demonstrated that the likelihood for CD increases with increasing IgA anti-tTG concentrations [10], but this has not yet been investigated for anti-DGP antibodies. The additional information provided by the antibody concentration can be taken into account by using likelihood ratios for different test result intervals. The likelihood ratio (LR) is the ratio of the probability that a patient with the disease has a particular test result over the probability that a patient without the disease has the same
0009-9120/$ – see front matter © 2010 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.clinbiochem.2010.09.026
P. Vermeersch et al. / Clinical Biochemistry 44 (2011) 248–250
We calculated LRs for test result intervals for the IgG anti-DGP assays from EUROIMMUN, (Lübeck, Germany; IgG Anti-Gliadin GAF-3X) and INOVA (San Diego, USA; Quanta Lite Gliadin IgG II) in 229 CD patients and 900 disease control patients. Serologic data were derived from two previously published studies by two independent groups [3,4]. The study by Prause et al. included only pediatric patients, while the study by Vermeersch et al. included both adults and children. The CD group consisted of 57 adult patients (aged 18 years or older) and 172 pediatric patients and the disease control group consisted of 519 adult patients and 381 children. All CD patients and disease control patients aged 3 months or older underwent an intestinal biopsy (on a normal glutencontaining diet). The LRs for the IgA anti-DGP assays from the same manufacturers were calculated in 229 CD patients and 505 disease controls. Only 346 of the 519 adult disease control patients and none of the 222 disease control children from the study by Vermeersch et al. were tested. Results The LR for CD depends on the IgG and IgA anti-DGP concentration The overall sensitivity, specificity and LR of the IgG anti-DGP assays was 82.5%, 98.9% and 74.3 for the EUROIMMUN test and 87.8%, 98.8%, and 71.9 for the INOVA test. For IgA anti-DGP, the overall sensitivity, specificity, and LR was, respectively, 81.7%, 95.4%, and 17.9 for the EUROIMMUN assay and 84.7%, 94.3%, and 14.8 for the INOVA assay. The LRs for the different test result intervals are shown in Table 1. The two independent studies gave similar results. For the two IgG anti-DGP assays, the LR for CD markedly increased with increasing IgG anti-DGP concentrations: from 12.9 for results between 25 and 50 U/
Table 1 Overview of the LRs as a function of antibody concentration for the different IgG and IgA anti-DGP assays. Assay
Cut-off
Interval
LR (95% confidence interval)
EUROIMMUN IgG anti-DGP
25 U/mL
INOVA IgG anti-DGP
20 U/mL
b 10 U/mL 10 to b 25 U/mL 25 to 50 U/mL N 50 to 100 U/mL N 100 U/mL b 10 U/mL 10 to b 20 U/mL 20 to 30 U/mL N 30 to 50 U/mL N 50 U/mL b 10 U/mL 10 to b 25 U/mL 25 to 50 U/mL N 50 to 100 U/mL N 100 U/mL b 10 U/mL 10 to b 20 U/mL 20 to 30 U/mL N 30 to 50 U/mL N 50 U/mL
0.09 2.0 12.9 60.9 531 0.11 0.48 10.5 29.9 +∞ 0.19 1.7 5.5 8.5 32.0 0.14 0.86 2.2 4.4 59.9
EUROIMMUN IgA anti-DGP
INOVA IgA anti-DGP
25 U/mL
20 U/mL
(0.06 to 0.14) (1.2 to 3.3) (5.6 to 29.7) (14.7 to 253) (74.6 to 3774) (0.07 to 0.16) (1.19 to 1.20) (4.1 to26.5) (11.9 to75.0) (0.13 to 0.29) (1.1 to 2.8) (2.2 to 14.0) (3.8 to 19.2) (17.2 to 59.5) (0.08 to 0.23) (0.55 to 1.33) (1.1 to 4.4) (1.9 to 10.2) (26.9 to 133)
Use of LRs improves clinical interpretation The posttest probabilities for the different test result intervals for the IgG anti-DGP assays as a function of pretest probabilities can be calculated through application of Bayes' Theorem. The graphical representations of the two IgG anti-DGP assays are shown in Fig. 1. The graphical representations of the two IgA anti-DGP assays are available upon request. The use of LRs for different test result intervals can improve the clinical interpretation of serologic testing for CD. We illustrate this for the EUROIMMUN IgG anti-DGP assay. For a pretest probability of 2.4% (estimated prevalence of CD in patients who underwent IgA anti-tTG testing at the University Hospitals Leuven), the posttest probability of CD will be 0.2%, 4.6%, 24.1%, 59.9%, and 92.9% for a test result of, respectively, b10 U/mL, 10–b25 U/mL, 25–50 U/mL, N50–100 U/mL, and N100 U/mL. For a pretest probability of 7.3% (prevalence of CD in patients who underwent IgA anti-tTG testing and an intestinal biopsy), the probabilities will be, respectively, 0.7%, 13.6%, 50.4%, 82.7%, and 97.6%.
EUROIMMUN IgG anti-DGP
1
Posttest probability
Materials and methods
mL up to 531 for results N100 U/mL for EUROIMMUN and from 10.5 for results between 20 and 30 U/mL (“weak positive” according to the manufacturer's instructions) to + ∞ for results N50 U/mL for INOVA. For the two IgA anti-DGP assays, the LR also increased with increasing antibody concentration, although the LRs were lower than for IgG anti-DGP (Table 1). For EUROIMMUN, a test result between 10 and 25 U/mL had a LR of 2.0 for IgG and 1.7 for IgA. For INOVA, a test result between 10 and 20 U/mL had a LR of 0.48 for IgG and 0.86 for IgA. This means that such test results indicate only small differences in pretest to posttest probabilities. Values b10 U/mL had a LR of 0.09 (INOVA) and 0.11 (EUROIMMUN) for IgG antibodies and 0.19 (EUROIMMUN) and 0.14 (INOVA) for IgA antibodies. These results demonstrate that the LR for CD depends on the IgG and IgA anti-DGP concentration.
0.8 >100 50-100 25-<50 10-<25 <10
0.6 0.4 0.2 0 0
0.2
0.4
0.6
0.8
1
Pretest probability INOVA IgG anti-DGP 1
Posttest probability
test result. A LR of 10, for example, means that a patient with the disease is 10 times more likely to have a certain test result than a patient without the disease. In the present paper, we demonstrate that the use of likelihood ratios (LR) for different IgG and IgA anti-DGP test result intervals improves the clinical interpretation of serologic testing for CD. In contrast to the positive and negative predictive value, LRs are not dependent on the prevalence of the disease and allow to calculate the posttest probability for any given pretest probability through application of Bayes' Theorem [11].
249
0.8 >50 30-50 20-<30 10-<20 <10
0.6 0.4 0.2 0
0
0.2
0.4
0.6
0.8
1
Pretest probability Fig. 1. Posttest probability for celiac disease as a function of the pretest probability and IgG anti-DGP concentration.
250
P. Vermeersch et al. / Clinical Biochemistry 44 (2011) 248–250
Discussion In the present study, we demonstrate that the LR for CD increases with increasing IgG and IgA anti-DGP antibody concentrations and that interpreting anti-DGP testing as negative/positive only results in a significant loss of clinically relevant information. The increase in LRs as a function of the antibody concentration was lower for IgA antiDGP than for IgG anti-DGP antibodies, confirming the better diagnostic performance of IgG anti-DGP antibodies for the diagnosis of celiac disease [5–8]. The accuracy of a diagnostic test is traditionally reported in terms of sensitivity and specificity in scientific papers and systematic reviews. The use of sensitivity and specificity does, however, result in the loss of diagnostic accuracy information if the likelihood for CD disease increases with increasing antibody concentration. The use of likelihood ratios for different test result intervals allows to take into account this additional information. Calculating the posttest probability using Bayes' Theorem is impractical and difficult for clinicians, but a graphical representation of the posttest probability as a function of the pretest probability can overcome this problem [11]. In the present paper, we provide a graphical representation for the IgG and IgA anti-DGP assays for different test result intervals. This graphical representation could be provided to clinicians together with the test results in order to help the clinician to better interpret a given test result. While such an approach is ill suited for printed laboratory reports, the advances in information technology today allow laboratories to provide this graphical representation “one mouse click away” to clinicians when results are checked electronically. Moreover, a graphical representation of the pretest probability as a function of the posttest probability is the most efficient way to convey diagnostic accuracy information to clinicians [11]. Using sensitivity and specificity, doctors tend to overestimate the posttest probability of disease, particularly when the pretest probability is low. In conclusion, interpreting anti-DGP results using LRs for different test result intervals improves the clinical interpretation for the diagnosis of CD in children and adults. A graphical representation of the posttest probability as a function of the pretest probability improves clinical interpretation. Conflict of interest Part of this the study was supported by a local grant from the University Hospital, Leipzig, Germany.
Mothesand H. Uhlig described sequences used as antigens for detection of antibodies against deamidated gliadin peptides (Osman et al. 2000, Schwertz et al. 2004) (patented as T. Mothes, A.A. Osman, H.H. Uhlig, T. Günnel, A. Dietl: Peptide und Verfahren zur Diagnostik von Zöliakie und Dermatitis herpetiformis) and T. Mothes belongs to inventors of the patent “Verfahren und Immunabsorbentien zur spezifischen Detektion und Absorption Zöliakie- und Dermatitis herpetiformis assoziierter Antikörper” (EUROIMMUN AG, Lübeck, Germany) submitted 2008. The other authors report no conflicts of interest. Acknowledgments We thank EUROIMMUN (Lübeck, Germany) and INOVA (San Diego, CA) for the generous donation of assays. References [1] Aleanzi M, Demonte AM, Esper C, Garcilazo S, Waggener M. Celiac disease: antibody recognition against native and selectively deamidated gliadin peptides. Clin Chem 2001;47:2023–8. [2] Vermeersch P, Geboes K, Marien G, Hoffman I, Hiele M, Bossuyt X. Diagnostic performance of IgG anti-deamidated gliadin peptide antibody assays is comparable to IgA anti-tTG in celiac disease. Clin Chim Acta 2010;411:931–5. [3] Prause C, Richter T, Koletzko S, Uhlig HH, Hauer AC, Stern M, et al. New developments in serodiagnosis of childhood celiac disease: assay of antibodies against deamidated gliadin. Ann N Y Acad Sci 2009;1173:28–35. [4] Prause C, Ritter M, Probst C, Daehnrich C, Schlumberger W, Komorowski L, et al. Antibodies against deamidated gliadin as new and accurate biomarkers of childhood coeliac disease. J Pediatr Gastroenterol Nutr 2009;49:52–8. [5] Volta U, Granito A, Fiorini E, Parisi C, Piscaglia M, Pappas G, et al. Usefulness of antibodies to deamidated gliadin peptides in celiac disease diagnosis and followup. Dig Dis Sci 2008;53:1582–8. [6] Niveloni S, Sugai E, Cabanne A, Vazquez H, Argonz J, Smecuol E, et al. Antibodies against synthetic deamidated gliadin peptides as predictors of celiac disease: prospective assessment in an adult population with a high pretest probability of disease. Clin Chem 2007;53:2186–92. [7] Sugai E, Vazquez H, Nachman F, Moreno ML, Mazure R, Smecuol E, et al. Accuracy of testing for antibodies to synthetic gliadin-related peptides in celiac disease. Clin Gastroenterol Hepatol 2006;4:1112–7. [8] Sugai E, Moreno ML, Hwang HJ, Cabanne A, Crivelli A, Nachman F, et al. Celiac disease serology in patients with different pretest probabilities: is biopsy avoidable? World J Gastroenterol 2010;7:3144–52. [9] Vermeersch P, Coenen D, Geboes K, Marie G, Hiele M, Bossuyt X. Use of likelihood ratios improves clinical interpretation of IgA anti-tTG antibody testing for celiac disease. Clin Chim Acta 2010;411:13–7. [10] Bossuyt X. Clinical performance characteristics of a laboratory test. A practical approach in the autoimmune laboratory. Autoimmun Rev 2009;8:543–8. [11] Vermeersch P, Bossuyt X. Comparative analysis of different approaches to report diagnostic accuracy. Arch Intern Med 2010;170:734–5.