- Email: [email protected]
Urine antigen test for diagnosis of HIV-associated tuberculosis – Authors' reply
Correspondence
Tuberculosis
Sputum AFB
Determine TB-LAM
Determine TB-LAM and sputum AFB
Xpert MTB/RIF (1 sample)
Determine TB-LAM and Xpert MTB/RIF
Positive Prevalence
Positive Sensitivity
Positive Sensitivity
Positive Sensitivity
Positive Sensitivity
Positive Sensitivity
<50 (n=64)
18
28·1% (17·6–40·8)
6
33·3% (13·3–59·0)
12
66·7% (41·0–86·7)
13
72·2% (46·5–90·3)
13
72·2% (46·5–90·3)
15
83·3% (58·6-96·4)
50–99 (n=64)
11
17·2% (8·9–28·7)
4
14·5% (10·9–69·2)
3
27·3% (6·0–61·0)
6
54·5% (23·4–83·3)
9
81·8% (48·2–97·7)
9
81·8% (48·2–97·7)
100–49 (n=96)
17
17·7% (10·7–26·8)
6
35·3% (14·2–61·7)
6
35·3% (14·2–61·7)
8
47·1% (23·0–72·2)
11
64·7% (38·3–85·8)
11
64·7% (38·3–85·8)
150–99 (n=101)
13
12·9% (7·0–21·0)
2
15·4% (1·9–45·4)
2
15·4% (1·9–45·1)
4
30·8% (9·1–61·4)
4
30·8% (9·1–61·4)
5
38·5% (13·9–68·4)
≥200 (n=189)
25
13·2% (8·9–19·3)
6
24·0% (9·4–45·1)
1
4·0% (1·1–20·4)
6
24·0% (9·4–45·1)
11
44·0% (24·4–65·1)
11
44·0% (24·4–65·1)
Data are number and prevalence (95% CI) or sensitivity (95% CI). AFB=acid-fast bacilli.
Table: Sensitivity of different diagnostic assays for patients with tuberculosis stratified by discreet CD4 cell count
We read with interest the Article1 by Stephen Lawn and colleagues examining the diagnostic accuracy of the urine lipoarabinomannan antigen assay (Determine TB-LAM; Alere, Waltham, MA, USA) for HIV-associated adult pulmonary tuberculosis. This assay could be a promising addition to diagnostic algorithms for tuberculosis in view of its low cost (estimated at US$3·50 per test strip), ease of collection of urine samples, short turnaround time (30 min), and ability to be done at the point of care without the need for instruments. A 2011 systematic review2 by Minion and co-workers, however, concluded that the LAM urine assay has inadequate sensitivity for the diagnosis of active tuberculosis in unselected cohorts.2 The LAM assay works best in HIVinfected patients with advanced immunosuppression 2–4—individuals in whom diagnosis of tuberculosis is especially challenging. Lawn and colleagues’ study is of great interest because it offers further insights into the diagnostic accuracy of LAM by providing results stratified by CD4 cell count and by comparing the test with other point-of-care diagnostics, including sputum smear microscopy and Xpert MTB/RIF (and combinations thereof). Their results show that the sensitivity of the LAM assay is highest in patients 826
with CD4 counts of fewer than 50 cells per μL (66·7%) and gradually falls with increasing CD4 cell count. Stratification of result by cumulative CD4 count categories (as done by Lawn and co-workers) could result in incorrect conclusions about which HIV-infected people with suspected tuberculosis would optimally benefit from use of the LAM assay. When we recalculated diagnostic accuracy on the basis of discreet CD4 categories, we noted a pronounced (rather than gradual) decrease in sensitivity with increasing CD4 count, from 66·7% in individuals with fewer than 50 CD4 cells per μL to 27·2% in those with 50–99 CD4 cells per μL. By contrast, the sensitivity of Xpert MTB/RIF remained high for both count methods (point estimates of 72·7% for patients with counts <50 cells per μL and 81·8% for those with counts of 50–99 cells per μL) (table). These findings, however, are limited by the small sample size, resulting in wide 95% CIs. In conclusion, Lawn and colleagues’ results show that definition of the role of urine LAM in diagnostic algorithms for HIV-infected people with suspected tuberculosis and CD4 counts of more than 50 cells per μL will necessitate a large study powered for stratification by discreet CD4 cell count categories.
*Colleen F Hanrahan, Annelies Van Rie [email protected] Department of Epidemiology, Gillings School of Global Public Health, University of North Carolina, Chapel Hill, NC 27599, USA 1
2
3
4
Lawn SD, Kerkhoff AD, Vogt M, Wood R. Diagnostic accuracy of a low-cost, urine antigen, point-of-care screening assay for HIV-associated pulmonary tuberculosis before antiretroviral therapy: a descriptive study. Lancet Infect Dis 2012; 12: 201–09. Minion J, Leung E, Talbot E, et al. Diagnosing tuberculosis with urine lipoarabinomannan: systematic review and meta-analysis. Eur Respir J 2011; 38: 1398–405. Lawn SD, Edwards DJ, Kranzer K, et al. Urine lipoarabinomannan assay for tuberculosis screening before antiretroviral therapy diagnostic yield and association with immune reconstitution disease. AIDS 2009; 23: 1875–80. Gounder CR, Kufa T, Wada NI, et al. Diagnostic accuracy of a urine lipoarabinomannan enzymelinked immunosorbent assay for screening ambulatory HIV-infected persons for TB. J Acquir Immune Defic Syndr 2011; 58: 219–23.
Authors’ reply We thank Jonathan Peter and colleagues and Colleen Hanrahan and Annalies Van Rie for their interest in our work. We have assessed the diagnostic accuracy of three sequentially derived assays that detect lipoarabinomannan (LAM) in urine to diagnose HIV-associated tuberculosis.1,2 Two versions of the initial format of the test (ie, ELISA) and the subsequent Determine TB-LAM Ag point-of-care strip tests have yielded remarkably consistent data when screening for HIV-associated tuberculosis in our study cohort in South Africa.1,2
www.thelancet.com/infection Vol 12 November 2012
Correspondence
Peter and colleagues raise questions about the assay cutoff. The assay is manufactured with a qualitative, binary read-out. In accordance with the manufacturer’s instructions, we scored samples examined in our study as positive or negative by direct visual comparison of the test strip bands with the faintest positive band on the assay reference card.2 When strips were read independently by two masked researchers, agreement was excellent (κ=0·97), as was the agreement between Determine TB-LAM and the objectively measured ELISA results (0·84). Thus, we concluded that results could be reproducibly and accurately categorised. We agree that several plausible explanations exist for the low interobserver agreement noted by Peter and colleagues in their study, including the proportion of samples with very low LAM concentrations, observer technique, assay production batch and shelf-life, and methods of urine collection and storage. These issues should be investigated. Furthermore, operational research needs to assess how readily this assay can be implemented in clinical settings and robustness when used by the appropriate cadres of health-care workers. We agree with the points made by Hanrahan and Van Rie. However, irrespective of how the sensitivity data are stratified, the usefulness of the assay will clearly be restricted to patients with fewer than 200 CD4 cells per μL; sensitivity was at best only moderate in such patients but was highest in those with the lowest CD4 cell counts.2 Despite suboptimum sensitivity, the assay had high specificity. Furthermore, the assay permitted rapid diagnosis of tuberculosis in patients with poor prognostic features and high mortality risk.3 Determine TB-LAM clearly should not be used as a standalone test but rather as the initial point-of-care screen for tuberculosis in HIV-infected patients with advanced immunodeficiency, such as those
enrolling in antiretroviral therapy services or those who are sick and need to be admitted to inpatient care.3,4 This strategy would allow rapid point-ofcare screening in patients at greatest risk of dying pending the results of additional diagnostic tests. Such patients have the most to gain from immediate treatment for tuberculosis. Rather than large-scale studies of diagnostic accuracy, which will almost certainly confirm that overall sensitivity is suboptimum, perhaps the more important question is whether use of this assay can reduce mortality in these vulnerable patients. Studies of clinical effects are needed. This assay shows great promise but careful optimisation of its use is needed.4 SDL was funded by the Wellcome Trust, London, UK. RW was funded in part by the International Epidemiologic Database to Evaluate Aids with a grant from the National Institute of Allergy and Infectious Diseases (NIAID: 5U01AI069924-02), Cost-Effectiveness of Preventing AIDS Complications (CEPAC) funded by the National Institutes of Health (NIH, 5 R01AI058736-02), USAID Right to Care (CA 674 A 00 08 0000 700), and the South African Centre for Epidemiological Modelling and Analysis (SACEMA). ADK declares that he has no conflicts of interest.
*Stephen D Lawn, Andrew D Kerkhoff, Robin Wood [email protected] Desmond Tutu HIV Centre, Institute for Infectious Disease and Molecular Medicine, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa (SDL, ADK, RW); Department of Clinical Research, Faculty of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, UK SDL); and George Washington University School of Medicine and Health Sciences, Washington DC, USA (ADK) 1
2
3
www.thelancet.com/infection Vol 12 November 2012
Lawn SD, Edwards DJ, Kranzer K, Vogt M, Bekker LG, Wood R. Urine lipoarabinomannan assay for tuberculosis screening before antiretroviral therapy diagnostic yield and association with immune reconstitution disease. AIDS 2009; 23: 1875–80. Lawn SD, Kerkhoff AD, Vogt M, Wood R. Diagnostic accuracy of a low-cost, urine antigen, point-of-care screening assay for HIV-associated pulmonary tuberculosis before antiretroviral therapy: a descriptive study. Lancet Infect Dis 2012; 12: 201–09. Lawn SD, Kerkhoff AD, Vogt M, Wood R. Clinical significance of lipoarabinomannan (LAM) detection in urine using a low-cost point-of-care diagnostic assay for HIV-associated tuberculosis. AIDS 2012; 26: 1635–43.
4
Lawn SD. Point-of-care detection of lipoarabinomannan (LAM) in urine for diagnosis of HIV-associated tuberculosis: a state of the art review. BMC Infect Dis 2012; 12: 103.
Tests for latent tuberculosis infection and isoniazid preventive therapy Molebogeng Rangaka and colleagues1 showed that the interferon-γ release assays (IGRAs) and the tuberculin skin test (TST) predict active tuberculosis with an incidence rate ratio of 2. Their results not only show the limitations of tests for latent tuberculosis infection, but also have a far more profound implication. The most important role of testing for latent tuberculosis is to identify people who would benefit from isoniazid preventive therapy. Rangaka and colleagues’ findings enable calculation of the attributable fraction of incident tuberculosis that might be averted through provision of isoniazid preventive therapy to latently infected people. Some assumptions are necessary to make this calculation—namely, that isoniazid preventive therapy is not effective in patients who are TST negative,2 completion of isoniazid preventive therapy reduces the incidence of tuberculosis by 60–70% in TST-positive patients,2,3 TST and IGRA have similar predictive ability in screening patients for isoniazid preventive therapy, and worldwide prevalence of TST positivity is roughly one in three. If, as suggested by Rangaka and colleagues, patients with a positive TST or IGRA have an incidence rate ratio for active tuberculosis of 1·6–2·1 relative to TST-negative or IGRA-negative patients, then 44–51% of patients with incident tuberculosis would be TST or IGRA positive. Reduction of this fraction by 65% through isoniazid 827
Tuberculosis
Sputum AFB
Determine TB-LAM
Determine TB-LAM and sputum AFB
Xpert MTB/RIF (1 sample)
Determine TB-LAM and Xpert MTB/RIF
Positive Prevalence
Positive Sensitivity
Positive Sensitivity
Positive Sensitivity
Positive Sensitivity
Positive Sensitivity
<50 (n=64)
18
28·1% (17·6–40·8)
6
33·3% (13·3–59·0)
12
66·7% (41·0–86·7)
13
72·2% (46·5–90·3)
13
72·2% (46·5–90·3)
15
83·3% (58·6-96·4)
50–99 (n=64)
11
17·2% (8·9–28·7)
4
14·5% (10·9–69·2)
3
27·3% (6·0–61·0)
6
54·5% (23·4–83·3)
9
81·8% (48·2–97·7)
9
81·8% (48·2–97·7)
100–49 (n=96)
17
17·7% (10·7–26·8)
6
35·3% (14·2–61·7)
6
35·3% (14·2–61·7)
8
47·1% (23·0–72·2)
11
64·7% (38·3–85·8)
11
64·7% (38·3–85·8)
150–99 (n=101)
13
12·9% (7·0–21·0)
2
15·4% (1·9–45·4)
2
15·4% (1·9–45·1)
4
30·8% (9·1–61·4)
4
30·8% (9·1–61·4)
5
38·5% (13·9–68·4)
≥200 (n=189)
25
13·2% (8·9–19·3)
6
24·0% (9·4–45·1)
1
4·0% (1·1–20·4)
6
24·0% (9·4–45·1)
11
44·0% (24·4–65·1)
11
44·0% (24·4–65·1)
Data are number and prevalence (95% CI) or sensitivity (95% CI). AFB=acid-fast bacilli.
Table: Sensitivity of different diagnostic assays for patients with tuberculosis stratified by discreet CD4 cell count
We read with interest the Article1 by Stephen Lawn and colleagues examining the diagnostic accuracy of the urine lipoarabinomannan antigen assay (Determine TB-LAM; Alere, Waltham, MA, USA) for HIV-associated adult pulmonary tuberculosis. This assay could be a promising addition to diagnostic algorithms for tuberculosis in view of its low cost (estimated at US$3·50 per test strip), ease of collection of urine samples, short turnaround time (30 min), and ability to be done at the point of care without the need for instruments. A 2011 systematic review2 by Minion and co-workers, however, concluded that the LAM urine assay has inadequate sensitivity for the diagnosis of active tuberculosis in unselected cohorts.2 The LAM assay works best in HIVinfected patients with advanced immunosuppression 2–4—individuals in whom diagnosis of tuberculosis is especially challenging. Lawn and colleagues’ study is of great interest because it offers further insights into the diagnostic accuracy of LAM by providing results stratified by CD4 cell count and by comparing the test with other point-of-care diagnostics, including sputum smear microscopy and Xpert MTB/RIF (and combinations thereof). Their results show that the sensitivity of the LAM assay is highest in patients 826
with CD4 counts of fewer than 50 cells per μL (66·7%) and gradually falls with increasing CD4 cell count. Stratification of result by cumulative CD4 count categories (as done by Lawn and co-workers) could result in incorrect conclusions about which HIV-infected people with suspected tuberculosis would optimally benefit from use of the LAM assay. When we recalculated diagnostic accuracy on the basis of discreet CD4 categories, we noted a pronounced (rather than gradual) decrease in sensitivity with increasing CD4 count, from 66·7% in individuals with fewer than 50 CD4 cells per μL to 27·2% in those with 50–99 CD4 cells per μL. By contrast, the sensitivity of Xpert MTB/RIF remained high for both count methods (point estimates of 72·7% for patients with counts <50 cells per μL and 81·8% for those with counts of 50–99 cells per μL) (table). These findings, however, are limited by the small sample size, resulting in wide 95% CIs. In conclusion, Lawn and colleagues’ results show that definition of the role of urine LAM in diagnostic algorithms for HIV-infected people with suspected tuberculosis and CD4 counts of more than 50 cells per μL will necessitate a large study powered for stratification by discreet CD4 cell count categories.
*Colleen F Hanrahan, Annelies Van Rie [email protected] Department of Epidemiology, Gillings School of Global Public Health, University of North Carolina, Chapel Hill, NC 27599, USA 1
2
3
4
Lawn SD, Kerkhoff AD, Vogt M, Wood R. Diagnostic accuracy of a low-cost, urine antigen, point-of-care screening assay for HIV-associated pulmonary tuberculosis before antiretroviral therapy: a descriptive study. Lancet Infect Dis 2012; 12: 201–09. Minion J, Leung E, Talbot E, et al. Diagnosing tuberculosis with urine lipoarabinomannan: systematic review and meta-analysis. Eur Respir J 2011; 38: 1398–405. Lawn SD, Edwards DJ, Kranzer K, et al. Urine lipoarabinomannan assay for tuberculosis screening before antiretroviral therapy diagnostic yield and association with immune reconstitution disease. AIDS 2009; 23: 1875–80. Gounder CR, Kufa T, Wada NI, et al. Diagnostic accuracy of a urine lipoarabinomannan enzymelinked immunosorbent assay for screening ambulatory HIV-infected persons for TB. J Acquir Immune Defic Syndr 2011; 58: 219–23.
Authors’ reply We thank Jonathan Peter and colleagues and Colleen Hanrahan and Annalies Van Rie for their interest in our work. We have assessed the diagnostic accuracy of three sequentially derived assays that detect lipoarabinomannan (LAM) in urine to diagnose HIV-associated tuberculosis.1,2 Two versions of the initial format of the test (ie, ELISA) and the subsequent Determine TB-LAM Ag point-of-care strip tests have yielded remarkably consistent data when screening for HIV-associated tuberculosis in our study cohort in South Africa.1,2
www.thelancet.com/infection Vol 12 November 2012
Correspondence
Peter and colleagues raise questions about the assay cutoff. The assay is manufactured with a qualitative, binary read-out. In accordance with the manufacturer’s instructions, we scored samples examined in our study as positive or negative by direct visual comparison of the test strip bands with the faintest positive band on the assay reference card.2 When strips were read independently by two masked researchers, agreement was excellent (κ=0·97), as was the agreement between Determine TB-LAM and the objectively measured ELISA results (0·84). Thus, we concluded that results could be reproducibly and accurately categorised. We agree that several plausible explanations exist for the low interobserver agreement noted by Peter and colleagues in their study, including the proportion of samples with very low LAM concentrations, observer technique, assay production batch and shelf-life, and methods of urine collection and storage. These issues should be investigated. Furthermore, operational research needs to assess how readily this assay can be implemented in clinical settings and robustness when used by the appropriate cadres of health-care workers. We agree with the points made by Hanrahan and Van Rie. However, irrespective of how the sensitivity data are stratified, the usefulness of the assay will clearly be restricted to patients with fewer than 200 CD4 cells per μL; sensitivity was at best only moderate in such patients but was highest in those with the lowest CD4 cell counts.2 Despite suboptimum sensitivity, the assay had high specificity. Furthermore, the assay permitted rapid diagnosis of tuberculosis in patients with poor prognostic features and high mortality risk.3 Determine TB-LAM clearly should not be used as a standalone test but rather as the initial point-of-care screen for tuberculosis in HIV-infected patients with advanced immunodeficiency, such as those
enrolling in antiretroviral therapy services or those who are sick and need to be admitted to inpatient care.3,4 This strategy would allow rapid point-ofcare screening in patients at greatest risk of dying pending the results of additional diagnostic tests. Such patients have the most to gain from immediate treatment for tuberculosis. Rather than large-scale studies of diagnostic accuracy, which will almost certainly confirm that overall sensitivity is suboptimum, perhaps the more important question is whether use of this assay can reduce mortality in these vulnerable patients. Studies of clinical effects are needed. This assay shows great promise but careful optimisation of its use is needed.4 SDL was funded by the Wellcome Trust, London, UK. RW was funded in part by the International Epidemiologic Database to Evaluate Aids with a grant from the National Institute of Allergy and Infectious Diseases (NIAID: 5U01AI069924-02), Cost-Effectiveness of Preventing AIDS Complications (CEPAC) funded by the National Institutes of Health (NIH, 5 R01AI058736-02), USAID Right to Care (CA 674 A 00 08 0000 700), and the South African Centre for Epidemiological Modelling and Analysis (SACEMA). ADK declares that he has no conflicts of interest.
*Stephen D Lawn, Andrew D Kerkhoff, Robin Wood [email protected] Desmond Tutu HIV Centre, Institute for Infectious Disease and Molecular Medicine, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa (SDL, ADK, RW); Department of Clinical Research, Faculty of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, London, UK SDL); and George Washington University School of Medicine and Health Sciences, Washington DC, USA (ADK) 1
2
3
www.thelancet.com/infection Vol 12 November 2012
Lawn SD, Edwards DJ, Kranzer K, Vogt M, Bekker LG, Wood R. Urine lipoarabinomannan assay for tuberculosis screening before antiretroviral therapy diagnostic yield and association with immune reconstitution disease. AIDS 2009; 23: 1875–80. Lawn SD, Kerkhoff AD, Vogt M, Wood R. Diagnostic accuracy of a low-cost, urine antigen, point-of-care screening assay for HIV-associated pulmonary tuberculosis before antiretroviral therapy: a descriptive study. Lancet Infect Dis 2012; 12: 201–09. Lawn SD, Kerkhoff AD, Vogt M, Wood R. Clinical significance of lipoarabinomannan (LAM) detection in urine using a low-cost point-of-care diagnostic assay for HIV-associated tuberculosis. AIDS 2012; 26: 1635–43.
4
Lawn SD. Point-of-care detection of lipoarabinomannan (LAM) in urine for diagnosis of HIV-associated tuberculosis: a state of the art review. BMC Infect Dis 2012; 12: 103.
Tests for latent tuberculosis infection and isoniazid preventive therapy Molebogeng Rangaka and colleagues1 showed that the interferon-γ release assays (IGRAs) and the tuberculin skin test (TST) predict active tuberculosis with an incidence rate ratio of 2. Their results not only show the limitations of tests for latent tuberculosis infection, but also have a far more profound implication. The most important role of testing for latent tuberculosis is to identify people who would benefit from isoniazid preventive therapy. Rangaka and colleagues’ findings enable calculation of the attributable fraction of incident tuberculosis that might be averted through provision of isoniazid preventive therapy to latently infected people. Some assumptions are necessary to make this calculation—namely, that isoniazid preventive therapy is not effective in patients who are TST negative,2 completion of isoniazid preventive therapy reduces the incidence of tuberculosis by 60–70% in TST-positive patients,2,3 TST and IGRA have similar predictive ability in screening patients for isoniazid preventive therapy, and worldwide prevalence of TST positivity is roughly one in three. If, as suggested by Rangaka and colleagues, patients with a positive TST or IGRA have an incidence rate ratio for active tuberculosis of 1·6–2·1 relative to TST-negative or IGRA-negative patients, then 44–51% of patients with incident tuberculosis would be TST or IGRA positive. Reduction of this fraction by 65% through isoniazid 827