LAM Test Performance Among All Participants
Figure 1 shows qualitative LAM results by final TB diagnostic classification. LAM test sensitivity was 59% (95% CI: 52 to 66) in participants with confirmed TB and 46% (40 to 52) for confirmed TB plus possible TB groups combined. Among individuals categorized as “not TB”, specificity was 96% (91 to 99). Positive predictive value for confirmed TB (PPV) and negative predictive value were, respectively, 73% (65 to 80) and 34% (29 to 39) among all participants and 96% (90 to 99) and 60% (52 to 67) when indeterminate and possible TB patients were excluded. Twenty-three of 499 study participants (4.6%) were receiving ongoing TB treatment at the time of enrollment; 6 (26%) had confirmed TB, 7 (30%) had possible TB, 4 (17%) did not have TB, and 6 (26%) were indeterminate. Among these 23 participants, LAM test sensitivity was 33% (4.3 to 77) in those with confirmed TB and 15% (1.9 to 45) for confirmed TB plus possible TB groups combined.
Quantitative LAM results are shown in Figure 2. Median (interquartile range) ODs were as follows: not TB 0.012 (0.003-0.03); indeterminate 0.02 (0.008-0.075); possible TB 0.0175 (0.002-0.059); and confirmed TB 0.19 (0.026-1.3). Median coefficient of variation for duplicate wells was 2.0% (interquartile range 0.8-3.7).
LAM Test Performance Stratified by HIV-Infection Status
LAM test sensitivity and specificity, stratified by HIV infection status, are shown in Figure 1. Overall LAM sensitivity was 67% (59 to 74) and specificity was 94% (87 to 98) among HIV-positive participants with confirmed TB or no TB, respectively. For HIV-positive patients, LAM PPV for confirmed TB and negative predictive value were, respectively, 75% (67 to 82) and 30% (25 to 36) among all participants and 96% (90 to 99) and 60% (51 to 68) when indeterminate and possible TB patients were excluded. Among 47 HIV-negative participants, the LAM test was positive in 3, including 2 of 14 (14%) with confirmed TB and 1 of 5 (20%) indeterminate participants; specificity was 100% (23 of 23) in the small number of HIV-negative participants classified as “not TB”.
Among HIV-positive participants with confirmed TB, LAM test sensitivity differed by CD4 category (P < 0.001). LAM sensitivity was 55% (41 to 69) for those with CD4 counts greater than 200, 14% (3.6 to 58) for CD4 counts of 150-200, 56% (30 to 80) for CD4 counts of 100-150, 71% (51 to 87) for CD4 counts of 50-100, and 85% (73 to 93) for CD4 counts less than 50.
Factors Associated With a Positive Urine LAM Test
HIV infection (AOR 13.4, P < 0.01), positive mycobacterial blood culture (AOR 3.21, P = 0.01), and positive sputum smear (AOR 2.42, P < 0.01) were independently associated with a positive LAM test in confirmed TB patients (Table 2). In a separate analysis including only HIV-positive individuals with confirmed TB, independent predictors of a positive LAM test were positive sputum smear, (AOR 2.39, P = 0.03), and CD4 count <50 (AOR 4.04, P < 0.01). Among HIV-positive participants with confirmed TB, several indicators of higher bacillary burden were associated with higher ODs. In multiple linear regression, those with CD4 count less than 50 had an OD of 0.80 (0.45-1.15) greater than those with CD4 count greater than 200 (P < 0.001); those with positive sputum smear had an OD of 0.42 (0.13-0.71) greater than those with negative sputum smear (P = 0.004); and those with positive mycobacterial blood cultures had an OD of 0.49 (0.15-0.83) greater than those with negative cultures (P = 0.005). Age, sex, and death at 2 months were not statistically significantly associated with a difference in OD.
LAM Test Performance Compared With Sputum Smear Microscopy in Confirmed TB Cases
Sensitivity of the LAM test was compared with that of sputum smear microscopy, a test with rapid turn-around-time (Table 3). Of 193 confirmed TB cases, 52 (27%) were positive by both the LAM test and smear microscopy, 49 (25%) were negative by both assays, 62 (32%) were positive by the LAM test alone, and 30 (16%) were positive by smear microscopy alone. The LAM test was more sensitive than sputum smear microscopy [42% (82 of 193), P < 0.01], and the LAM test was positive in 56% (62 of 111) of confirmed TB patients with a negative sputum smear. The combined sensitivity of sputum smear plus LAM test (either or both positive) was 75% (144 of 193, 95% CI: 68 to 81; P < 0.01 compared with smear alone and P < 0.01 compared with LAM test alone). LAM test sensitivity was higher than that of sputum smear microscopy for confirmed TB cases with HIV infection (67% vs 40%, P < 0.01) or who died (72% vs 48%, P = 0.03).
Sensitivity and specificity were optimal at the manufacturer's recommended cutoff of OD 0.1 (see Figure, Supplemental Digital Content 1, http://links.lww.com/QAI/A22). Decreasing the OD cutoff below 0.1 resulted in marked reduction in sensitivity with little gain in specificity. Increasing the threshold for a positive test from an OD of 0.1-0.51 increased the specificity to 100% with a reduction in sensitivity to 38%.
For the diagnosis of active TB in a setting of high HIV prevalence, the LAM test had a sensitivity of 59% in confirmed TB cases and specificity of 96% among individuals classified as “not TB”. LAM test sensitivity was higher in HIV-positive TB patients than HIV-negative TB patients and was highest in the subgroup of HIV-positive TB patients with CD4 counts less than 50. HIV-related immunosuppression and high overall bacillary burden (as reflected by positive mycobacterial blood cultures and positive sputum smears) were associated with LAM test positivity among confirmed TB patients. An important attribute of the LAM test was its ability to detect over half of confirmed TB cases not detected by sputum smear microscopy. The combination of sputum smear plus LAM testing identified 75% of confirmed TB cases. Rapid detection of active TB is essential for managing patients with advanced HIV infection and permits earlier initiation of TB therapy and institution of infection control procedures.
How does LAM test performance in our study compare with performance reported by other investigators? Boehme et al14 used a prior version of the existing urine LAM assay (Chemogen, South Portland, ME) to evaluate 231 TB suspects (69% HIV positive) and 103 healthy controls in Tanzania. Sensitivity was 80.3% among individuals with M. tuberculosis isolated from sputum culture and was unaffected by HIV status; specificity was 99% in healthy controls. Boehme et al14 used unprocessed fresh urine, whereas we used concentrated frozen urine. Whether this was a major factor in the sensitivity difference between studies is unclear. Corbett et al1 recently evaluated accuracy of the Clearview TB ELISA test in TB patients and suspects in Harare and found sensitivity of 52% among HIV-infected, TB culture-positive individuals.17 In HIV-infected outpatients screened for TB during enrollment in an antiretroviral treatment program in Cape Town, Lawn et al18 found Clearview TB ELISA test sensitivity of 38% overall in TB culture-positive individuals and 67% in the subgroup with CD4 <50; specificity was 100%. These findings are in agreement with our finding that LAM sensitivity was highest in patients with the lowest CD4 counts.
In our study, LAM test sensitivity was low in the group of participants designated as “possible TB”. There are several possible explanations. First, these individuals could have had TB disease with low mycobacterial burden that was insufficient to result in a positive LAM test. Alternatively, “possible TB” patients may not have had TB. Many received treatment directed against bacterial pathogens in addition to anti-TB treatment, and improvement could have been due to non-TB treatment. In addition, at 2-month follow-up, we collected subjective information on clinical improvement; objective parameters such as chest radiographs may have been useful. In all likelihood, the “possible TB” group is a heterogeneous one that includes individuals with and without TB.
A new rapid TB diagnostic test with very high sensitivity is desperately needed but elusive. The LAM test, with modest sensitivity, might nevertheless meet an important need in HIV-prevalent, resource-constrained settings. Dowdy et al19 used decision analysis to explore the potential cost-effectiveness of a hypothetical new point-of-care TB test. Cost-effectiveness depended most strongly on specificity and price and was maximized in circumstances in which existing TB diagnostic capacity was poor (eg, HIV-prevalent settings in which sputum smear microscopy has a low yield). The LAM test's high specificity, potentially low price, and ability to detect TB in individuals with negative sputum smears are therefore attractive features. Although a dipstick or other point of care test format would have advantages over the current test format, the current assay could be integrated into laboratories equipped for ELISA-based HIV testing.
Our study has important limitations. A definitive diagnosis could not be established in a substantial minority of study participants, a challenge not unique to our study or clinical practice in settings of high TB/HIV burden. To nevertheless maximize study interpretability, we report results for the “indeterminate” and “possible TB” groups despite uncertainties about final diagnosis. Second, the studied population had few individuals from whom nontuberculous mycobacteria (NTM) were isolated, and therefore, we were unable to assess whether the LAM test discriminates M. tuberculosis from NTM in clinical practice. In preclinical testing, the current assay had highest analytical sensitivity for M. tuberculosis complex, with substantially less reactivity to NTM.14 Our study has several key strengths. Importantly, it was performed in a setting of high TB-HIV prevalence, and results are therefore generalizable to similar settings where the need for improved TB diagnostics is great. It should be noted, however, that the LAM test PPV may be reduced in settings with lower TB prevalence. TB diagnostic accuracy was enhanced by the use of mycobacterial blood cultures, and HIV status was determined for almost all participants.
In conclusion, the urine LAM test detected a subset of HIV-positive patients with severe TB and high mortality in whom smear microscopy has suboptimal sensitivity. The combination of urine LAM testing and smear microscopy is attractive for use in settings with high HIV burden. However, the apparent low LAM test sensitivity in HIV-negative TB patients may limit this test's utility in settings of low HIV prevalence. Further studies are warranted to determine if implementation of LAM testing would result in improved clinical outcomes.
The authors thank Ms. Lolo Rafedile, Mr. Kagisho Baepanye, and Mr. Sello Obakile for recruitment and follow-up of study participants and Ms Msandiwe, Dr. Mpolokeng Melamu, and Dr. David Dowdy for valuable support.
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diagnosis; HIV; infections; tuberculosis; mycobacterium
Supplemental Digital Content
© 2009 Lippincott Williams & Wilkins, Inc.