Chapter 1: Introduction
1.4 Confirmatory tests for active tuberculosis
1.4.5 Urine lipoarabinomannan-based tests
Lipoarabinomannan (LAM) is a cell wall lipopolysaccharide found in mycobacteria. LAM is released from mycobacteria and is subsequently filtered by the kidney and can therefore be detected in urine.63 The first commercially available urine LAM test was the laboratory-based urine Clearview TB-ELISA.64 Subsequently, the AlereLAM assay, a urine point-of-care lateral-flow LAM (LF-LAM) assay, was developed as a confirmatory test for LAM. The AlereLAM assay is currently the only commercially available LF-LAM assay. It costs only US $3.50, gives a result in only 25 minutes or less, poses minimal biohazard risk, and is easy to perform since the test requires minimal technical expertise and urine is readily available and easy to collect (Figure 1-8). In 2015, WHO recommended the use of AlereLAM in symptomatic inpatient or outpatient PLHIV who have a CD4 cell count ≤100 cells/µL or who are ‘seriously ill’ (i.e., respiratory rate > 30/minute, temperature >39°C, heart rate >
120/minute, or unable to walk unaided).65 In 2019, WHO broadened its recommendation for inpatient PLHIV to those who are symptomatic, have a CD4 cell count ≤ 200 cells/µL, have advanced HIV disease, or who are ‘seriously ill’.66
16
Figure 1-8: AlereLAM test strip (A). Urine is applied to the test strip and the result is read 25 minutes later. Reference card (B). The reference card is used to determine if a test is positive and to “grade” the test result.67
WHO has only recommended AlereLAM in subgroups, because AlereLAM has suboptimal sensitivity in all PLHIV. Sensitivity of AlereLAM is highly dependent on setting and degree of immunodeficiency (Table 1-4). In a 2019 Cochrane systematic review, the pooled
sensitivity of AlereLAM among outpatient PLHIV irrespective of tuberculosis symptoms and signs was only 31% (18, 47) while specificity was 95% (87, 99);33 in symptomatic outpatient PLHIV, sensitivity was similar (29% [17, 47]). The sensitivity of AlereLAM was higher in inpatient PLHIV irrespective of tuberculosis symptoms and signs (62% [41, 83]) and in symptomatic HIV-positive inpatients (52% [40, 64]), but specificity was only 84% (48, 96) and 87% (78, 93), respectively. Among symptomatic PLHIV, the sensitivity in those with CD4 cell count >100 cells/µL and CD4 cell count >200 cells/mm was only 17% (10, 27) and 16% (8, 31), respectively, but higher in those with CD4 cell count ≤100 cells/µL (54% [38, 69) and CD4 cell count ≤200 cells/µL (45% [31, 61]), respectively. Data on diagnostic accuracy by CD4 cell count among PLHIV irrespective of tuberculosis symptoms and signs was limited.
17
Table 1-4: Pooled sensitivity and specificity of AlereLAM for the diagnosis of tuberculosis in PLHIV overall, as well as by setting and CD4 cell count33
Setting Symptomatic PLHIV PLHIV irrespective of tuberculosis symptoms and signs
Pooled sensitivity
(95% CI) Pooled specificity
(95% CI) Pooled sensitivity
(95% CI) Pooled specificity (95% CI) Overall accuracy
All participants 42% (31%, 55%) 91% (85%, 95%) 35% (22%, 50%) 95% (89%, 98%) By setting
Inpatients 52% (40%, 64%) 87% (78%, 93%) 62% (41%, 83%) 84% (48%, 96%) Outpatients 29% (17%, 47%) 96% (91%, 99%) 31% (18%, 47%) 95% (87%, 99%) By CD4 cell count
CD4 > 200 16% (8%, 31%) 94% (81%, 97%) Not applicable Not applicable CD4 ≤ 200 45% (31%, 61%) 89% (77%, 94%) 26% (9%, 56%) 96% (87%, 98%) CD4 > 100 17% (10%, 27%) 95% (89%, 98%) 20% (10%, 35%) 98% (95%, 99%) CD4 ≤ 100 54% (38%, 69%) 88% (77%, 94%) 47% (40%, 64%) 90% (77%, 96%) CD4 101‐199 24% (14%, 38%) 90% (77%, 96%) Not applicable Not applicable
The higher sensitivity in inpatients and those with more advanced immunodeficiency is likely because these groups have higher mycobacterial burden and higher rates of haematogenous dissemination of tuberculosis with subsequent renal involvement.68 The low specificity of AlereLAM in some studies is likely a result of an imperfect microbiological reference standard, since most studies did not collect multiple samples for culture and/or Xpert from both pulmonary and extra-pulmonary sites.69 Disseminated nontuberculous mycobacteria may also reduce specificity but are uncommon.70,71
Although AlereLAM has lower sensitivity compared with sputum Xpert,67 diagnostic yield (i.e., proportion of total tuberculosis cases with a positive confirmatory test) is higher in some populations, such as inpatients, because urine is more readily available. For example, in 2 cohorts of HIV-positive inpatients who were enrolled regardless of tuberculosis symptoms and signs, only 57% and 63% of inpatients were able to produce sputum for confirmatory testing, respectively, while >99% were able to produce urine for AlereLAM testing.18,25 In 1 cohort, sputum Xpert diagnosed 27% of all tuberculosis cases (vs 38% for AlereLAM),25
18
while in the other cohort sputum Xpert diagnosed 40% of all tuberculosis cases (vs 75% for AlereLAM).18
AlereLAM rapidly identifies those at high risk of mortality who may benefit from prompt treatment. The risk of mortality in PLHIV with LAM positive tuberculosis was 2.3 times that of PLHIV with LAM negative tuberculosis.72 Furthermore, two randomised trials have demonstrated a reduction in all-cause mortality among HIV-positive medical inpatients with the use of AlereLAM in addition to routine diagnostics (pooled RR 0.85 [0.76, 0.94]).18,66,73 One trial assessed HIV-positive medical inpatients irrespective of tuberculosis symptoms and signs,18 while the other assessed medical inpatients with a positive W4SS (who typically comprise >90% of all HIV-positive medical inpatients).18,73 In a subgroup analysis, the trial conducted in HIV-positive inpatients irrespective of tuberculosis signs and symptoms found that AlereLAM reduced mortality in 3 pre-specified subgroups: those with a CD4 cell count
<100 cells/µL, severe anaemia, and clinically suspected tuberculosis.18
Recently, a novel urine-based LF-LAM test has been developed – the Fujifilm SILVAMP TB LAM (FujiLAM).74 In an IPDMA of 5 studies, the pooled sensitivity was 71% (59, 81) for FujiLAM but only 35% (20, 51) for AlereLAM.75 Compared with the sensitivity of
AlereLAM, the sensitivity of FujiLAM was 28 and 43 percentage points higher in outpatients and inpatients, respectively. FujiLAM showed slightly lower specificity compared with AlereLAM. Since FujiLAM detects lower concentrations of LAM,76 the higher false positive results with FujiLAM may reflect the inability of the reference standard to correctly classify active tuberculosis at a lower mycobacterial burden. Nontuberculous mycobacteria may also reduce the specificity of FujiLAM but were found in only 4% of participants with a false- positive FujiLAM test.75 Although studies in the IPDMA used bio-banked urine samples, these samples showed similar results compared with fresh samples.74 In two recent large multicentre diagnostic accuracy studies, FujiLAM sensitivities were 55% (49, 60) and 60%
(51, 69).77,78 However, accuracy varied significantly by lot number. This variability likely needs to be addressed before FujiLAM can be commercially available.