Lipoarabinomannan in urine during tuberculosis treatment: association with host and pathogen factors and mycobacteriuria

Abstract

Background: Detection of lipoarabinomannan (LAM), a Mycobacterium tuberculosis (Mtb) cell wall antigen, is a potentially attractive diagnostic. However, the LAM-ELISA assay has demonstrated variable sensitivity in diagnosing TB in diverse clinical populations. We therefore explored pathogen and host factors potentially impacting LAM detection.

Methods: LAM-ELISA assay testing, sputum smear and culture status, HIV status, CD4 cell count, proteinuria and TB outcomes were prospectively determined in adults diagnosed with TB and commencing TB treatment at a South African township TB clinic. Sputum TB isolates were characterised by IS61110-based restriction fragment length polymorphism (RFLP) and urines were tested for mycobacteriuria by Xpert® MTB/RIF assay.

Results: 32/199 (16.1%) of patients tested LAM-ELISA positive. Median optical density and proportion testing LAM positive remained unchanged during 2 weeks of treatment and then declined over 24 weeks. LAM was associated with positive sputum smear and culture status, HIV infection and low CD4 cell counts but not proteinuria, RFLP strain or TB treatment outcome. The sensitivity of LAM for TB in HIV-infected patients with CD4 counts of ≥ 200, 100-199, 50-99, and < 50 cells/μl, was 15.2%, 32%, 42.9%, and 69.2% respectively. Mycobacteriuria was found in 15/32 (46.9%) of LAM positive patients and in none of the LAM negative controls.

Conclusions: Urinary LAM was related to host immune factors, was unrelated to Mtb strain and declined steadily after an initial 2 weeks of TB treatment. The strong association of urine LAM with mycobacteriuria is a new finding, indicating frequent TB involvement of the renal tract in advanced HIV infection.

Figure 1

LAM Study Consort Diagram. Disposition by HIV and TB sputum bacteriological status of 200 consecutive patients presenting to a South African township TB clinic and undergoing LAM-ELISA urine testing

Figure 2

Proportion of LAM positive patients testing LAM positive at days 1-7, week 2, week 8, week 16 and week 24 and median LAM-ELISA optical density at each time point. Proportions are represented as bars (values shown on left-hand vertical axis) and median optical density is represented as a solid line (values shown on right-hand vertical axis). The number of patients who tested at each time point, median OD and IQR are shown in corresponding table below the figure

Figure 3

Protein: Creatinine Ratios of 32 LAM positive patients and 32 randomly chosen LAM negative patients at Day 1 and Week 24 of TB treatment. There was an overall statistically significant difference in P:C ratios between day 1 and week 24 (p < 0.001, Wilcoxon signed rank test*). There was a statistically significant difference in P:C ratios between LAM positive and LAM negative patients at day 1 (p = 0.011, Wilcoxon rank sum test **) but not at week 24 (p = 0.289, Wilcoxon rank sum test **)

Figure 4

Proportions of urine samples from 85 HIV positive patients with laboratory confirmed TB who were LAM positive and Xpert^® MTB/RIF positive stratified into 4 CD4 cell strata: < 50 cells/μl, 50-99 cells/μl, 100-199 cells/μl, and ≥ 200 cells/μL. The proportions of LAM positive urine samples were 69.2%, 42.9%, 32%, 15.2% and Xpert^®

Figure 5

Three proposed models of the possible fate of LAM molecules released from systemic or urinary Mtb organisms. A. Systemically released LAM is bound to an antibody to form an immune complex within the circulation that impedes renal filtration of LAM across the intact glomerular membrane. This model gives rise to a negative LAM test in the absence of Mtb. B. Circulating LAM unattached to a specific anti-LAM antibody is freely filtered through the kidney into the urine, which gives rise to a positive LAM test in the absence of Mtb. C. Mtb within in the renal tract releases LAM directly into urine, which gives rise to a positive LAM test in the presence of Mtb.