Point-of-care detection of lipoarabinomannan (LAM) in urine for diagnosis of HIV-associated tuberculosis: a state of the art review

Abstract

Detection of Mycobacterium tuberculosis antigens in urine is attractive as a potential means of diagnosing tuberculosis (TB) regardless of the anatomical site of disease. The most promising candidate antigen is the cell wall lipopolysaccharide antigen lipoarabinomannan (LAM), which has been used to develop commercially available enzyme-linked immunosorbent assays. Although highly variable diagnostic accuracy has been observed in different clinical populations, it is now clear that this assay has useful sensitivity for diagnosis of HIV-associated TB in patients with advanced immunodeficiency and low CD4 cell counts. Thus, this assay is particularly useful when selectively used among patients enrolling in antiretroviral treatment services or in HIV-infected patients requiring admission to hospital medical wards. These are the very patients who have the highest mortality risk and who stand to gain the most from rapid diagnosis, permitting immediate initiation of TB treatment. A recently developed low-cost, lateral-flow (urine 'dip-stick') format of the assay provides a result within 30 minutes and is potentially a major step forward as it can be used at the point-of-care, making the possibility of immediate diagnosis and treatment a reality. This paper discusses the likely utility of this point-of-care assay and how it might best be used in combination with other diagnostic assays for TB. The many further research studies that are needed on this assay are described. Consideration is particularly given to potential reasons for the variable specificity observed in existing field evaluations of LAM ELISAs. Whether this might be related to the assay itself or to the challenges associated with study design is discussed.

Figure 1

Cartoon showing the basic structure of mycobacterial lipoarabinomannan (LAM) and its three main domains. (i) The glycophospholipid anchor binds the molecule to the plasma membrane of the organism. (ii) The mannan core is attached to this and is highly conserved across mycobacterial species. (iii) The variable branching arabinan side chains and the variable mannose capping of these side chains gives rise to the diversity of LAM molecules.

Figure 2

Graph showing the sensitivity of a commercially available enzyme-linked immunosorbent assay (ELISA) to detect lipoarabinomannan (LAM) within urine samples to diagnose tuberculosis (TB) in a cohort of patients accessing antiretroviral treatment (ART) in a South African township. The sensitivity (%) of sputum microscopy and the LAM ELISA are shown individually and combined (either positive) compared with a gold standard of automated liquid culture of two sputum samples. Data are stratified by CD4 cell count (cells/μL) and show that the sensitivity of the LAM ELISA was substantially greater in the patients with the lowest CD4 cell counts. Figure reproduced from Lawn et al. 2011 [5] and data originally from Lawn et al. 2009 [31].

Figure 3

Photograph of a Determine TB-LAM test strip showing the sample pad to which 60 μL of the test urine is applied. After 25–35 minutes of incubation at room temperature, the control band is checked and the sample test result is read. Reading may be facilitated by comparison to the reference card. The presence of a band in the sample window of similar or greater intensity to the weakest positive on the reference card indicates the presence of lipoarabinomannan (LAM) in the urine.

Figure 4

Graph showing the diagnostic sensitivity for sputum culture-positive tuberculosis using the Determine TB-LAM assay for lipoarabinomannan (LAM) when used alone or in combination with other diagnostics, including sputum smear microscopy (smear) and Xpert MTB/RIF (Xpert). Data are stratified by CD4 cell count (cells/μL) and show that the sensitivity of the LAM was substantially greater in the patients with the lowest CD4 cell counts. There was important incremental sensitivity when microscopy and Determine TB-LAM were combined and this was similar to the sensitivity of Xpert in patients with the lowest CD4 cell counts.