Diagnostic accuracy of induced sputum LAM ELISA for tuberculosis diagnosis in sputum-scarce patients

Abstract

Objective: To examine whether a lipoarabinomannan (LAM) enzyme-linked immunosorbent assay (ELISA) that offers diagnostic utility using urine in patients with tuberculosis (TB) and human immunodeficiency virus (HIV) co-infection can be used in induced sputum to diagnose sputum-scarce patients with suspected TB.

Design: LAM was measured in induced sputum samples obtained from 61 consecutively recruited sputum-scarce TB suspects in a tertiary hospital respiratory clinic in South Africa. Liquid culture positivity for Mycobacterium tuberculosis was used as the reference standard. Receiver operating characteristic analysis was used to assess alternative LAM concentration cut-offs.

Results: A total of 87% (53/61) of study patients had a valid M. tuberculosis culture result; 49% (23/53) were HIV-infected and 17% (9/53) were culture-positive for M. tuberculosis. Induced sputum smear microscopy and LAM ELISA had an overall sensitivity of 56% (95%CI 27-81); however, the specificity of LAM ELISA was 48% (95%CI 34-62), while the positive and negative predictive values were respectively 18% (95%CI 8-36) and 84% (95%CI 65-94). An optimal rule-in cut-off selected by receiver operating characteristic (LAM concentration >5.73 ng/ml) increased test specificity to 98% and reduced sensitivity to 22%. Normalisation of the assay for sample total protein or cell count did not improve diagnostic accuracy.

Conclusions: In this proof-of-concept study, the ELISA was not clinically useful for TB diagnosis using induced sputum.

Figure A1

Alternative analysis study flow diagram. Patients classified according to composite reference standard outlined in Appendix text. TB = tuberculosis.

Figure A2

Normalisation experiments for sputum LAM to attempt to improve discriminatory performance. Induced sputum LAM concentrations (ng/ml) normalised for sample protein concentration (mg/ml). LAM content per mg protein (ng/mg) in M. tuberculosis culture-positive vs. -negative patients with medians and interquartile range indicated. LAM = lipoarabinomannan.

Figure A3

Induced sputum LAM concentrations (ng/ml) normalised for total number of mononuclear cells in sample. LAM content in ng per cell in M. tuberculosis culture positive vs. negative patients (medians only indicated). LAM = lipoarabinomannan.

Figure 1

Study flow diagram outlining patient groups used for assessment of diagnostic accuracy. TB = tuberculosis; LAM = lipoarabinomannan; ELISA = enzyme-linked immunosorbent assay.

Figure 2

Two-way scatter plot of induced sputum LAM concentrations (ng/ml) in M. tuberculosis culture-positive vs. negative sputum-scarce patients. Median (IQR) LAM concentrations are illustrated. According to the manufacturer, any LAM concentration > 0 ng/ml is considered to be a positive test. LAM = lipoarabinomannan; IQR = interquartile range.

Figure 3

ROC for sputum LAM ELISA and diagnostic accuracy measures (sensitivity and specificity) for different LAM concentration cut-points. AUROC = area under ROC; ROC = receiver operating characteristic; LAM = lipoarabinomannan; ELISA = enzyme-linked immunosorbent assay.