Diagnostic accuracy of induced sputum LAM ELISA for tuberculosis diagnosis in sputum-scarce patients
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.
Document Type: Research Article
Affiliations: 1: Lung Infection and Immunity Unit, Division of Pulmonology, University of Cape Town Lung Institute and Clinical Immunology, Department of Medicine, University of Cape Town, Cape Town, South Africa 2: Lung Infection and Immunity Unit, Division of Pulmonology, University of Cape Town Lung Institute and Clinical Immunology, Department of Medicine, University of Cape Town, Cape Town, South Africa; Institute of Infectious Diseases and Molecular Medicine, University of Cape Town, Cape Town, South Africa; and Centre for Infectious Diseases and International Health, Department of Infection, University College London Medical School, London, UK
Publication date: August 1, 2012
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