Sputum smears mistake other bacteria for TB in HIV+ Nigerians

Sputum smear remains the standard test to diagnose Mycobacterium tuberculosis infection in many resource-poor regions. But molecular diagnostics for M tuberculosis DNA have proved superior in confirming TB diagnosis.

To assess the reliability of sputum smear in HIV-positive people suspected of TB, researchers collected sputum samples from HIV-positive people making regular clinic visits over a 12-month period at two Nigerian clinics. They tested samples by sputum smear microscopy and by the Genotype MTBDRplus test. The researchers identified other bacteria by 16S rRNA sequencing.

Sputum smear determined that 415 HIV-positive people had M tuberculosis infection, but molecular testing confirmed the diagnosis in only 233 people (56%). People taking antiretroviral therapy were 75% less likely than untreated people to have M tuberculosis infection (odds ratio [OR] 0.25, P = 0.003).

Multivariate logistic regression analysis identified two independent predictors of M tuberculosis infection. Abnormal respiratory findings on auscultation more than tripled the odds of TB (OR 3.28, P = 0.03), and a direct sputum smear greater than grade 3/100 raised the odds more than 6 times (OR 6.4, P < 0.02).

Only the highest grades of concentrated sputum smear (2+ and 3+) reliably predicted M tuberculosis infection.

Among 65 sputum samples not confirmed as M tuberculosis by molecular testing, 32 (49%) contained other possibly novel pathogens, including atypical Mycobacteria, Rhodococcus species, Nocardia species, and Corynebacterium species.

The researchers suggest that “use of molecular diagnostics could reduce unnecessary or inappropriate treatment and improve identification of pathogens in resource-limited settings with high HIV burden.”

Source: Lana Dinic, Oni E. Idigbe, Seema Meloni, Holly Rawizza, Patrick Akande, Geoffrey Eisen, Dan Onwujekwe, Oche Agbaji, Agatha Ani, Phyllis J. Kanki. Sputum smear concentration may misidentify acid-fast bacilli as Mycobacterium tuberculosis in HIV-infected patients. JAIDS. 2013; 63: 168-177.

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Source: IAS