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Faster Diagnosis of MDR-TB

The microscopic-observation drug-susceptibility assay is faster and more sensitive than existing methods for diagnosing TB and multidrug-resistant TB.

TB kills 1.7 million people worldwide every year. Conventional methods for Mycobacterium tuberculosis culture and susceptibility testing are barriers to rapid detection of TB and multidrug-resistant TB (MDR-TB), especially in resource-limited settings. The microscopic-observation drug-susceptibility (MODS) assay, which relies on early microscopic detection of typical cord formation in broth culture, has been shown in preliminary studies to be rapid and sensitive for TB and MDR-TB detection. Now, investigators have compared this assay against two conventional diagnostic methods in Lima, Peru.

From April 2003 through July 2004, sputum samples were collected from 1570 consecutive patients with suspected TB at 10 government clinics, 253 patients prescreened for high risk of TB and MDR-TB, and 157 hospitalized HIV patients at 2 hospitals. All samples were processed by three methods: MODS assay, Löwenstein-Jensen (L-J) culture, and automated mycobacterial culture using the MBBacT broth system.

Of 3760 samples, 401 (10.7%) yielded cultures positive for M. tuberculosis. The MODS assay was more sensitive than either the automated mycobacterial culture or the L-J culture (98% vs. 89% and 84%; P<0.001) and showed shorter median time to culture positivity (7 days vs. 13 and 26 days; P<0.001). The addition of a second sputum sample for MODS did little to improve sensitivity, especially for the group prescreened for high TB risk. Susceptibility results for the MODS assay were >92% concordant with reference testing for each of the four drugs examined (isoniazid, rifampin, ethambutol, and streptomycin).

Comment: The MODS assay is an affordable, rapid method for culture-based detection of TB, including MDR-TB. As an editorialist points out, however, manipulating broth cultures requires a TB laboratory with biosafety level 3 standards. Bringing more such laboratories into countries with a high prevalence of TB and MDR-TB is necessary before this assay’s advantages can be fully realized.

— Daniel J. Diekema, MD, MS

Published in Journal Watch Infectious Diseases October 11, 2006

Citation(s):

Moore DAJ et al. Microscopic-observation drug-susceptibility assay for the diagnosis of TB. N Engl J Med 2006 Oct 12; 355:1539-50.

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• Medline abstract (Free)

Iseman MD and Heifets L. Rapid detection of tuberculosis and drug-resistant tuberculosis. N Engl J Med 2006 Oct 12; 355:1606-8.

• Original article (Subscription may be required)
• Medline abstract (Free)