Diagnostic Kit Distinguishes Non-TB Pulmonary Disease from TB

The bacterium that causes TB, Mycobacterium tuberculosis, originates from a larger family of mycobacteria. The most common pathogenic nontubercular mycobacteria are known together as Mycobacterium avium complex (MAC). Distinguishing MAC-related pulmonary disease (MAC-PD) from TB is difficult, and can take up to eight weeks or even longer. Because MAC bacteria are ubiquitous in the environment, a positive culture may mean nothing more than specimen contamination.

A multicenter study has shown that differentiating MAC-PD from TB can be accomplished in just a few hours using an assay that can identify antibodies specific to MAC. To test the efficacy of the immunoassay kit, specimens were acquired from six centers between June 2003 and December 2005. The samples came from 70 patients with MAC-PD; 18 with MAC contamination, 36 with pulmonary TB, 45 with other lung disease, and 76 from healthy patients.

The investigators found that found that serum antibody levels to the MAC-specific antigen were higher in patients with MAC pulmonary disease compared to those with other respiratory diseases, including tuberculosis. The sensitivity and specificity of the serologic test were 84.3% and 100%, respectively. The test took only hours as opposed to the four to eight weeks it takes to determine conventional culture results.

Distinguishing between MAC and TB has largely relied on a range of clinical signs and obtaining repeatedly positive sputum cultures, a process both unwieldy and often unreliable. "About 20% of the time the physician might make the wrong determination,” said Dr. Alvin Teirstein, professor of medicine at Mount Sinai School of Medicine (New York, NY, USA).

Dr. Teirstein pointed out that to be validated the kit must perform well with different populations and in different locations, as MAC strains can vary from place to place. This is the first multicenter demonstration of the efficacy of such a kit, raising the hope that it may solve the problem of distinguishing MAC-PD from TB. This would increase the accuracy and efficiency in treating patients with MAC-PD and TB.

The study was published in the first issue for April 2008 of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.


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