TB is an infection caused by the slow-growing bacterium Mycobacterium tuberculosis. After many years of decline in its frequency, TB is increasing, especially in persons with HIV infection and in developing countries. Worldwide, more than one and a half million people die each year from tuberculosis, a treatable disease. More and more infections are caused by strains of TB that are resistant to many of the drugs used to treat it; these strains are called multi-drug resistant, or MDR.

Cultures to detect the presence of TB take a long time; the best current methods take an average of 13 days to recognize the bacteria, and traditional cultures (used in many developing countries) average 26 days to get a positive culture. If performed, tests to identify the best drug treatment take an additional period almost as long and are often not performed in developing countries.

Workers from the UK, the USA, and Peru have shown that a new liquid culture method called Microscopic-Observation Drug-Susceptibility (MODS) assay has advantages over both of these methods. It takes only about 7 days to diagnose TB and at the same time it finds the best treatment. As well as being much faster, it is cheaper and seems to perform at least as well as the more widely used methods.

The initial test for recognizing TB, a stain of a smear of sputum that identifies TB and related bacteria (but not more common bacteria), may miss up to half of cases. The shorter time for TB cultures to become positive allows more rapid diagnosis, especially in those patients who had a negative sputum smear examination. Earlier recognition of the presence of MDR TB infections can lead to proper treatment at a much earlier stage, rather than waiting months to see if the person responds to treatment. However, the authors of the NEJM study also point out that the accuracies of the tests for drug resistance are not as high as for the culture itself, and they did not, at this point, recommend using it as the sole way to determine drug treatment.

An editorial in the same issue of the NEJM pointed out other limitations of the test that need to be addressed before the test could be recommended. Some other mycobacteria can grow in a way that mimics the growth of TB, which could lead to false diagnosis of TB. Also, because TB is highly contagious, laboratories in developed countries are required to use strict (and expensive) safety practices to prevent infection of others in the laboratory and the health care facility when TB is cultured. All laboratories should meet the safety standards for handling mycobacterial cultures in order to perform the test. Even so, the authors of the editorial believe that the availability of such rapid tests and laboratories to perform them safely should become an urgent priority in the effort to control TB world-wide.

Sources
Moore, DAJ et al. Microscopic-Observation Drug-Susceptibility Assay for the Diagnosis of TB. New England Journal of Medicine 2006;355:1539-1550.

Iseman, MD and Heifets, LB. Editorial: Rapid Detection of Tuberculosis and Drug-Resistant Tuberculosis. New England Journal of Medicine 2006;355:1606-1608.