The World Health Organization (WHO) believes the test has potential to revolutionize TB care because of its quick results and potential for directing appropriate, immediate treatment and preventing spread of the disease. WHO has made the test a top priority and is currently consulting with experts to decide about endorsing use of the test, according to The Associated Press.
TB is caused by Mycobacterium tuberculosis. These bacteria most often cause an infection in the lungs but can spread to almost any organ in the body. TB is a major public health problem not only because it is so widespread, but also because it is difficult to diagnose and treat. The current testing methods for pulmonary infections, which commonly involve microscopic examination of a stained sputum sample on a slide (acid-fast smear) and culture, are not ideal. An acid-fast smear often fails to identify the presence of M. tuberculosis when the organisms are in low numbers. The bacteria grow slowly, so detection and identification by culture may take days to several weeks to report a positive result, and cultures must be held 6 to 8 weeks to confirm a true negative sample. Additional tests are also necessary to diagnose TB caused by mycobacteria that are resistant to the commonly prescribed drugs.
Worldwide, few of the estimated 500,000 patients whose infections are resistant to multiple drugs and the 1.37 million patients who have both TB and HIV have access to both sufficiently sensitive tests and those that determine drug resistance, say researchers. According to the U.S. Centers for Disease Control, the U.S. has about 11,500 reported TB cases annually. The TB rate in foreign-born U.S. residents is 11 times that of the rate for people born here.
Writing in the September 1 New England Journal of Medicine, researchers from the U.S., Germany, and four developing countries detail a study of a test that detects genetic material of TB-causing bacteria in sputum samples from people with symptoms and suspected active infections. The test, which requires minimal training and 15 minutes of manual labor, also determines if a positive case of TB is caused by bacteria resistant to rifampin, one of the first-line drugs used, and allows patients to begin treatment before they leave clinics.
To evaluate the performance of the new test, the researchers compared it to culture in 1,730 patients with suspected TB in Peru, Azerbaijan, South Africa, and India. The test identified 98% of all confirmed TB cases and 98% of ones resistant to rifampin. The test correctly identified about three quarters of the TB cases that were mistakenly declared negative after microscopic exam and accurately ruled out the disease in 99% of people who were not infected.
An accompanying editorial by Peter M. Small, M.D., and Madhukar Pai, M.D., Ph.D. of The Bill and Melinda Gates Foundation and McGill University, respectively, points to some of the new test's drawbacks. It is relatively costly, compared to conventional tests, and cannot tell if a TB infection is resistant to drugs other than rifampin, they note. Also, as done in the study, the tests were performed in special reference laboratories, not where the patients were actually seen. Nevertheless, if the test is shown to be as accurate outside research settings and is made affordable for resource-poor areas, it promises to make an impact on the prevalence of TB worldwide. In addition to the review by WHO, the test is will undergo scrutiny by the Food and Drug Administration for possible approval in the US in the next year or so.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
Peter M. Small, M.D. and Madhukar Pai, M.D., Ph.D. Tuberculosis Diagnosis — Time for a Game Change. New England Journal of Medicine. PDF available for download at http://www.nejm.org/doi/pdf/10.1056/NEJMe1008496 through http://www.nejm.org. Issued September 1, 2010. Accessed September 22, 2010.
C. Winston, et al. Decrease in Reported Tuberculosis Cases -- United States, 2009. Morbidity and Mortality Weekly Report. Available online at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5910a2.htm through http://www.cdc.gov. Issued March 19, 2010. Accessed September 22, 2010.
Marilyn Marchione. "New test seen as big advance in diagnosing TB." The Associated Press. Issued September 1, 2010. Accessed September 22, 2010. (No longer available online.)