TB is an infectious disease caused by Mycobacterium tuberculosis. Nearly 9 million people develop TB every year, and TB kills nearly 1.4 million individuals each year. While the incidence of TB has steadily declined in the west, it remains a huge public health problem in many developing countries, with India having the highest burden.
Although many countries have met the Stop TB Partnership’s targets of 70% case detection and 85% cure rate by 2005 (70/85 targets), TB incidence is still not falling as quickly as expected. One important reason is that TB patients are not diagnosed and cured quickly enough. When TB patients are not diagnosed and cured quickly, they may unknowingly spread their infection to their families and communities – further exacerbating the epidemic. Without early and better TB diagnosis, we are unlikely to achieve TB elimination.
What methods have previously been used to test for TB?
In most high TB incidence countries, sputum smear microscopy is the most widely used test for TB. It is inexpensive and can be done even in peripheral laboratories. This test has been used for nearly a century. Sputum is smeared on a glass slide, stained with dyes, and read under a microscope by a trained technician.
Sputum smears detect highly infectious patients and have high specificity. However, sensitivity of the test is modest (about 50 – 60%), although higher sensitivity can be obtained by performing fluorescence (e.g. auramine) staining and with the use of light-emitting diode microscopy. Sputum microscopy generally performs poorly in HIV-infected persons. It is of lesser value in young children (who often cannot produce sputum) and in extrapulmonary TB.
There are better tests for TB such as liquid culture and nucleic acid amplification tests (NAATs). However, these technologies, until recently, have been challenging to scale-up in resource-limited settings. They are expensive and require sophisticated laboratories and expertise that is often lacking in developing countries.
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February 15, 2013