Smear campaign

4th March 2013

There is a new tool in the arsenal to fight multidrug-resistant tuberculosis (MDR-TB): a rapid diagnostic test that can function in high-burden settings such as public health clinics. MDR-TB is increasingly on the rise, and spreads most rapidly through vulnerable communities that are already riven by HIV and poverty. One of the biggest barriers to appropriate treatment is the lengthy diagnostic process of conventional techniques that is not well-suited to public health settings serving vulnerable populations.

"With continued delay of testing results, and thus, treatment, the patient will likely transmit the infection to those persons they come in close contact with," said Richard O'Brien, M.D., senior investigator from the Foundation for Innovative New Diagnostics, a nonprofit funded in part by USAID and the Bill & Melinda Gates Foundation, which sponsored the study. "TB is already the leading cause of death among AIDS patients worldwide. This association is particularly lethal when drug-resistant TB is being transmitted. MDR-TB, with resistance to the most important anti-TB drugs, isoniazid and rifampicin, is even more lethal."

The study is reported in first issue for April of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society. Dr. O'Brien and his colleagues found that using a molecular assay test, as opposed to conventional culture techniques, may revolutionise the diagnosis and reporting of MDR-TB in these high-infection areas by delivering the critical results more quickly, and perhaps even more accurately-thus enabling proper treatment to begin promptly.

The study took place in a national health laboratory in Cape Town, South Africa, which serves over 4.2 million people and processes about 400,000 specimens annually. The testing was performed on residual portions of specimens originally collected for other purposes. They were able to obtain interpretable results within 1-2 days in 97 percent of smear-positive samples, and had a better than 98 percent specificity and sensitivity rate for multidrug resistance.

"The advantage of this test, based on its performance in the lab in South Africa are its equivalent or perhaps increased accuracy compared to standard methods and an increase in the number of interpretable results, as standard techniques are subject to contamination," said Dr. O'Brien. "Additionally, it only takes 1-2 days, as opposed to the mean turnaround time of 42 days with conventional cultures."

The two to three months that conventional techniques can take to determine drug resistance can be fatal to an HIV-infected patient. Complicating the picture is the looming threat of MDR-TB becoming XDR-TB-extensively drug resistant. "If patients receive inappropriate treatment because of the lack of proper screening, the risk of developing increasingly resistant and consequently, virtually untreatable strains of TB in these already vulnerable populations is much higher," said Dr. O'Brien.

While this diagnostic method had been in existence, until now it had not been tested under real-world conditions in a high-volume public health laboratory in a low-income, disease-endemic country. "We evaluated this test in response to the call for implementation of rapid tests by the World Health Organization and its partners," said Dr. O'Brien.

"This test is now being evaluated in program conditions in a number of countries where MDR-TB is epidemic," said Dr. O'Brien. "We expect that more widespread use of this test will be a great improvement in our ability to address this serious problem that threatens the global control of tuberculosis."

Such an improvement in diagnosis in public health settings could take clinicians treating TB in high-burden areas out of "the dark," said Max Salfinger, M.D. and David Ashkin, A.G. in an editorial in the same issue of the journal.





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