TB test detects even drug-resistant strains
[NEW DELHI] Notoriously low detection rates for one of the developing world's key diseases could soar as a result of a new test — but its cost, and dependence on a constant power supply, could limit its use.
Tuberculosis (TB) remains one of the world's leading infectious diseases. In 2008 there were 9.4 million new cases and 1.8 million deaths from the disease.
The current test, which dates from the nineteenth century, requires a technician to examine a culture of sputum from the lung under a microscope. It misses almost half of normal cases, as well as most multidrug-resistant cases and up to half of infections in people who already have HIV.
The new test uses polymerase chain reaction (PCR) to isolate and make multiple copies of the genetic material of the mycobacterium that causes tuberculosis. It can detect strains resistant to the most powerful anti-TB drug, rifampicin, according to research published in the New England Journal of Medicine this month (1 September).
The test was developed in a public–private partnership between US-based diagnostics firm Cepheid and the University of Medicine and Dentistry of New Jersey, United States.
The report says that a trial by the non-profit organisation Foundation for Innovative New Diagnostics (FIND), on more than 1,700 patients across Azberbaijan, India, Peru and South Africa, found that it detected almost 98 per cent of cases, 97 per cent of cases resistant to rifampicin and 93 per cent of those in HIV sufferers.
But there are obstacles to overcome before the test can be widely used, the report cautions. First, the trial used hospital reference centres with good facilities. In contrast, microscopy centres, health posts and local treatment centres often have erratic power supplies and variable temperatures.
In addition, each machine costs almost US$18,000 and requires trained personnel, although Lakshmi Sundaram, FIND's advocacy officer, said that the cost is expected to be cut by three-quarters for developing countries.
Alexander Yule, a UK-based consultant who has studied TB diagnostic tests, said that commercial diagnostic tests for TB have been around for almost 15 years but are yet to make an impact in Sub-Saharan Africa and Asia, where they are most needed.
"TB diagnosis and control is complex and improved diagnostic testing is only one small part of the solution," he added. "Although the need for a sensitive test is indisputable, a very costly test would mean diversion of already scant resources from other components of TB management."
The WHO's scientific and technical advisory group will decide later this month whether to recommended the new test.
Link to full paper in New England Journal of Medicine
N Engl J Med 2010; 363:1005-1015