TB or not TB, the inaccuracy of blood tests…

On the 20th July 2011 the World Health Organisation called for countries to ban the use of serological (blood) tests to diagnose active TB disease; citing “inconsistent and imprecise estimates” and recommending “that these tests not be used for the diagnosis of pulmonary and extra-pulmonary TB.”

Mode of action of common tuberculosis rapid diagnostic test

How the TB tests work

Tuberculosis, the common bacterial infection caused by Mcyobacterium tuberculosis is a worldwide killer. Coughing up blood and sputum, chest pains, weakness, weight loss, fever and night sweats are common symptoms of the respiratory disease, treated only with a six month course of antibiotics. Left untreated, each person with the disease will infect between 10 and 15 people every year. With one-third of the world’s population infected with TB, the disease is most prevalent in developing countries of sub-Saharan Africa and South-east Asia. Two factors complicate this already persistent disease — HIV coinfection and drug resistant TB. In Africa, HIV is the single most important factor contributing to the increase in the incidence of TB since 1990. We have only been fighting this disease for the last 50 years — before then, there were no medicines to cure TB. Resistant strains have appeared in every country surveyed; and strains of TB resistant to all major anti-TB drugs have emerged. And with this recent WHO recommendation, diagnosing the disease just got a little more complicated for endemic communities.

As an exercise and a lesson on the translation of research into policy — research done as early as in 2007, showed the lack of accuracy of such diagnostic tests. 19 of the commercial rapid diagnostic tests were deemed unacceptable. TropIKA.net adds “Unfortunately, after these studies were published, the evidence was never translated into policy. While positive results often generate publicity and excitement, negative findings rarely evoke action.”

The report concluded to say “the majority of products had poor specificity (<80%) when tested in TB suspects from endemic settings”. However, the most intriguing part suggested that the performance of such tests is diminished by HIV co-infection.

It seems greater regulatory oversight is what is needed. The tests are sold and used in endemic countries, developing countries, and with very little evidence of effectiveness. No body, governmental or otherwise, exists to regulate. Leading to misdiagnosis and clinical mismanagement — curbing the global health effort.

WHO recommendations sometimes fall short of the reality on the ground. The gap from research to policy is only bettered by the gap between policy and implementation. Malaria parasite resistance prompted the wide-scale shift in first-line treatments against malaria, under recommendation from the WHO. However, many countries continued to use the ineffective mono-therapy treatments, due, in part, to the disparity in costs between the more conventional chloroquine and sulfadoxinepyrimethamine based therapies and the recommended Artemisinin combination drugs. In reality, the front-line decisions usually comes down to the economics of the situation. The case for TB should be smoother, as findings show that the estimated total cost of serologic testing (including treatment of newly diagnosed cases) was approximately four times that of microscopy, at $47.5 million versus $11.9 million.

The WHO’s policy recommendation is unprecedented. This is the first time WHO has issued an explicit “negative” policy recommendation against a practice that is widely used. More R&D into accurate point-of-care tests for TB are ongoing and the WHO policy strongly encourages future research to develop novel or improved point-of-care tests.


18 Responses to “TB or not TB, the inaccuracy of blood tests…”

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    Remember, tests are done to determine the cause of your symptoms or disorder. Since the blood goes around the body, it can carry traces of the illness you have wherever it is, brain, hands, feet, anywhere within your body. Next time a doctor explain the results of your blood test, don’t be shy to ask for explanations, indications and what does those names and numbers mean…

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