TB blood tests ‘put patients at risk’

Standard microscopy and sputum tests are more reliable than commercial blood tests in diagnosing TB Copyright: Flickr/trygveberge

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Blood tests for tuberculosis (TB) diagnosis may be putting patients’ lives at risk through providing misleading results, and should not be used, according to a WHO policy statement.

Instead, the WHO recommends several other diagnostics — even though these may take longer to provide a result, or are still very expensive — and calls for more research on alternative diagnostics.

Commercial tests to detect TB antibodies in the blood can lead to up to 50 per cent of patients being misdiagnosed, according to Karin Weyer, coordinator of laboratory strengthening at WHO’s Stop TB Department.

"We evaluated 92 studies, and among these were 32 studies carried out in developing countries," Weyer told SciDev.Net. "We looked at the evidence from all angles: the reliability and accuracy of tests, and what the impact would be on patients and public when there is a false positive or false negative result."

False negative results can lead to transmission of the disease to others and delayed or inadequate treatment, while false positives wrongly diagnose TB in people, causing unnecessary treatment while their real ailment remains undetected.

WHO’s policy statement applies to commercial blood tests, also known as serodiagnostic or serological tests, which are not approved by any regulatory body and are commonly used in many of the highest TB burden countries. Dozens of such tests exist and more than a million diagnoses are carried out every year, mostly in private clinics where patients pay up to US$30 per test.

"These tests are in use largely because they are being marketed aggressively by the private sector," said Weyer. "In many countries, there is a belief that healthcare is better in the private than the public sector."

The tests are marketed as quick and easy — with same-day results — compared to tests recommended by the WHO, such as standard microscopy or culture tests, which can take days to provide diagnosis.

Two molecular methods are also recommended, but they are still expensive and require sophisticated laboratory equipment. No cheap and rapid test is yet available for TB in remote regions of developing countries.

The policy also calls for "targeted further research to identify new/alternative point-of-care tests for TB diagnosis and/or serological tests with improved accuracy ".

"There is a huge gap and little research investment in point-of-care tests," said Weyer.

Mark Perkins, chief scientific officer at the Foundation for Innovative New Diagnostics (FIND), told SciDev.Net that several alternative tests were in the pipeline. For example, FIND is working on a test to detect TB in urine that could be tested in 2012.

Some researchers are calling for more funding and indigenous innovation in TB diagnostics in high-burden developing countries, such as India and China. At a conference in Bangalore, ‘TB diagnostics in India: from importation and imitation to innovation’ later this month (25–26 August), researchers will look at how India can improve diagnosis and could even take the lead in the field.

Madhukar Pai, associate professor at McGill University, Canada, and a co-organiser of the conference, told SciDev.Net that research and development (R&D) in TB diagnostics has been neglected by rich countries and the industry.

He said emerging economies affected by the disease, such as China, India and South Africa, should invest in R&D to bring down the cost.

WHO’s negative recommendation, published last month (20 July), does not apply to tests for inactive TB infection, which is currently under WHO review.

TB kills 1.7 million people every year.

Link to WHO policy statement  [292kB]