Using blood tests to diagnose active tuberculosis (TB) often leads to misdiagnosis, say the World Health Organization (WHO), who urge all countries to stop using the tests and rely instead on accurate WHO-recommended microbiological or molecular tests.

This is the first time the global health agency has issued a “negative” policy recommendation relating to TB, something they say “underscores the Organization’s determination” to advise governments when there is strong evidence that widespread practice is doing more harm than good.

The WHO’s new policy recommendation, released from its headquarters in Geneva on Wednesday, states that the currently available commercial blood (serological) tests are unreliable and inaccurate. This is because they look for antibodies or antigens in the blood, which is a notoriously difficult thing to do.

Sometimes these blood tests don’t find antibodies in patients who have active TB and return a negative result, and sometimes they find antibodies to other diseases that are mistaken for TB antibodies and return a positive result.

Using blood tests to test for active TB is “bad practice” that leads to misdiagnosis, mistreatment and potential harm to public health, say the WHO.

Dr Mario Raviglione, Director of the Stop TB Department at the WHO, said:

“In the best interests of patients and caregivers in the private and public health sectors, WHO is calling for an end to the use of these serological tests to diagnose tuberculosis.”

“A blood test for diagnosing active TB disease is bad practice. Test results are inconsistent, imprecise and put patients’ lives in danger,” he added.

The agency pointed out that the new policy recommendation only applies to blood tests for active TB. They are in the process of reviewing the blood tests for inactive TB, which is also referred to as dormant or latent TB.

The new policy recommendation follows 12 months of analysis of evidence by the WHO and global experts on TB, who evaluated 94 studies, including 67 for pulmonary TB. The “overwhelming” evidence from these studies showed that the blood tests return an unacceptable level of either false positives or false negatives, compared to tests recommended by the WHO.

For example, many of the studies found that the commercial blood tests had “low sensitivity” and returned negative results when they should have been positive. This means patients who think they are in the clear don’t get treated and there is a greater risk they will pass on the disease, or even die from untreated TB.

The evidence also showed that the commercial blood tests have “low specificity”, which means too many patients receive a positive result, indicating they have active TB, when they do not. This results in them being treated for an illness they don’t have, and perhaps more alarmingly, not being diagnosed or treated for an illness they do have, which could be serious and even fatal.

And it’s not as though these tests are cheap: many patients pay as much as 30 US dollars per test, of which there are 18 different brands, mostly made in Europe and the US, not approved by any regulatory body, and sold in a global market where more than a million are carried out every year to test for active TB.

As Dr Karin Weyer, WHO Stop TB Department’s Coordinator of TB Diagnostics and Laboratory Strengthening, explained:

“Blood tests for TB are often targeted at countries with weak regulatory mechanisms for diagnostics, where questionable marketing incentives can override the welfare of patients.”

“It’s a multi-million dollar business centred on selling substandard tests with unreliable results,” she added.

Every year, 1.7 million people die of TB. It is the number one killer of people with HIV. Improving the diagnosis of TB is a priority for the WHO and the community that is fighting TB worldwide.

Researchers are currently working on ways to make better, faster, more accurate tests that are easier to administer, said the WHO.

Source: WHO.

Written by: Catharine Paddock, PhD