20 July 2011 - Geneva - The use of currently available commercial blood tests to diagnose active tuberculosis (TB) often leads to misdiagnosis, mistreatment and potential harm to public health, the World Health Organization (WHO) said today.
WHO is urging countries to ban the inaccurate and unapproved blood tests and instead rely on accurate microbiological or molecular tests. The WHO recommendation applies only to blood tests for active TB. Blood tests for inactive, or latent TB infection are currently under review by WHO.
Testing for active TB through antibodies or antigens found in the blood is extremely difficult. Patients can have different antibody responses suggesting that they have active TB even when they do not. Antibodies may also develop against other organisms which again could wrongly indicate they have active TB. In addition, different organisms share the same antigens, making tests results unreliable. These factors can result in active TB not being identified or wrongly diagnosed.
"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," said Dr Mario Raviglione, Director of WHO Stop TB Department. "A blood test for diagnosing active TB disease is bad practice. Test results are inconsistent, imprecise and put patients' lives in danger."
The new recommendation is the result of 12 months rigorous analysis. WHO and global experts evaluated 94 published and unpublished studies, including 67 on pulmonary tuberculosis (TB in the lungs) and 27 on extrapulmonary tuberculosis (TB elsewhere in other organs). The review produced overwhelming evidence that blood tests produce an unacceptable level of wrong results compared to tests endorsed by WHO.
The research revealed "low sensitivity" in commercial blood tests which leads to an unacceptably high number of patients wrongly being given the 'all clear' (i.e. a false-negative when in reality they have active TB). This can result in the transmission of the disease to others or even death from untreated tuberculosis. It also revealed "low specificity", which leads to an unacceptably high number of patients being wrongly diagnosed with TB (i.e. a false-positive when in reality they do not have active TB). Those patients may then undergo unnecessary treatment, while the real cause of their illness remains undiagnosed, which may then also result in premature death.
More than a million of these inaccurate blood tests are carried out annually to diagnose active TB, often at great financial cost to patients. Many patients pay up to US$30 per test. There are at least 18 of these blood tests available on the market. Most of these tests are manufactured in Europe and North America, even though the blood tests are not approved by any recognized regulatory body.
"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," said Dr Karin Weyer, Coordinator of TB Diagnostics and Laboratory Strengthening for the WHO Stop TB Department. "It's a multi-million dollar business centred on selling substandard tests with unreliable results."
This is the first time WHO has issued an explicit "negative" policy recommendation against a practice that is widely used in tuberculosis care.
Improving the early and effective diagnosis of TB to ensure more lives are saved is a priority action for WHO and the international TB community. Research is currently under way to bring better and more rapid TB tests that are easy to administer, effective and accurate.
WHO's Commercial Serodiagnostic Tests for Diagnosis of Tuberculosis - Policy Statement and other related documents are available at: http://www.who.int/tb/laboratory/policy_statements/en