The World Health Organisation recently issued a recommendation against the use of blood tests based on antibody response for diagnosing active tuberculosis, both pulmonary and extra-pulmonary. India's Revised National Tuberculosis Control Programme (RNTCP) has endorsed WHO's advice. Unfortunately, with TB, unlike diseases like HIV, the presence or absence of antibodies does not reflect true disease condition. So these blood tests lack the necessary sensitivity or specificity or both to be used as an effective diagnostic tool. When the sensitivity is low, many people with active TB are likely to be wrongly diagnosed and hence will go untreated (false negative); tests with low specificity will result in people with no active TB getting treated with toxic drugs (false positive). That explains why serological testing based on antibody responses has so far not been recommended by the U.S. Food and Drug Administration and other reputed regulators. However, considering that in high prevalence settings like India, while the specificity of sputum smear microscopy testing is better than 99 per cent, the sensitivity is only 60-70 per cent, finding more reliable and inexpensive diagnostic blood tests for TB is a public health imperative.
The world body's recommendation is based on a body of evidence, including a study by its special programme for research & training in tropical diseases (TDR), which looked at 19 commercially available rapid blood tests. The WHO concluded that patient safety is adversely impacted by “commercial serological tests [which] provide inconsistent and imprecise findings.” Unfortunately, notwithstanding the unreliability and prohibitive cost, the private sector in India uses the blood tests on “at least 1.5 million people every year,” according to a report in The Lancet published in January 2011. These tests appear to have been done mostly at the expense of sputum examination. For example, a 2006 WHO report, “Diagnostics for tuberculosis — global demand and market potential,” found that in Tamil Nadu and Delhi, less than one-third of patients underwent sputum examination despite multiple visits while doctors based in west India relied solely on chest X-rays for diagnosis. That the government has to curb the availability of serological antibody tests and undertake awareness-building campaign among doctors and the public on the benefits of sputum tests cannot be over-emphasised. This is vital to public health — considering that in India the incidence rate of TB is a high 168 per 100,000 population and the disease kills two people every three minutes.