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CDC recommends new tuberculosis blood test

Florida Fred Charatan

The US Centers for Disease Control and Prevention (CDC) is recommending a new blood test for tuberculosis that it believes is more accurate than the existing test. The QuantiFERON-TB Gold test, made by the Australian company Cellestis, has been used by the San Francisco Department of Public Health since February 2005. Officials there said that it was “one of the first [tuberculosis] advancements made since the discovery of antibiotics” and that it represented a considerable advance over the first generation QuantiFERON-TB test previously used, which began in 2001.

Guidelines produced by the San Fransisco Department of Public Health explain that, for the test, a blood sample drawn from a patient in a listed high risk population is mixed with antigens including ESAT-6 (early secreted antigenic target 6 kDa protein) and CFP-10 (culture filtrate protein 10) proteins specific to the Mycobacterium tuberculosis complex. If the patient is infected, the patient’s lymphocytes will recognise the antigens and release interferon γ in response. The test results are based on this amount of interferon γ. Details of the national guidance on using the test were issued in the Morbidity and Mortality Weekly Report, published by the CDC (16 Dec 2005: pp 49-55).

The advantages of the latest test, says the CDC, are higher sensitivity and specificity than the 100 year old tuberculin skin test, which is liable to false positives from non-tuberculosis bacteria and from patients with AIDS or other immune deficiency conditions. Also, the patient only needs to attend once for the test.

According to Masae Kawamura, director of the tuberculosis control section at the San Francisco Health Department, introducing the new test has led to a big drop in false positive readings. She said that the more accurate diagnoses could result in reduced healthcare costs. San Francisco’s guidelines caution, however, that the new test “is a useful but imperfect diagnostic aide. It should not replace clinical judgment.”

Kenneth Castro, director of the CDC’s Division of Tuberculosis Elimination, said, “We’re saying that this can be used instead of the tuberculin [skin] test. But we’re not going as far as saying it should replace it.”