In a landmark study electronically published in the American Journal of Respiratory and Critical Care Medicine, a blood test for detecting TB infection, QuantiFERON(R)-TB Gold (QFT(TM)), has been shown to be six times more accurate than the conventional tuberculin skin test (TST) at predicting which tuberculosis (TB)-exposed individuals will go on to develop TB disease. This study has important implications for the worldwide effort to eradicate TB, as accurately indentifying TB-infected individuals allows health authorities to treat them before they develop disease, and the greater accuracy of the new test makes this more efficient as well as cheaper and safer (see also Tuberculosis).
Globally, up to two billion people are infected with TB. In most cases the TB bacteria is contained by the body’s immune system (latent TB infection) without any symptoms of the infection. But 9 million people, most originating from this large pool of individuals with latent TB infection, are reported every year by The World Health Organization as developing active and infectious TB disease.
In the developed world, TB eradication strategies have focused on identifying and treating people with latent TB infection. Until recently the tuberculin skin test (TST; Mantoux test) has been the only test available to detect infected people. However, the TST often produces false-positive results, meaning many uninfected people are offered unneeded preventative treatment for TB. This treatment is for 6 to 9 months and may produce damaging side effects. For improved TB control, a test that accurately detects those truly infected, enabling treatment for only those at risk of developing TB disease, has major implications. The QFT test has now been shown to have this desired characteristic.
In the German city of Hamburg, Dr. Roland Diel and colleagues used both TST and QFT on 601 people who may have been infected by contact with people with TB disease. 40% had a positive TST, but only 11% (66) of the exposed persons were found infected by QFT and offered TB treatment — 41 declined. Over the next two years 6 people developed TB disease, all were QFT positive and had declined treatment. In the past, all 243 TST positive people would need to have had anti-TB treatment to prevent only 5 of these cases, as one was TST negative. In scientific terms, QFT had a predictive value for developing TB disease of 15%, more than 6 times greater than the 2.3% for the TST.What does this mean? Using QFT, doctors can now treat only a fraction of the people they would have with the TST, with the knowledge that they are preventing TB disease to at least the same extent, and likely better.