A new blood test, Enzyme-linked immunospot (ELISpot), is found to be more effective in diagnosing
tuberculosis (TB) than the traditional skin tests.
Tuberculosis is an infectious bacterial disease, which typically attacks the lungs and affects about 9.2 million more people each year, killing an estimated 1.7 million. Most of the victims are in developing countries where limited means of screening for the disease lead to delay in treatment.
Traditional testing for TB involves injecting a person with components of the TB bacterium and resultant swelling of the skin is considered to be a signal to dormant tuberculosis. Such skin tests, however, are prone to false positives as at times, people have been wrongly identified as needing treatment and conversely, people actually infected with TB have wrongly found to be free of the infection.
This immunoassay detects a chemical secreted by a type of lymphocytes (T-cells) in response to a protein not found in the BCG vaccine and most environmental Mycobacteria. The test results are therefore not confounded by previous BCG vaccination, conferring higher specificity than the tuberculin skin test. Moreover, results are available the next day.
To find out the effectiveness of the new ELISpot in comparison to the skin test, researchers from Britain studied 908 healthy
children exposed to tuberculosis in their homes. The researchers used the traditional skin test and the newly developed ELISpot test for screening the children. A little over half the participants were tested positive for latent TB using the two tests.
The skin test suggested 580 children required drugs to ward off active TB, but the blood test pointed to just 380. Twelve children developed active TB even with treatment. This suggests that ELISpot is 1.5 times better at spotting tuberculosis carriers than the
skin test.
The researchers noted that findings are really useful in the sense that by using the blood test one only needs to treat 380 children instead of 550 children to prevent the same number of active cases.
The next step is to make the new test even more accurate and establish its use in the developing world.
Annals of Internal Medicine
October 2008
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