Tuberculin Skin Test - Procedure and Result of Tuberculin Skin Test

Tuberculin Skin Test detects the 'infected' cases of tuberclosis by eliciting a hypersensitivity reaction in patients who are already infected with tubercle bacilli. In this test, more precisely the Mantoux test, purified proteins isolated from tubercle bacilli called 'tuberculins' are injected intracutaneously on the upper surface of the forearm. Out of several tuberculin tests, the Mantoux is most commonly used, since by and large its results are highly reliable. This test will have little value in places where there is a high incidence of tuberculosis, i.e., at such places almost every person can be expected to be infected with tubercle bacilli. However it has value otherwise. Those cases which are tuberculin negative, may not be suffering from tuberculosis, and in such cases, one should think of other pathology in a concerned patient.

In about 48-72 hours, following the injection of the test, an allergic reaction occurs, as a raised, hard red spot at the site of injection. the size of this area should be carefully measured, and an arear of 10 mm, roughly about half an inch or more in diameter, indicates that the reaction is positive. Only the induration (raised area) should be measured with regard to its width. The redness around the raised area must be avoided while taking measurements. Ideally, a trained and experienced technician, especially one who specifically deals in carrying out tuberculin tests, is required. An induration of less than 7 mm is considered negative, while induration between 7-10 mm, may be considered a borderline or a doubtful case of tuberculin positive, and such cases need further tests, for exclusion, or for establishing a diagnosis of tuberculosis.

However this test has its own limitations. Although a positive reaction does not indicate that the person is infected with tubercle bacilli, it does not indicate whether the disease is active or not.

In a nutshell, this test may not have any value in detecting the hidden/symptomless, especially active cases of tuberculosis, when a large scale programme is carried out; even though this test has its value in individual cases. therefore this test may not find a place in a national policy programme. However, an awareness about it is important since it is one of the routine tests.