by Dr Pandula Siribaddana

Created on: February 24, 2010

TB test or the Mantoux test is the industry standard in screening for Tuberculosis in many parts of the world. The test makes use of a Purified Protein Derivative of Tuberculin and injects 0.1 ml of it in to the skin over the inner aspect of the forearm. The injection site will be re-assessed in 48 to 72 hours and the changes in the area will be noted. For the test to be significant, an induration or a hard swelling should be formed at the inoculation site rather than just a reddening.

Interpreting the TB test is a job for the clinician and the Center for Disease Control has stated the necessary guidelines based on medical research in order to accurately interpret the results. Anyone who wants to interpret the results of a TB test should also keep in mind that, a positive test per se does not mean the person is suffering from TB and a negative test does not really mean the person is free from TB.

According to the guidelines, an induration of more than 15 mm is considered positive in any person and would warrant further investigations in order to identify the primary focus and to initiate treatment.

If the induration is less than 5 mm, usually the test is considered to be negative.

If the induration ranges between 5 mm and 10 mm, the test should be taken as positive in patients with, HIV infection, has had a contact with a TB positive patient in the recent past, a person who is immunosuppresed or else instances where the person shows fibrotic lung changes in the x-ray. At the same time, the CDC guidelines also state that a patient with organ transplant should also be taken as positive if fallen within this category.

In instances where the induration falls between 10 and 15 mm, the interpretation of being positive should be made in instances of,

*Patients with clinical signs and symptoms

*Children who are less than 4 years of age

*Children and adolescents who are exposed to TB patient

*Persons who are working in laboratory settings in which TB tests are done

*Persons who are residents in an institutional settings…etc.

As mentioned earlier, there can be false positive skin tests among persons who are having infections from mycobacterium other than tuberculosis as well as in persons who are given the BCG vaccine in the recent past. At the same time, an erroneous measurement, wrong administration and incorrect use of the solution can all give rise to positive results in the absence of TB.

A false negative result can be seen in instances of immune-suppression, recent TB infection, very old TB infection, some viral infections, very young age and an overwhelming TB disease.

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