The technical details about tuberculins and how to administer and read a tuberculin test are beyond the scope of this book. Clinical tuberculosis (Crofton, Horne & Miller) gives a good account.The standard amount of tuberculin used is 5 units, injected as 0.1 ml into the anterior surface of the forearm at the junction of the middle and upper thirds. It is very important that the tuberculin is injected intradermally so that it is well localized. If correctly given, the injection should raise a small bump of 5 mm or more in diameter, which disappears within 1-2 hours.This is not easy in a vigorous and protesting child. Poor injection technique can cause a false-negative reaction.
A tuberculin test is not significant, or "negative", when the diameter of skin induration is less than 10 mm (or less than 5 mm in an HIV-infected child). This is regardless of whether the child has had BCG. A negative tuberculin skin test does not exclude TB.Thus, it is of no help in deciding that someone does not have TB.The table below shows the conditions that can suppress a tuberculin skin test in a person with active TB.
Conditions that may suppress the tuberculin skin test
HIV infection malnutrition severe bacterial infections, including TB itself viral infections, e.g. measles, chickenpox, glandular fever cancer immunosuppressive drugs, e.g. steroids incorrect injection of PPD
The criterion for a significant or "positive" tuberculin test depends on whether a child has previously had BCG vaccination or not. This is because a reaction to tuberculin is usual after a previous BCG, at least for several years.The reaction is usually weaker (diameter often less than 10 mm) than the reaction to natural infection with M. tuberculosis. A tuberculin test is considered significant or positive when the diameter of skin induration is 10 mm or more. However, if the child is HIV-infected, the tuberculin test is considered positive if the induration is 5 mm or more. A positive tuberculin test is only one piece of evidence in favour of the diagnosis of TB. The younger the child and the greater the diameter of induration, the stronger is that one piece of evidence.
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