The Schick test detects immunity to diphtheria, either acquired through previous infection or vaccination.
A small amount of diphtheria toxin in 0.2ml is injected intradermally into the left forearm. An equivalent amount of heat-inactivated toxin in 0.2ml is injected into the right forearm. The test is read at 1-4 days, the maximal reaction occurring on day 4.
Those who are Schick negative will have no skin reaction. This is due to sufficient antitoxin being present within the circulation to neutralise the toxin. A Schick negative person is immune and need not be immunised or reinforced. At worst, if they become infected, they will only suffer a mild attack.
A Schick positive test is an erythematous reaction developing at the site of the toxin injection after a day or two and persisting for seven or more days before fading. Schick positive individuals are susceptible to diphtheria.
Occasionally, a mild reaction may occur in the right arm only. This is a non-specific reaction to bacterial protein. It disappears by day 4. The individual is Schick negative.
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