Schick test
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Schick test diphtheria
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Schick test is an intradermal test formerly used to determine whether a person is susceptible to diphtheria.
Principle
It detects whether the person has enough circulating diphtheria antitoxin antibodies to neutralize a very small amount of injected diphtheria toxin.
Method
- A small dose of diluted diphtheria toxin is injected intradermally into one forearm.
- A control, consisting of heated and inactivated toxin, is injected into the other forearm.
- The reaction is read after about 48-72 hours, with later observation if needed.
Interpretation
| Result | Finding | Meaning |
|---|---|---|
| Positive | Erythema and induration at the test site, usually appearing after 24-48 hours and persisting for several days | No protective antitoxin present. The person is susceptible to diphtheria. |
| Negative | No reaction at the test site | Adequate antitoxin present. The person is immune. |
| Pseudo-positive | Similar early reactions at both test and control sites, then fading rapidly | Hypersensitivity to toxin constituents rather than lack of immunity. |
| Combined reaction | Early reaction at both sites, but test-site reaction persists while control fades | Both hypersensitivity and true susceptibility may be present. |
Current relevance
The Schick test is now mainly of historical importance and is not used routinely because diphtheria immunization is assessed from vaccination history and, when necessary, by measuring serum antitoxin antibody levels. Modern diagnosis of suspected diphtheria focuses on detecting toxin-producing Corynebacterium diphtheriae, including toxin testing such as the CDC-described Elek test.
Textbook reference: Quick Compendium of Clinical Pathology, 5th ed., section 3.5.3.2.1.1.
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