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Controlled application of biological or chemical substances to the skin to test for allergy. Small amounts of diluted test substances applied under a patch of cloth or soft paper and an impermeable membrane are left in place for 48 hours, and the skin reaction is then examined and scored from 0 (none) to 4+ (severe blistering and redness).
This entry comes from Encyclopædia Britannica Concise. For the full entry on patch test, visit Britannica.com.