skin test
Meaning of skin test: A positive result proves the immune system remembers the substance, which is not the same as proving it causes symptoms.
Definition of skin test
A skin test introduces a small amount of a substance onto or into the skin and looks for a visible immune reaction at that spot. Redness, a raised weal or a firm swelling shows that the immune system already recognizes the substance. Different techniques probe different kinds of reaction, and the moment the result is read distinguishes them as much as the method does.
What are the main types?
| Type | How it is applied | When it is read | What it detects |
|---|---|---|---|
| Prick or scratch | A drop pricked into the surface | Within about a quarter of an hour | Immediate allergy |
| Intradermal | Injected into the skin layer | The same short interval | Immediate allergy, more sensitive |
| Tuberculin | Injected into the skin layer | After two to three days | Immune memory of tuberculosis bacteria |
| Patch | Taped against the back | After two days, often read again later | Delayed contact allergy |
Why does the timing separate them?
Because two different arms of the immune response are being read. An immediate reaction depends on antibodies already stuck to mast cells in the skin: the substance links them, the cells dump histamine on the spot, and an itchy weal rises within minutes. A delayed reaction has no antibody involved at all. Immune cells must travel to the site, recognize the substance and recruit others, which takes a day or two and produces a firm, deep swelling rather than a soft weal. That is why a tuberculin result is measured for hardness rather than for redness, and why a patch test stays taped in place across days.
What does a positive result prove?
Only that the immune system has met the substance before. It does not establish that the substance causes symptoms. Plenty of people react on testing to foods they eat without trouble, so a result is interpreted next to the history rather than on its own. The same gap applies to tuberculosis: a positive tuberculin test shows infection at some point in the past, not active disease, and prior vaccination or exposure to related environmental bacteria can produce one. Antihistamines taken beforehand blunt immediate reactions and can flatten a genuine response, which is why a control site is included for comparison.
Also known as
Used in a sentence
A skin test was placed on the forearm and read two days later.
Good to know
A positive skin test shows that the immune system has encountered the substance before, not that it currently causes disease; sensitization without symptoms on real exposure is common, and a positive tuberculin result indicates infection at some point rather than active tuberculosis.