Allergy Skin Test Interpreter Calculator

The standard clinical cutoff is a net wheal -- the allergen wheal diameter minus the negative (saline) control diameter -- of at least 3mm. This calculator computes that net wheal automatically and reports "clinically positive" once it reaches 3mm, which is the threshold most widely used in skin-prick-test interpretation because it balances reproducibility of measurement against sensitivity to true reactions.

Standard skin-prick-test grading is based on the wheal -- the raised, firm bump -- not the surrounding flare (erythema), which tends to be more variable and less reproducible between observers. This calculator records flare diameter for reference and flags it separately as "significant" past 10mm, but the reaction grade, net wheal, and clinically-positive determination are all computed from wheal measurements alone.

Two controls need to behave as expected: the positive histamine control should produce a wheal of at least 3mm, confirming the skin can mount a normal reaction (a smaller histamine wheal can mean antihistamine medication is still active or the skin is hyporeactive), and the negative saline control should stay small, generally no more than about 2mm, ruling out irritant skin or dermatographism producing a false positive. This calculator checks both conditions before reporting the test as valid.

A larger wheal-to-histamine ratio generally does track with a stronger immediate hypersensitivity reaction, which is why this calculator's reaction grade and sensitivity level both scale with wheal size relative to the histamine control. But skin test size does not translate directly to clinical severity -- a large wheal indicates sensitization, not necessarily how severe a real-world allergic reaction would be, so results are always interpreted alongside a patient's clinical history.