Skin prick test

Principle

A small amount of allergen introduced into the epidermis triggers mast cells coated with specific IgE to release histamine, producing a wheal and flare (type I hypersensitivity).

Procedure

• Place drops of allergen extracts on the forearm or back, with histamine (positive) and saline (negative) controls.
• Prick through each drop with a lancet; read after 15–20 minutes.

Interpretation

• Wheal 3 mm or larger than the negative control = positive

Precautions

• Stop antihistamines beforehand (usually several days).
• Resuscitation equipment must be available because anaphylaxis can occur.