Skin Prick Testing

Skin prick testing detects IgE-mediated sensitisation to aeroallergens, foods, insect venoms and some drugs. A drop of allergen extract is placed on the skin, and a lancet pierces the epidermis through the drop. A wheal and flare within 15 to 20 minutes indicates IgE binding to mast cells in the skin.

A positive result shows sensitisation, not clinical allergy. A negative result has a high negative predictive value, so IgE sensitisation to that allergen is unlikely when the test is valid.

It is used in allergic rhinitis, asthma triggers, atopic eczema with suspected food or aeroallergen triggers, food allergy evaluation, venom allergy, and as the first step in penicillin allergy assessment.

Component resolved diagnostics. Some food and venom allergens are tested as purified components, which can separate true sensitisation from cross-reactivity. Component results are interpreted with the prick result and the history.

Indications. Allergic rhinitis, asthma triggers, atopic eczema, food allergy evaluation, venom allergy and penicillin allergy assessment.

Preparation and technique. Second generation antihistamines are usually stopped about 5 to 7 days beforehand, and first generation antihistamines about 2 to 3 days beforehand, with local timings confirmed. Tricyclic antidepressants may suppress reactions. The test site, usually the forearm or back, must be free of dermatitis and dermographism that obscures the reading.

Sites are marked at least 2 cm apart. Each drop is placed, the skin is pricked through the drop at about 45 degrees, and excess fluid is wiped away. Wheal size is measured at 15 minutes, using the mean of the longest diameter and the diameter at right angles.

Controls. A positive control (histamine) and a negative control (diluent, usually saline or glycerol) are tested with each series. A valid test requires a clear histamine wheal. Without it, a negative result is uninterpretable.

Contraindications and precautions. Beta blockers increase the severity of anaphylaxis and reduce response to adrenaline, so testing is deferred or performed only under specialist supervision. Severe uncontrolled asthma and recent systemic reactions require specialist judgement.

Interpretation.

• A positive result is a mean wheal diameter at least 3 mm greater than the negative control, with a valid histamine control.
• Wheal size correlates with the probability of clinical reactivity. Predictive thresholds exist only for a few foods, including peanut and hen's egg, and they vary between populations and extracts.
• Discordant results between prick testing, specific IgE and the history prompt either an intradermal test or an oral challenge, depending on the question.

Causes of a false negative result. Antihistamines, expired or poorly stored extracts, recent systemic reactions, skin with reduced reactivity, and poor technique.

Causes of a false positive result. Dermographism, which is common and can produce large wheals without allergy, and skin trauma from over vigorous pricking.

Critical values and findings needing immediate action.

• Systemic reactions can occur, although they are rare. Adrenaline and resuscitation equipment must be immediately available.
• The patient is observed for a period after testing, and any early systemic sign requires immediate treatment.

Limitations and artefacts. Testing on skin with recent sunburn, eczema or irritation lowers reliability. Pricking through a drop of blood or excess fluid changes the wheal. Extracts lose potency with poor storage.

Follow up tests. Specific IgE testing, component resolved diagnostics, intradermal testing and oral food challenge are used depending on the allergen and the strength of the history. Positive results are linked to the exposure history and to avoidance advice where clinical allergy is confirmed.

Special groups. Young infants have limited skin reactivity, and food sensitisation patterns differ in early life, so interpretation requires experience. Older adults with cardiovascular disease need careful consideration of the severity of any systemic reaction.