Skin prick testing

What it tests

In vivo demonstration of allergen-specific IgE on cutaneous mast cells
Drop of standardised allergen extract on volar forearm, pricked with a lancet, read at 15-20 minutes
First-line test for IgE-mediated allergy - cheaper, faster and more sensitive than serum specific IgE
Detects sensitisation, not clinical allergy - interpret only against the history

Sensitisation on SPT is far commoner than clinical allergy - up to 40-50% of adults have >=1 positive test

Allergen cross-links IgE on dermal mast cells -> degranulation -> histamine
-> triple response of Lewis: local vasodilatation (erythema), axon reflex flare, and increased permeability (wheal)
Wheal reflects mast cell reactivity as well as IgE quantity

Positive control - histamine 10 mg/mL. A flat positive control invalidates the test - usually antihistamine effect
Negative control - saline/glycerin diluent. A positive negative control means dermatographism - the whole test is uninterpretable
Prick sites >=2 cm apart, read at 15-20 min

Positive: wheal >=3 mm greater than the negative control
High negative predictive value (~90-95%) - a negative test is powerful for ruling out IgE-mediated allergy
Poor positive predictive value (~50%) - a positive test alone never diagnoses allergy
Wheal size correlates with probability of reaction, not with severity of the next reaction
Intradermal testing - more sensitive, less specific; used for venom and drug allergy after a negative SPT, never for foods (risk of systemic reaction and false positives)

Absolute

Antihistamine use (cease 3-5 days; long-acting agents up to 7)
Tricyclic antidepressant use (potent H1 blockade; cease 2 weeks) - also mirtazapine, some antipsychotics
Topical corticosteroid to the test site within 72 hours
Severe eczema - no unaffected skin
Dermographism - uninterpretable
Poor cooperation

Relative

Pregnancy (systemic reaction risks the fetus)
Persistent or unstable asthma - the strongest predictor of a systemic reaction; defer until FEV1 >70% predicted
Severe prior anaphylaxis to the specific allergen being tested - use specific IgE instead, or test at high dilution

Beta-blockers and ACE inhibitors do not prevent testing but make a systemic reaction harder to treat - review beforehand
Oral and inhaled corticosteroids and montelukast do not suppress SPT and need not be ceased
Age extremes give smaller wheals - infants and the very elderly

Antihistamines cannot be ceased, extensive skin disease, dermographism, high-risk anaphylaxis, poor cooperation

Perform where adrenaline, resuscitation equipment and 30 minutes of observation are available
Systemic reactions are rare (<0.1% for aeroallergens) but higher with food, venom, drug and intradermal testing
Discordant history and test -> supervised oral or drug provocation challenge, which outranks both
Report as sensitisation with a clinical interpretation, not as "allergic to X"
Unnecessary avoidance from an uninterpreted positive causes nutritional harm and may cause loss of tolerance

Allergic rhinitis, asthma, atopic dermatitis, food allergy
Venom allergy - SPT plus specific IgE, as either alone misses cases
Drug allergy - validated reagents essentially limited to penicillin determinants
Dermographism and mastocytosis - false positives

Wheal size falls as tolerance develops - supports timing a food challenge
Penicillin skin test reactivity is lost over time: ~80% negative by 10 years - the basis of delabelling
Persistent large wheals to peanut, tree nut, fish and shellfish predict persistent allergy

11 more sections, plus exam facts

Premium unlocks every note across every specialty, and the full exam fact library behind it.