Skin Prick Testing

A skin prick test places a drop of allergen extract (or, in “prick-to-prick” testing, the fresh food itself) on the forearm or back and pricks the skin through it with a small lancet. A raised wheal measured after about 15 minutes, compared against a saline negative control and a histamine positive control, shows that the child has IgE antibodies to that allergen. This is sensitisation, not allergy. The same information can be obtained from a blood test (Specific IgE Testing). Toddlers tolerate SPT well because it is quick and does not need a blood draw, but antihistamines have to be stopped for several days beforehand, and the tester needs patches of clear, unaffected skin, which can be scarce in severe Atopic Dermatitis. The NIAID addendum guideline says SPT should be done and read by a specialist who can manage reactions (Togias 2017).

Skin Prick Testing [defines] Food Allergy Skin Prick Testing [relates] Specific IgE Testing Skin Prick Testing [relates] Atopic Dermatitis

False positives in eczema. Children with eczema often have very high total IgE because of their damaged skin barrier, so many foods come back “positive” even though the child eats them without problems. A 2022 JAAD International review reports that up to 53% of children with AD have a positive food SPT and/or sIgE, but only up to 15% react on an Oral Food Challenge. SPT has an excellent negative predictive value (>95%) but only a moderate positive predictive value (30–50%). Braun et al. (2025) add that IgE plays only a marginal role in most AD, so tests should be done only when there is a matching immediate reaction such as hives or swelling, not for eczema alone. Larger wheals are more predictive. In the Australian HealthNuts cohort (12–18 months), a peanut wheal of ≥8 mm had a 95% PPV for a positive challenge. In LEAP, only 3 of 301 infants with 0–2 mm wheals reacted at a baseline peanut challenge, compared with 4 of 17 with 3–4 mm wheals (Togias 2017).

Atopic Dermatitis [worsens] Skin Prick Testing Oral Food Challenge [contradicts] Skin Prick Testing LEAP Trial [supports] Skin Prick Testing

Why panel testing misleads. Ordering a broad “food panel” for a toddler with eczema and no history of reactions produces a list of false positives. Parents then remove foods the child was already eating safely. In 2012 the American Academy of Allergy, Asthma & Immunology’s Choosing Wisely list said not to perform “an indiscriminate battery of immunoglobulin E (IgE) tests”, because “skin and blood testing to foods has a high rate of showing positive results in people who don’t have food allergy”. In 2014 it added a more specific item: “Don’t perform food IgE testing without a history consistent with potential IgE-mediated food allergy”. The NIAID panel likewise advises against food panels because their poor PPV leads to over-diagnosis and unnecessary dietary restriction. Removing tolerated foods can itself cause harm. Chang et al. (2016) found that 19% of children on Elimination Diets with no prior immediate reactions went on to develop new immediate reactions, and 30% of those reactions were anaphylaxis. The 2022 AAAAI/ACAAI meta-analysis found test-guided elimination no better than empiric elimination (Oykhman 2022). IgG Food Testing is not a valid allergy test at all.

Choosing Wisely [opposes] Skin Prick Testing Skin Prick Testing [causes] Elimination Diets Elimination Diets [causes] Food Allergy

Unsupervised food elimination after a "positive" test

A positive SPT or blood test in a toddler with eczema is not proof of allergy. Removing several foods on the strength of a panel risks nutritional gaps and slower growth, and it can turn a sensitisation the child currently tolerates into a true immediate allergy (Chang 2016). Ask for an allergist review and, where needed, an Oral Food Challenge before cutting foods out.

Where it does help. SPT is useful when there is a clear story: hives, swelling or vomiting within about two hours of eating a food. It is also used for risk-stratifying infants with severe eczema before peanut introduction (NIAID 2017 wheal categories: ≤2 mm, introduce at home; 3–7 mm, supervised feed or challenge; ≥8 mm, specialist management), as in the LEAP Trial and the Dual-Allergen Exposure Hypothesis work. NICE CG57 reserves allergy testing for eczema that is not controlled despite optimal management, or for children with a history of immediate reactions (see When to Refer).

Skin Prick Testing [precedes] Oral Food Challenge Dual-Allergen Exposure Hypothesis [relates] Skin Prick Testing

Connections

• Food Allergy — sensitisation ≠ allergy; low PPV in AD, source: Braun 2025
• Elimination Diets — test-guided elimination no better than empiric, source: Oykhman 2022
• Dual-Allergen Exposure Hypothesis — NIAID tiers use SPT wheal size before peanut introduction, source: NIAID 2017
• LEAP Trial — stratification by 0 vs 1–4 mm wheal, source: Du Toit 2015
• Pets and Eczema — dog/cat sensitisation interacts with ownership, source: Epstein 2011
• Oral Food Challenge — reference standard that confirms or refutes a positive SPT, source: JAAD Int 2022
• Specific IgE Testing — blood-test equivalent with the same low PPV in AD, source: Togias 2017
• Choosing Wisely — AAAAI: no indiscriminate IgE batteries / no food IgE without consistent history, source: AAAAI 2012
• IgG Food Testing — invalid test, not a substitute, source: AAAAI 2012
• When to Refer — allergy testing is a specialist decision, source: NICE CG57