The question parents most often ask after a food reaction in a child is: how do we know what caused it, and how do we know what to avoid? The desire for certainty is understandable. Allergy testing can provide useful information, but it is less definitive than most parents expect, and the interpretation of results requires clinical judgement rather than a simple positive/negative answer.
The fundamental limitation of food allergy tests – whether skin prick or blood test – is that they measure sensitisation: the presence of IgE antibodies to a food. Sensitisation and clinical allergy are not the same thing. Many sensitised individuals can eat the food they are sensitised to without any reaction at all.
Healthbooq (healthbooq.com/apps/healthbooq-kids) covers allergy and immune conditions in children.
Types of Food Allergy
IgE-mediated food allergy involves an immediate immune response driven by IgE antibodies. Symptoms occur within minutes to 2 hours of eating the food and can include hives, angioedema, vomiting, and in severe cases anaphylaxis. This is the type of allergy that skin prick and specific IgE blood tests are designed to detect.
Non-IgE-mediated food allergy involves a different immune mechanism and produces delayed reactions: gastrointestinal symptoms (vomiting, diarrhoea, abdominal pain) or eczema worsening, typically 2 to 72 hours after eating. Skin prick and blood tests are not useful for non-IgE-mediated allergy. Diagnosis relies on dietary elimination followed by reintroduction.
Many children have conditions that mix both mechanisms, particularly food protein-induced enterocolitis syndrome (FPIES) and atopic dermatitis with food triggers.
Skin Prick Test
A skin prick test (SPT) involves placing a small drop of food allergen extract on the forearm, then pricking through the drop with a lancet. A raised wheal of 3mm or more at 15 minutes indicates sensitisation. The test is quick, has immediate results, and can test multiple foods simultaneously.
SPT sensitivity (correctly identifying true allergy) is around 70-85%; specificity (correctly identifying those without allergy) is lower, around 30-70%, meaning a substantial proportion of positive tests are false positives. A negative SPT is more informative than a positive one: a negative SPT makes IgE-mediated allergy unlikely, though not impossible.
SPT can be performed on infants, including very young babies, though wheal sizes tend to be smaller and interpretation requires specialist experience. Antihistamines must be stopped before SPT as they suppress the wheal response.
Specific IgE Blood Tests
A specific IgE blood test (also called RAST, or ImmunoCAP for the Thermo Fisher Scientific assay) measures the level of IgE antibodies to specific food proteins in the blood. Results are reported as a concentration (kU/L) and a class (0-6). Class 0-1 generally indicates low sensitisation; class 3 and above indicates higher sensitisation.
A higher specific IgE level is associated with a higher probability of clinical allergy, but the relationship is not linear and varies by food. For peanut, a specific IgE above 15 kU/L has a positive predictive value for clinical allergy of around 95%; below 2 kU/L, the positive predictive value drops to around 50%. For milk and egg, the predictive values at different thresholds have been established in research by George du Toit (King's College London) and colleagues.
Component-resolved diagnostics (CRD) – testing for specific protein components rather than whole food extracts – improves predictive accuracy for some foods. For peanut, Ara h 2 sensitisation is strongly associated with systemic reactions; Ara h 8 sensitisation is associated with pollen-food allergy syndrome, which causes mild oral symptoms only. CRD helps distinguish clinically significant sensitisation from cross-reactive, low-risk sensitisation.
Oral Food Challenge
The oral food challenge (OFC) is the gold standard for confirming or excluding food allergy. The child is given increasing amounts of the food in a controlled clinical setting, under medical supervision with resuscitation equipment available, and observed for reactions. A positive OFC (reaction occurs) confirms allergy; a negative OFC (no reaction) effectively rules it out for that dose.
OFCs are performed by allergy services and are time-intensive, requiring dedicated clinic time, trained staff, and typically a half or full day. They are recommended when: the clinical history and tests are inconsistent; when allergy is suspected but unconfirmed; when a child has been avoiding a food based on test results alone without a clinical reaction; and to assess whether a child has outgrown their allergy (particularly relevant for milk and egg, which many children outgrow).
The Limits of Testing in Practice
A common and harmful scenario is unnecessary food avoidance based on positive blood tests ordered without a clinical history of reaction. Children who have never reacted to a food may test positive because they have eaten the food rarely, because they have a related sensitisation (pollen cross-reactivity), or simply due to test imprecision. Restricting foods unnecessarily in childhood has nutritional consequences and may actually increase the risk of maintaining allergy rather than developing tolerance.
The BSACI (British Society for Allergy and Clinical Immunology) position is that allergy testing should be guided by clinical history and interpreted by a clinician with allergy expertise, not used as a screening tool in children without suggestive symptoms.
GP referral to a paediatric allergy service is appropriate for any child with a convincing history of immediate allergic reaction, anaphylaxis, or unexplained eczema or gastrointestinal symptoms potentially driven by food.
Oral Immunotherapy
Oral immunotherapy (OIT) for peanut allergy is now available through NHS England following approval of Palforzia, a licensed peanut OIT product. The PALISADE trial (Vickery et al., NEJM 2018) established that OIT can achieve desensitisation in the majority of peanut-allergic children aged 4-17, raising the reaction threshold rather than curing the allergy. OIT requires specialist allergy services and involves a slow up-dosing protocol with initial doses taken under medical supervision.
For egg and milk allergy, baked egg and baked milk challenges are part of standard management: many children who react to raw egg or milk can tolerate well-baked forms, and introducing baked milk and baked egg is associated with faster development of tolerance.
Key Takeaways
Food allergy affects approximately 5-8% of children in the UK, with peanut, tree nut, milk, egg, wheat, soy, fish, and shellfish accounting for the majority of reactions. Allergy testing – skin prick tests (SPT) and specific IgE blood tests (RAST or ImmunoCAP) – supports diagnosis but has significant limitations: a positive result indicates sensitisation (the immune system has made IgE to the food) but does not confirm clinical allergy or predict reaction severity. An oral food challenge (OFC) under medical supervision is the gold standard for confirming allergy. NICE guideline NG116 covers food allergy in children under 19.
