After a child has a food reaction — or even just suspected one — the natural next question is "what test will tell us?" The honest answer is more layered than parents are usually told. Allergy tests can be very useful, but they are diagnostic aids, not yes/no machines. A positive result in the wrong context leads to unnecessary food avoidance, which carries its own harms. Understanding what each test does, and what it can't do, prevents that.
Healthbooq covers allergy and immune conditions in children.For a comprehensive overview, see our complete guide to child health.
The Two Mechanisms — Why It Matters Which Type You're Testing For
Before any test makes sense, the type of suspected allergy needs to be clear.
IgE-mediated allergy. Immediate. Symptoms within minutes to 2 hours of eating. Hives, swelling, vomiting, breathing changes, or anaphylaxis. This is the type SPT and IgE blood tests detect.
Non-IgE-mediated allergy. Delayed, often 2 to 72 hours later. Symptoms are usually gastrointestinal (vomiting, diarrhoea, abdominal pain, blood-streaked stools, poor weight gain) or eczema flares. SPT and blood tests don't detect this. Diagnosis is by elimination and reintroduction trial under clinician guidance.
Mixed mechanisms are common — atopic dermatitis with food triggers, eosinophilic oesophagitis, FPIES (food protein-induced enterocolitis syndrome).
Ordering an SPT in a baby with delayed eczema flares and no immediate reactions will produce a useless or misleading answer. The first job of an allergy clinician is to figure out which mechanism is in play.
Skin Prick Test (SPT)
The mechanics:
- A drop of food extract is placed on the forearm
- The skin is pricked through the drop with a small lancet
- Results read at 15 minutes
- A wheal 3 mm or more beyond a negative control is considered positive
What it tells you:
- Whether IgE has been made to that food (sensitisation)
- Wheal size correlates roughly with allergy probability — bigger wheals mean higher probability of clinical reactivity, but it's not a reliable predictor of severity
The numbers:
- Sensitivity around 70 to 85% — most true allergies test positive
- Specificity around 30 to 70% — many positive tests are not clinically meaningful
- A negative SPT is more informative than a positive one — a negative test makes IgE allergy unlikely (though not impossible)
Practical points:
- Antihistamines suppress the wheal response — stop them for at least 3 days before
- Severe eczema in the test area can interfere — pick a different patch of skin
- Babies and young children can be tested but interpretation needs experience
- "Prick-prick" testing with the actual food (used for fresh fruits where extracts are unreliable) is sometimes done
Specific IgE Blood Test (ImmunoCAP / RAST)
The mechanics:
- A blood sample is sent to a lab
- IgE antibody to specific food proteins is measured
- Result reported as a concentration (kU/L) and a class (0 to 6)
- Results take days, not minutes
How to interpret:
- Higher levels mean higher probability of clinical allergy, but the relationship varies by food
- For peanut, an IgE above 15 kU/L has roughly 95% positive predictive value for clinical allergy; below 2 kU/L it's around 50%
- Cut-offs differ for milk, egg, and other foods — published probability curves (the work of George du Toit at King's College London and others) help interpret
- Class 0–1 is generally low; class 3+ is higher
When blood test is preferred over SPT:
- Severe eczema covering the test sites
- Antihistamines that can't be stopped
- A history of severe anaphylaxis where even a tiny SPT exposure carries risk
- Very young or very anxious children where the prick is impractical
Component-Resolved Diagnostics (CRD)
Newer testing breaks foods into individual proteins and measures IgE to each. The most clinically useful application is for peanut:
- Ara h 2 sensitisation is strongly associated with systemic reactions and clinical peanut allergy
- Ara h 8 sensitisation is associated with pollen-food allergy syndrome — usually only mild oral itching after fresh peanut, no systemic risk
- Whole-peanut IgE positive but Ara h 2 negative + Ara h 8 positive = the child probably has cross-reactivity to birch pollen, not true peanut allergy
CRD has changed practice in some clinics, particularly in distinguishing children who really need an EpiPen from those whose "positive" test is pollen cross-reactivity.
The Oral Food Challenge (OFC) — The Gold Standard
Tests measure sensitisation. The OFC measures actual reaction.
Mechanics:
- The child eats increasing amounts of the food in a clinical setting, under medical supervision, with resuscitation equipment available
- Doses given every 15 to 30 minutes
- Observation for several hours
- Half or full day in clinic
When it's needed:
- Test results don't match the clinical history
- A food has been avoided based on test results alone, with no real-world reaction
- Checking whether a child has outgrown an allergy (especially milk, egg, wheat, soy)
- Distinguishing "reacts to raw" from "tolerates baked" for milk and egg
A negative OFC (no reaction at full dose) effectively rules out IgE-mediated allergy at that dose. A positive OFC confirms it.
Most NHS allergy services run OFCs but waiting times vary.
What to Avoid: "IgG Food Intolerance Panels"
The private blood tests sold direct to consumers — typically £100 to £200 — measuring IgG antibodies to dozens or hundreds of foods are not validated, are not recommended by any UK or international allergy society (BSACI, EAACI, AAAAI), and routinely produce dramatic-looking lists of "food sensitivities."
What they're actually measuring is normal exposure — IgG antibodies to a food generally indicate that the person has eaten it. Restricting on the basis of these tests leads to nutritionally inadequate diets, food fear, and (critically) does not help with actual symptoms.
If a private clinic offers this test, it's not allergy testing. The same goes for hair analysis, bioresonance, kinesiology, "vega" testing, and similar — all unvalidated, all unhelpful.
Why Sensitisation Is Not the Same as Allergy
This is the single most important point in this article.
A child can have IgE to peanut, hazelnut, or wheat and eat those foods every day with no symptoms. The IgE tells you the immune system has noticed the food. It does not tell you that the food causes a reaction.
Why this matters in practice:
- "Just to be safe" panel testing of children who've never reacted produces lists of "positive" foods that the family then starts avoiding unnecessarily
- Restricting foods that the child actually tolerates increases the risk of losing tolerance
- Avoidance also has nutritional, social, and psychological costs
The BSACI position: allergy tests should be ordered to answer a specific clinical question raised by the child's history, not used as screening tests.
When to See an Allergy Clinician
GP referral to a paediatric allergy service is appropriate for:
- A convincing history of an immediate allergic reaction to a food
- Any episode of anaphylaxis
- Severe or worsening eczema not responding to standard treatment, where food triggers are suspected
- Persistent gastrointestinal symptoms (vomiting, blood-streaked stools, failure to thrive) suspected of being food-related
- Feeding difficulties suspected to be CMPA
- Suspected oral allergy syndrome (mouth itch with fresh fruit/veg, often pollen-related)
- Wanting to introduce peanut at home but eczema is severe or there's existing egg allergy
- Considering oral immunotherapy
NHS waiting times vary. If a child has had anaphylaxis, the GP can prescribe adrenaline autoinjectors at the first appointment without waiting for specialist clinic.
Treatment: Beyond Avoidance
For most food allergies, the management remains: avoid the food, carry adrenaline if at risk of anaphylaxis, treat reactions promptly. Two important newer options:
Oral Immunotherapy (OIT) for peanut allergy. Palforzia is licensed and available through NHS specialist allergy services for selected children aged 4 to 17 with confirmed peanut allergy. The PALISADE trial (NEJM 2018, Vickery et al.) showed OIT achieves desensitisation in around two-thirds of treated children — meaning their reaction threshold is raised, often by 10 to 100 times. It is not a cure. It involves slow dose escalation over months, with the first dose of each new level given under medical observation.
Baked egg and baked milk introduction is part of standard management for egg- and milk-allergic children. Many children who react to raw egg or fresh milk tolerate well-baked forms (cake, biscuits, muffins where the egg or milk is heated thoroughly through). Introducing baked forms is associated with faster development of full tolerance. This should be done under specialist guidance — not at home from a recipe blog.
Outgrowing Allergies
Many childhood allergies are outgrown:
- Cow's milk allergy — around 80% by age 5
- Egg allergy — around 70% by age 5
- Wheat and soy — most by school age
- Peanut and tree nuts — only around 20% are outgrown
- Fish and shellfish — usually persistent
Knowing whether your child has outgrown an allergy generally requires an oral food challenge or, in lower-risk cases, a careful at-home reintroduction under specialist guidance — not just a repeat blood test.
What to Bring to the Allergy Appointment
A few things that make a clinic visit much more useful:
- A timeline of any reactions: what was eaten, how much, how long after, what symptoms, how severe, what was given to treat, how long it took to resolve
- Photos of any visible reactions (hives, swelling, eczema flares) with timestamps
- A food diary for a week or two before the appointment if there are unclear chronic symptoms
- A list of all current treatments, including emollients and antihistamines
- The action plan the GP has given (if any)
Allergy clinic time is precious. Walking in with this information turns a 20-minute appointment into a useful one.
Key Takeaways
Food allergy affects roughly 5 to 8% of UK children. The two main tests — skin prick (SPT) and specific IgE blood test (ImmunoCAP/RAST) — measure sensitisation, which is not the same as clinical allergy. A positive test in a child who has never reacted to a food often does not mean they're allergic. The clinical history is the most important diagnostic tool. The gold standard is the oral food challenge under specialist supervision. Avoid private 'IgG food intolerance panels' — they are not validated and lead to unnecessary food avoidance. NICE NG116 covers food allergy in under-19s; BSACI provides UK clinical guidance. The newer Palforzia peanut immunotherapy is now available through NHS specialist allergy services for some children with peanut allergy.
