Working out what an infant is reacting to is harder than the parenting forums make it look. Babies meet five new foods in a fortnight, the worst eczema flare of the week is on a Tuesday and you can't remember what you cooked on Sunday, and "the test was negative but he still vomits the milk" is a daily clinic conversation. The reliable approach is structured, not exhaustive — a careful history, the right test for the right kind of allergy, and a planned reintroduction. For a wider view, see our complete guide to child health.
Start With History — It Beats Testing
Before any blood is taken, the history does most of the diagnostic work. The questions worth answering specifically:
- When did symptoms start, and what changed in the days before? First formula introduced? Mum ate something unusual? New cat in the house?
- What's the timing from exposure to reaction? Within 30 minutes — hives, lip swelling, vomiting, sudden distress — is the IgE pattern. Hours to days — eczema flare, reflux, blood-streaked or mucousy stool, persistent loose stools, fussiness with feeds — is the non-IgE pattern.
- What has the baby been exposed to? For a breastfed baby, this includes mum's diet — small amounts of cow's milk protein, egg and peanut transfer through breast milk and can be enough to provoke a reaction in a sensitised infant.
- Is it reproducible? A reaction to one milk feed could be coincidence; the same response after three exposures, on three separate days, is a pattern.
- What's the home environment? Pets, damp, age of the mattress, recent renovation work — these all matter for eczema and respiratory symptoms.
- Family history. A first-degree relative with eczema, asthma, allergic rhinitis or food allergy roughly doubles a baby's risk.
Keep a 1–2 week diary before the GP appointment. Three columns is enough: time, what was eaten or encountered, what happened. Patterns that are obvious in the diary are almost invisible from memory.
Tests for IgE-Mediated Allergy
Skin prick test. A drop of standardised allergen extract goes on the forearm; a small lancet pricks through it. After 15 minutes you measure the wheal — the raised pale bump. Anything ≥3 mm larger than the negative control counts as positive. Quick, immediate, well tolerated even by small babies. Limited by widespread eczema (no clean skin to test on) and by recent antihistamine use, which suppresses the reaction.
Specific IgE blood test (ImmunoCAP, formerly RAST). Measures IgE antibodies against a specific food protein. Useful when skin testing isn't practical — severe eczema, dermatographism, antihistamine on board, very young infant. Result comes back as a number in kU/L, and food-specific cut-offs exist (for example, an egg-white IgE above about 6 kU/L in a child under 2 carries a high probability of clinical allergy on challenge).
Component-resolved testing breaks an allergen into its individual proteins. The peanut panel (Ara h 1, 2, 3, 6, 8, 9) is the best-known example: a positive Ara h 2 above about 0.35 kU/L predicts true peanut allergy reasonably well, while isolated Ara h 8 positivity usually means birch pollen cross-reactivity and a much milder picture.
The honest limits. Both tests show sensitisation — that the immune system has produced IgE — not necessarily allergy. A child can have a positive skin prick to egg and eat scrambled eggs every Sunday with no problem. About 8% of children with a positive food panel will tolerate the food on a properly conducted oral challenge. So the test is a piece of evidence, never the verdict.
Neither test is useful for non-IgE-mediated allergy. If the picture is delayed eczema flares, reflux, or persistent loose stools, the bloods will come back unhelpful and you still won't know.
Elimination and Reintroduction
For suspected non-IgE-mediated allergy — most often cow's milk protein allergy in young infants:
- Eliminate the suspected food for 2–4 weeks. For a formula-fed baby that means a hydrolysed formula such as Nutramigen or Aptamil Pepti, or, if symptoms continue, an amino-acid formula like Neocate or Alfamino. For a breastfed baby it means mum cuts out all cow's milk protein from her diet — and that's not just milk. It's cheese, yoghurt, butter, hidden milk in bread, biscuits and processed foods. Get a dietitian if you can; reading labels properly takes practice.
- Track symptoms. Eczema severity, stool pattern, sleep, feeding distress, weight gain. The diary continues. Most infants who are going to respond do so within 2–3 weeks.
- Plan the reintroduction. This is the step parents skip and clinicians insist on. Without reintroduction you don't know whether the food was actually the cause, and you may end up restricting unnecessarily for years. For mild non-IgE-mediated symptoms, reintroduction is at home, gradually, following something like the iMAP milk ladder. If symptoms come back, the link is real.
For anything that looked IgE-mediated — hives, swelling, breathing difficulty — reintroduction is not a kitchen experiment. It's an oral food challenge in a hospital allergy clinic with adrenaline and resus to hand.
Environmental Allergens — Often the Quiet Driver
Not everything is food. In babies with eczema and respiratory symptoms:
- House dust mite lives in mattresses, bedding, and soft toys. Allergy presents as worsening eczema after sleep, night cough, persistent runny nose. Hot-wash bedding (60°C), allergen-proof encasings, hard flooring, humidity under 50%.
- Cat dander (Fel d 1) is sticky, airborne, and lingers in a house for months after the cat has gone. Cat out of the bedroom; HEPA vacuum twice weekly.
- Mould spores grow wherever there's chronic damp. Worse in late summer/autumn. Treat the building, not the child — extractor fans, fix leaks, dehumidifier.
These can be tested too. Skin prick or specific IgE works the same way for environmental allergens.
When to Get Specialist Help
In the UK, the GP is the first stop. They can take a history, arrange initial allergy bloods, prescribe a hydrolysed or amino-acid formula on FP10, and refer onwards.
Reasonable thresholds for a paediatric allergy clinic:
- Any episode that looked like anaphylaxis (breathing difficulty, swelling beyond local hives, collapse).
- Two or more suspected food allergies.
- Allergy with poor weight gain or growth faltering.
- Diagnostic uncertainty after a careful workup.
- Severe eczema not controlled on standard treatment.
What to avoid: removing four or five foods on a hunch, especially in a breastfeeding mother. The nutritional and psychological cost of empirical multi-food elimination is real, the diagnostic yield is low, and you end up not knowing what the trigger was anyway. Pick the most likely culprit, eliminate it cleanly, give it 2–4 weeks, and reintroduce.
Key Takeaways
Before any test, a two-week food and symptom diary will tell you more than a blood panel — note exactly what was eaten, when, and how long it took for symptoms to appear. Reactions within minutes (hives, swelling, vomiting) are typically IgE-mediated; reactions hours to days later (eczema flare, reflux, blood-streaked stool, loose stools) are non-IgE-mediated. Skin prick wheals of ≥3 mm and specific IgE above the food-specific cut-off mean sensitisation, not allergy — they have to be read alongside history. Non-IgE allergy isn't picked up by either test; you diagnose it by elimination for 2–4 weeks and a planned reintroduction. Environmental triggers (dust mite, cat, mould) drive a lot of infant eczema and night cough, so don't put everything on food.
