Food allergy testing for babies and toddlers: what tests can tell you

The story comes first

The most useful “test” for a food allergy is a careful account of what happened. NICE, the UK guideline body, says allergy tests shouldn’t be done without first taking an allergy-focused history, and results should be read in the light of that history. ASCIA, the Australian allergy society, says the same: test results and history must be reviewed together.

So before any test, your pediatrician, GP or allergist will ask:

• what your child ate, and how much
• how soon symptoms started, what they looked like and how long they lasted
• whether it has happened before with the same food
• what treatment was needed (AAP)

Write it down while it’s fresh, and take photos of any rash. A food and symptom diary helps too (NHS).

The two main tests

Both look for IgE, the antibody behind the fast type of food allergy.

• Skin prick test. A drop of liquid with a tiny amount of the food goes on the arm or back, and the skin is pricked lightly through it. A raised, itchy bump means the test is positive. Results come in about 20 minutes (ACAAI). ASCIA says it’s usually well tolerated, even by small children. NICE says skin prick tests should only be done where staff can treat anaphylaxis.
• Specific IgE blood test. This measures IgE to the foods tested. ACAAI says it’s a bit less exact than skin testing, and results take about a week. It can help when skin testing isn’t practical, for example with severe eczema (ASCIA).

Your doctor chooses between them based on your child’s history and what is safe and practical (NICE).

A positive test isn’t the same as an allergy

This is the part that surprises most parents. A positive skin or blood test shows sensitization: the body has made IgE to that food. It doesn’t prove your child will react when they eat it.

• The 2010 US guidelines from NIAID say neither test alone can diagnose a food allergy, because many people have IgE to a food without having an allergy to it. Using the tests on their own can lead to over-diagnosis.
• ACAAI says a positive result doesn’t necessarily mean an allergy, though a negative result is useful for ruling one out.
• The AAP adds that a child can test positive and have no symptoms when they eat the food, or test negative and still have an allergy.

That’s why a doctor reads the result alongside the story. A child who already eats egg every week without problems doesn’t need to stop because a test says “positive”: tell your doctor and ask before changing anything. If your child has had a reaction to a food, don’t offer it again until your doctor says so.

Don’t test before trying new foods, unless your doctor advises it

Most babies don’t need any allergy test before they start allergens like peanut and egg.

• The 2021 US and Canadian allergy consensus says screening before introduction is not required, though some families may prefer it.
• ACAAI says testing can’t be used to screen for food allergy: you should only be tested for a food you’ve had a reaction to.
• Broad “panels” that test lots of foods at once aren’t recommended. NIAID’s 2017 peanut guidelines advise against them because of their poor positive predictive value, which can lead to unnecessary food restrictions.

The main exception is peanut for high-risk babies. If your baby has severe eczema or an egg allergy, NIAID’s 2017 guidelines suggest their doctor strongly consider a peanut IgE blood test or skin prick test, and sometimes a supervised first feed, before peanut goes in at home. Ask early, because testing can take time.

If the test is positive, don’t give peanut at home. NIAID’s guidelines send these babies to a specialist, who may offer a supervised first feed or food challenge in the clinic. A baby can test positive without being allergic, and if the supervised feed goes well, peanut can then become part of their diet (NIAID). See peanut introduction and eczema and starting solids.

The oral food challenge

When the story and the tests don’t give a clear answer, an allergist may suggest an oral food challenge. ACAAI calls it the most accurate way to diagnose a food allergy. Your child eats tiny, increasing amounts of the food over a period of time, then is watched for a few hours.

It’s done only by experienced allergists, in a clinic or food challenge center, with emergency medicine and equipment on hand (ACAAI, ASCIA). Never try a “challenge” at home with a food your child has reacted to.

A passed challenge means your child can eat the food. A challenge can also show that a food avoided only because of a positive test is fine after all.

Delayed allergies: no skin or blood test

Skin prick and IgE blood tests look for the fast (IgE) type. They don’t diagnose the delayed (non-IgE) type.

• Delayed cow’s milk allergy is usually diagnosed by taking milk out for a short time, then bringing it back under your doctor’s guidance. NICE suggests 2 to 6 weeks, with a dietitian’s advice. See cow’s milk allergy.
• FPIES is diagnosed from the pattern of symptoms, sometimes with a supervised challenge. See FPIES.

Tests to skip

These tests are sold to parents, but allergy experts say they don’t diagnose food allergy:

• IgG food tests, often sold as “food sensitivity” panels. NICE says not to use them, and the AAAAI says they’ve never been shown to work.
• Kinesiology, hair analysis and Vega (electrodermal) tests. NICE says not to use them, and NIAID lists them with other unproven tests.
• Cytotoxic and Alcat tests, pulse testing and reflexology (ASCIA).
• Total IgE and intradermal (under the skin) food tests. NIAID advises against both for diagnosing food allergy.

ASCIA warns that unproven tests can lead to a wrong diagnosis and to costly, sometimes dangerous, diet restrictions. A baby on an unnecessarily limited diet can miss nutrients they need. For home intolerance kits, see food allergy vs intolerance.

If your baby reacts while you wait

Don’t wait for a test to act on a reaction. If it’s one mild sign, such as a few hives or mild puffiness around the eyes, stop the food and call your pediatrician or GP before offering it again.

Signs of anaphylaxis are trouble breathing, wheezing or a cough that won’t stop, swelling of the tongue or throat, lips or mouth that suddenly swell, a hoarse cry, turning pale or floppy, becoming very sleepy or unresponsive, or signs in more than one part of the body at once. If you see any of these and your baby has an epinephrine (adrenaline) auto-injector, use it right away, then call for an ambulance. Call 911.Call 999.Call your local emergency number (such as 112 in Europe or 000 in Australia). If you don’t have an auto-injector, call straight away. If you’re alone with your baby, use the auto-injector first if they have one, then call on speaker. Don’t leave your baby to find a phone. Keep your baby lying down, never standing or walking (on their side if they’re vomiting, head and shoulders a little raised if breathing is hard). See signs of anaphylaxis.

Where US and UK advice differs

• Testing before peanut. NIAID’s 2017 guidelines suggest testing for babies with severe eczema or egg allergy; the 2021 US and Canadian consensus says testing first isn’t required. The NHS page on allergies in babies doesn’t mention testing first; it asks you to talk to your GP or health visitor before introducing allergens if your baby has eczema or a food allergy, or there’s a family history of food allergy, eczema, asthma or hay fever.
• Who does the test. In the US, your pediatrician may order a blood test or refer you to an allergist. In the UK, a GP may refer your child to a specialist for skin prick or blood tests (NHS).

For first tastes of allergens, see how to introduce allergens.