Do I need a food allergy test?
Only if a food makes you react fast, every time. The IgE blood test, once called RAST, looks for fast allergies. But a positive result alone doesn’t prove you’re allergic. One word explains why.
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Do I need a food allergy test?
Only if your reaction fits a fast allergy. NHS lab advice says test only when all four STAR signs are there. That means the right symptoms, the right timing, a likely food, and it happens every time.
Fast means a rash that comes typically within an hour of eating. A rash that lasts for days isn’t this kind of allergy.
| Your reaction | Does it help? | What usually happens |
|---|---|---|
| Hives, swelling, wheeze or being sick within an hour of one food, every time | It may | Skin prick test or IgE blood test for that food. |
| Symptoms that start hours or days later | No | There’s no blood test for this kind of reaction. A trial without the food may be used. |
| Only bloating, wind or tummy pain | No | An IgE test isn’t advised for gut symptoms alone. The NHS lists these as intolerance signs. |
| Hives that come and go with no clear food | No | Allergy testing isn’t advised for hives with no trigger. |
Sources: MFT NHS lab (RCPath); Black Country Pathology; NHS; NICE CG116
Fit the first row? See a GP and say which food, how fast you reacted and what happened.
What is a RAST or IgE allergy blood test?
IgE is an antibody, a protein your immune system makes to fight things it sees as a threat. A specific IgE test measures how much you’ve made against one named food. RAST is just the old name for it.
The lab tests only the foods your doctor asks for. The same test is used for things like pollen, pets and dust mites. One NHS lab offers over 100 allergens to choose from.
The STAR check before an IgE test
- 1
Symptoms
Hives, swelling, wheeze, being sick, diarrhoea or low blood pressure.
- 2
Timing
The reaction started within 1 hour of the food.
- 3
Allergen
One likely food can explain it.
- 4
Reproducible
It happens every time you eat that food.
Source: MFT NHS lab (RCPath)
So a test for 20 foods “just to see” isn’t advised. NHS labs say there’s no role for screening, and mixed food panels don’t show which food is the problem.
Ate the food again since without trouble? That food doesn’t need testing. Leave it off your list.
What do my allergy blood test results mean?
Below 0.35 is negative at South Tees NHS lab.
| Grade | Result (kUA/L) | Lab label |
|---|---|---|
| 0 | Below 0.35 | Negative |
| 1 | 0.35 to 0.7 | Weak positive |
| 2 and 3 | 0.7 to 17.5 | Positive |
| 4 to 6 | Above 17.5 | Strong positive |
Sources: South Tees NHS; MFT NHS lab
A positive grade means you’re sensitised. That word is the catch.
Sensitised means your body has made IgE to the food. NHS labs say this can’t confirm allergy without a history that fits.
Think of a smoke alarm. A beep makes a fire more likely, but burnt toast sets it off too. A positive result suggests a higher chance of a reaction, but doesn’t prove the food caused yours.
So if you eat a food happily but test positive, ask before you cut it out. Testing to find what triggers eczema isn’t advised. It can give false positives that lead to cutting out foods for no reason.
A negative result doesn’t always rule allergy out either. If you reacted badly, tell your GP even after a negative test.
Are home food intolerance tests worth it?
The NHS says some you can buy are not recommended. There’s limited evidence they’re accurate, and they may tell you to avoid lots of foods, which can be harmful.
| Test | What it checks | UK advice |
|---|---|---|
| IgE blood test | Antibodies to a named food | Used by GPs and clinics for fast reactions. |
| Skin prick test | A drop of food on pricked skin | Used by specialists for fast reactions. |
| IgG food test | A different antibody | NICE: don’t use to diagnose food allergy in under 19s. |
| Vega, kinesiology, hair analysis | What NICE calls alternative tests | NICE: don’t use to diagnose food allergy in under 19s. |
Sources: NHS; NICE CG116; NHS: intolerance
An intolerance isn’t an allergy. It isn’t caused by your immune system, and symptoms like bloating usually start a few hours after the food.
The NHS route is a diet trial, blood tests, or a breath test for lactose. For gut symptoms, see blood tests for bloating, the IBS blood test and the coeliac blood test.
How do I get a food allergy test?
Start with your GP. They may arrange tests or refer you to an allergy clinic.
For children and young people, NICE says to read results alongside a careful history of the reaction. Your notes make that history.
Before your allergy appointment
- Keep a food and symptoms diary.
- Write down how soon after eating each reaction started.
- List the foods you’ve eaten since the reaction with no problems.
- Don’t cut foods out of your diet without speaking to a GP.
- If you’ve been prescribed adrenaline, carry 2 auto-injectors or nasal sprays at all times.
Sources: NHS; MFT NHS lab (RCPath)
Take the diary with you. It helps the doctor pick the one or two foods worth testing.
Never stop or change a prescribed medicine without your prescriber.
When to speak to a GP
Book an appointment if:
- You think you or your child may have a food allergy.
- You or your child have symptoms of an intolerance that keep coming back.
- A test says you’re allergic to a food you eat without trouble. Ask before cutting it out.
Call 999 if any of these happen:
- Your lips, mouth, throat or tongue suddenly swell.
- You’re breathing very fast or struggling to breathe, very wheezy, choking or gasping.
- Your throat feels tight or you’re struggling to swallow.
- Your skin, tongue or lips turn blue, grey or pale. On black or brown skin, check the palms or soles.
- You suddenly become very confused, drowsy or dizzy.
- Someone faints and can’t be woken, or a child is limp, floppy or not responding like normal.
Have an adrenaline auto-injector or nasal spray? Use it straight away, then call 999, even if you feel better. Don’t drive yourself to A&E.