Which allergy test do I need: skin prick or blood test?

Maybe it's a rash after a meal, a sneeze that won't settle, or a child who wheezes whenever the cat is around. Or perhaps a home kit has come back flagging half the kitchen cupboard, and you've no idea what to believe.

The short answer: if you don't yet know what's causing the reaction, start with a consultation, and let the immunologist pick the test. If you already know which test you want, you can often book it directly.

If you'd rather skip the reading, allergy consultations with Dr Pereira run on Tuesday afternoons at our Harley Street clinic, and cost £250. Book an allergy consultation online or call 0207 499 1991.

When is an allergic reaction an emergency?

Call 999 or go to A&E if a reaction involves difficulty breathing, swelling of the tongue or throat, or collapse. If you have an adrenaline auto-injector, use it, then call 999. Those reactions need emergency care straight away, and a clinic appointment is for everything that can wait.

Should I see a GP or an allergy specialist?

If your hay fever is mild, comes with the season and settles with pharmacy treatment, a GP is a sensible first stop. They can take a history, look at what you've tried and decide whether a specialist would add anything. Our GP service is in the same building.

A consultant immunologist earns their fee when the picture is less tidy. People see Dr Pereira with a rash they can't pin to a food, wheeze that might be dust or might be a pet, swelling after an insect sting, a child who reacts at school, or an old blood result nobody has explained.

You don't need a GP referral to book Dr Scott Pereira, Consultant Immunologist, GMC 1524080. He sees adults and children, and allergy consultations and skin prick testing run on Tuesdays from 14:00, with the last bookable window ending at 17:30.

Which allergy test is right for me?

Allergy tests at Medical Express Clinic.

Why does history come before the test?

Allergy UK estimates that over 21 million adults in the UK have at least one allergy, so it's no surprise that testing has become big business. The catch is that tests are most useful when someone has already asked the right questions. NICE guideline CG116, on food allergy in under 19s, puts an allergy-focused clinical history first and chooses tests in that light. It also leaves out food intolerance, such as lactose intolerance.

Dr Pereira applies the same logic to adults. He starts with what's been happening, then decides whether skin prick testing, ALEX3 or a targeted panel will help. A consultation is £250. Visits usually last 30 to 60 minutes, and testing can be done on the same visit when it's suitable. Medical Express Clinic has practised at 117a Harley Street since 1984 and is CQC-registered.

How does skin prick testing work?

Small drops of allergen extract go on your forearm, and the skin is lightly pricked through each one. If you react, a small raised bump called a wheal usually shows within about 15 to 20 minutes, and Dr Pereira talks you through the results in the same appointment. The visit costs £450 and includes the consultation and up to eight allergens. If you suspect something specific, bring a sample.

Antihistamines dampen the skin's response, so stop all of them 72 hours (three days) before your test. Keep taking your other prescribed medicines as usual, including asthma inhalers and nasal sprays. Some other medicines can affect skin tests too, so list everything you take when you book. If you can't stop antihistamines, tell us, because a blood test may suit you better.

What is ALEX3 and when is it worth it?

ALEX3 is one blood sample that screens your sensitivity to 290+ allergens, including foods, pollens, animal dander, moulds, dust mites and insect venoms. It costs £413.75, and results typically arrive within five working days with a written report.

It suits a broad history that touches several parts of the body, or anyone who wants that wide coverage. Because blood tests work whatever tablets you take, it's also the easy choice if you can't pause hay-fever medicine, or for a child who finds a blood sample easier than a row of skin pricks. You can book ALEX3 as a blood test without a separate consultation when that's clinically appropriate, or book the consultation and let Dr Pereira decide.

What about targeted panels and food intolerance?

Targeted IgE blood panels look at one group of allergens when your history already points in a direction: inhalants, a food group, insects, antibiotics, eczema-related allergens, or a children's panel. They're arranged at the consultation, and results typically take two to five working days, depending on the panel.

These panels measure an immune (IgE) response, which is a different thing from intolerance. A food panel won't diagnose lactose intolerance or other non-immune food reactions, and the gluten sensitivity panel is a sensitivity profile rather than an intolerance diagnosis. If bloating or delayed gut symptoms are the whole story, mention it when you book. Suspected coeliac disease needs a tTG-IgA blood test while you're still eating gluten.

What does a positive allergy test mean?

This is the part most home kits leave out. A raised result shows sensitisation, which means your immune system has made IgE antibodies to that allergen. On its own, that's a finding rather than a diagnosis of clinical allergy. Plenty of people are sensitised to things they eat happily every day, and plenty have convincing symptoms with a negative test to the thing they blamed. That's why a printout shouldn't become a diet sheet, and why a severe reaction still matters even if a test comes back clear.

Dr Pereira reads each result against timing, which parts of the body reacted, whether it happens every time, your medicines, asthma and any previous anaphylaxis. Only then does he advise avoidance, a change of medicine, or reassurance that a result can be left alone.

Once a diagnosis is clear, treatment is agreed at a separate consultation. That might mean avoiding a confirmed allergen, changing a medicine, or allergen desensitisation where he judges it appropriate. Omlyclo, a biosimilar of omalizumab (original brand Xolair), is a separate in-clinic treatment when licensed criteria are met, with every dose given in clinic by Dr Pereira only. It's a treatment for a confirmed problem, so testing comes first.

What should I bring, and how do I prepare?

Bring a list of your current medicines, any previous allergy results, a referral letter if you have one, and photographs of a rash or swelling if that's part of the story.

• If you're having skin prick testing, stop antihistamines 72 hours beforehand, keep using inhalers and nasal sprays, and bring a sample of anything you suspect.
• If you can't pause your tablets, choose a blood test instead.
• If you're bringing a child, a parent or legal guardian attends, and Dr Pereira chooses the method that's suitable, safe and acceptable for them.
• If your symptoms sound more like intolerance than allergy, say so when you book.

Full details, including what each test covers, are on the private allergy testing page. Related reading: when a child needs a GP, NHS 111 or A&E and when to see a private GP.