What happens at a skin prick test and how long does it take?

A skin prick test takes place on the inside of your forearm: a small drop of each allergen goes on the skin, a tiny prick lets it in, and you wait about 15 to 20 minutes while any reaction appears. At Medical Express Clinic the whole visit usually takes 30 to 60 minutes, and you leave knowing the result.

The test costs £450 and includes your consultation with Dr Scott Pereira, our Consultant Immunologist, and up to eight allergens. Clinics run on Tuesdays from 14:00 to 17:30 at 117a Harley Street. You can book a skin prick test here, and the steps below show what the afternoon looks like.

Before you come: stop antihistamine tablets 72 hours ahead

Antihistamines damp down the skin reaction the test looks for, so tablets need to stop three days before your appointment. Bring a list of everything else you take, because a few other medicines can also blunt the reaction, and Dr Pereira will tell you when you book if anything needs to pause. Asthma inhalers carry on as normal. If you can't pause your tablets, a blood test gives a reliable answer whatever you have taken, and we cover that choice at the end.

Step 1: the conversation

Dr Pereira starts with your story rather than the needle. What you ate or touched, how quickly it came on, what it looked like, how often it happens. That history decides which allergens go on your arm.

Step 2: the allergens go on your forearm

You roll up a sleeve. Drops of the chosen allergens are placed in a row on the skin of your forearm, each one labelled. The £450 price covers up to eight; if your history points to more, extra allergens are £10 each, up to five more.

Step 3: the pricks, and the two checks

Through each drop, a small lancet makes a light prick in the surface of the skin. Most people feel a scratch rather than pain.

Two further drops sit alongside your allergens. One contains histamine, which makes almost every healthy skin react, and one contains saline, which should cause nothing at all. They are the checks. If the histamine drop raises a bump and the saline drop stays flat, the skin is behaving normally and the allergen results can be trusted.

Step 4: the 15 to 20 minute wait

You sit and wait while the skin does its work. A positive allergen shows as a raised, itchy bump called a wheal, often with a patch of redness around it.

Step 5: reading the wheals with Dr Pereira

Dr Pereira measures each wheal against the two checks and explains what he sees there and then, with your history in front of him. A reaction on the skin shows that your immune system has made antibodies to that allergen. Together with what happens when you actually meet it, that tells him whether it is the cause.

Step 6: your plan

Before you leave, you have a plan: what to avoid, what to keep eating, and what to do if you react again. If the picture needs more detail, Dr Pereira can suggest a wider blood test. ALEX3 checks more than 290 allergens from one sample and costs £413.75, with results in about five working days. Our blood tests page lists the targeted panels, from two to five working days. For help choosing between the methods, read our guide to skin prick or blood test.

If hives or an unexplained rash brought you here, the hives guide explains what the skin is doing. A general appointment with one of our GPs is £150.

If you ever have difficulty breathing or swelling of the throat after eating or touching something, call 999.

Book a skin prick test

At Medical Express Clinic, skin prick testing runs every Tuesday afternoon at 117a Harley Street, £450 including your consultation. Book your appointment or call 0207 499 1991.