Skin Prick Test

How a skin prick test works

Immediate allergy — the type that causes hay fever, allergic asthma, and reactions within minutes of eating a particular food — is driven by an antibody called IgE. In someone who is sensitised, IgE specific to a particular allergen sits bound to mast cells in the skin and elsewhere.

A skin prick test takes advantage of that. A tiny amount of allergen is introduced into the surface of the skin. If specific IgE is present, it binds the allergen and triggers the mast cells to release histamine and other mediators. Locally, this produces a small raised bump — a weal — surrounded by redness, the flare. The reaction peaks at about fifteen minutes.

In effect, the test reproduces a miniature, controlled version of an allergic reaction in a few square millimetres of skin, where it can be measured safely.

What the test can and cannot tell you

This is the part most worth reading, because it is where allergy testing is most often misunderstood.

A positive result is not the same as an allergy

A positive weal shows that specific IgE is present — that you are sensitised. It does not establish that you react clinically. Depending on the allergen, roughly half of positive results occur in people who are not actually allergic to that substance. This is why a skin prick test is read alongside your history rather than on its own, and why indiscriminate testing tends to create confusion rather than answers.

It is not a general screen

Testing a long panel of allergens in someone with no suggestive history is not recommended. It reliably produces incidental positives, which then require explaining away, and can lead to unnecessary avoidance — particularly of foods.

Some conditions it does not answer

Skin prick testing is generally unhelpful for food intolerances, reactions to food additives, non-allergic drug reactions, and symptoms caused by respiratory irritants. It also has limited value in chronic urticaria and angioedema when the history gives no allergic pointer, and in atopic dermatitis its interpretation requires particular care.

What to expect, step by step

Before

The most important preparation is stopping antihistamines in advance, since they suppress the skin's ability to react and will invalidate the test — commonly around 72 hours for shorter-acting preparations, though this varies. Tell us about everything you take, including over-the-counter remedies, antidepressants, and any creams applied to the forearms.

Do not stop prescribed medication without advice. Some medicines — beta blockers and ACE inhibitors in particular — are not stopped for testing but do need to be known about beforehand.

During

1. Testing is usually done on the inner forearm. The back may be used where many allergens are being tested, or in small children.
2. The skin is marked so each allergen can be identified, with the marks spaced apart to prevent reactions merging.
3. A drop of each allergen solution is placed beside its mark, along with the positive and negative controls.
4. A fresh sterile lancet is used to make a superficial prick through each drop — a new lancet for every allergen, so nothing is cross-contaminated.
5. Excess solution is dabbed away, and the reactions are left to develop.

Reading the result

Reactions are measured at about 15 to 20 minutes. A positive response appears as a raised weal, often resembling a mosquito bite, with surrounding redness. The size is recorded and compared against the controls.

Afterwards

Any itching usually settles within an hour or two. You will discuss what the results mean in the context of your symptoms, and what follows — which may be avoidance advice, treatment, further investigation, or reassurance that a suspected trigger is not the culprit.

Is it safe?

Skin prick testing is widely practised precisely because it carries a very low risk of serious side effects, provides good-quality information when performed and interpreted properly, and is inexpensive.

The usual effects are local: itching, a raised bump, and redness at positive sites, settling within a few hours. Generalised reactions are rare. Because they are nonetheless possible, testing is carried out by appropriately trained staff, with emergency treatment for anaphylaxis immediately available.

Additional caution applies in pregnancy, in unstable asthma, and for people taking beta blockers or ACE inhibitors. Please mention any of these when booking so the appropriate approach can be planned.

Testing can be performed at any age. In children under two it is generally undertaken by a specialist, and skin reactivity tends to decline in older adults — both are taken into account when interpreting the result.

When a skin prick test may be appropriate

Consider discussing allergy testing if:

• You have persistent sneezing, nasal blockage or itchy eyes and want to know the trigger
• You have reacted within minutes of eating a particular food
• You have had an episode of acute urticaria or facial swelling with a suspected trigger
• You have had a significant reaction to a bee or wasp sting
• Latex allergy is suspected
• You have asthma and an allergic component is suspected
• You want to confirm — or rule out — a specific suspicion before making changes to your home or diet