Skin Prick Test

A skin prick test is a quick, controlled allergy test in which tiny amounts of suspected allergens (substances that can trigger an allergic reaction) are placed on the skin to see whether the immune system reacts to them.

During the test, a small drop of each allergen solution is placed on the forearm or back. A lancet (a small, fine needle) is then used to lightly prick the skin through each drop, allowing a microscopic amount to enter just below the surface. If the immune system recognises a substance as a threat, it releases histamine (a chemical that causes swelling and itching), producing a small raised bump called a wheal at that spot. The size of the wheal helps the doctor judge the degree of sensitivity to that particular allergen.

Doctors use the skin prick test to identify the specific substances responsible for a patient's allergic symptoms. It is most commonly requested when a patient has recurring symptoms that suggest an allergic cause but the trigger is not yet known.

• Allergic rhinitis (hay fever) — sneezing, a runny or blocked nose triggered by pollen, dust mites, or animal dander (tiny flakes of skin from pets)
• Allergic asthma — breathing difficulties linked to inhaled allergens
• Atopic dermatitis (eczema) — chronic itchy, inflamed skin that may worsen with certain allergens
• Urticaria (hives) — recurring episodes of itchy raised welts on the skin
• Food allergies — reactions to foods such as peanuts, tree nuts, shellfish, eggs, or milk
• Insect venom allergy — severe reactions following a bee or wasp sting
• Contact allergy screening — identifying substances that cause skin reactions on direct contact
• Pre-immunotherapy assessment — confirming which allergens to include before starting allergen immunotherapy (desensitisation treatment)

The skin prick test is not suitable for everyone. Doctors usually avoid it or choose an alternative in certain situations.

• Patients who have had a severe anaphylactic reaction (a life-threatening whole-body allergic reaction) very recently
• Active, widespread skin conditions such as severe eczema or psoriasis (a scaly skin disease) covering the test area
• Patients taking antihistamines (anti-allergy medicines), certain antidepressants, or other medicines that suppress the skin response — these may need to be paused under medical guidance before testing
• Pregnancy, in some cases — the doctor will weigh the benefit against any risk
• Young infants — skin sensitivity is different and results can be harder to interpret

The skin prick test is a diagnostic procedure that carries very little risk for most patients; the amounts of allergen used are extremely small and stay near the skin surface.

• Local itching, redness, and a small wheal (raised bump) at the prick sites — this is the expected reaction and usually fades within an hour
• Lingering local irritation or swelling at one or more test spots, lasting a few hours
• Dermographism (skin writing) — some people's skin raises a line wherever it is scratched, which can make results harder to read
• Delayed skin reaction appearing several hours after the test, which is uncommon
• Systemic allergic reaction — a mild generalised reaction such as itching away from the test site or mild hives; this is uncommon
• Severe anaphylactic reaction — very rare, but testing is always done in a clinical setting with trained staff and emergency medicine available for this reason

Preparation for a skin prick test is straightforward, but following the instructions carefully helps ensure the results are accurate.

Before the test, the most important step is stopping medicines that can dampen the skin's allergic response. Antihistamines are the main group to pause — your doctor will advise exactly how many days in advance this should happen, as it varies by medicine. Certain antidepressants known as tricyclics, some stomach medicines, and some heartburn medicines can also interfere. Always inform the allergy team about every medicine, supplement, and herbal remedy you take. There is no fasting requirement for this test. Patients are usually asked not to apply creams, lotions, or sunscreen to the test area (forearm or back) on the day of the appointment.

Before scheduling the test, the doctor may review relevant records or order a blood test called specific IgE (a protein the immune system makes in response to allergens) to help plan which allergens to test. No other special investigations are usually needed beforehand.

On the day of the test, the procedure itself typically follows these steps:

• The nurse or doctor cleans the test area — usually the inner forearm, or the back for a larger panel — with alcohol and lets it dry.
• Small marks are drawn on the skin with a pen to label where each allergen drop will be placed.
• A drop of each allergen solution, a negative control (saline solution that should cause no reaction), and a positive control (histamine solution that should always cause a small wheal) are placed at the marked spots.
• A lancet is used to make a light, shallow prick through each drop. This is not a deep needle insertion — it barely breaks the surface.
• The drops are gently blotted off after a short wait.
• The patient waits, usually around 15 to 20 minutes, while any reactions develop.
• The nurse or doctor measures the diameter (width) of any wheals that have appeared and records the results.
• The test area is cleaned and the marks are removed.

Recovery after a skin prick test is almost immediate for most people, and no hospital stay is needed. Patients are observed at the clinic for a short period after the test — usually at least 20 to 30 minutes — to make sure no delayed reaction develops before they leave. The doctor or nurse will explain the results before discharge and discuss what the findings mean for further management.

• Any itching or redness at the prick sites usually fades on its own within one to two hours; a cool compress can help soothe the skin.
• Avoid scratching the test sites, as this can prolong irritation.
• Prescribed medicines that were paused before the test can generally be restarted as directed by the doctor.
• No wound care is needed — the pricks are so shallow that they do not require dressings or antiseptic.
• There are no activity restrictions; most people return to normal daily activities straight away.
• A follow-up appointment is usually arranged so the allergist (allergy specialist) can review the full results, correlate them with your symptoms, and plan the next steps — which may include further blood tests, an elimination diet, or allergen immunotherapy.
• If any symptoms such as widespread hives, swelling of the lips or throat, difficulty breathing, or dizziness develop after leaving the clinic, seek emergency medical care immediately.