Allergy testing is a quick, straightforward office visit that usually takes about an hour from start to finish. The most common form, the skin prick test, involves tiny pricks on your forearm or back, and you’ll have results within 15 to 20 minutes. Knowing what happens before, during, and after the appointment can help you show up prepared and leave with clear answers.
How to Prepare Before Your Appointment
The most important step is stopping certain medications ahead of time. Antihistamines are the main concern because they suppress the immune response your doctor is trying to measure, which can produce falsely negative results. You’ll typically need to stop them seven days before your test. This includes common over-the-counter options like Benadryl, Zyrtec, Claritin, Allegra, and Xyzal, as well as prescription antihistamines like hydroxyzine.
Cold, cough, and flu medicines are easy to overlook. Most contain hidden antihistamines, so check the active ingredients or call your allergist’s office if you’re unsure. Allergy nasal sprays that contain antihistamines also need to be paused, though steroid nasal sprays (like fluticasone) are generally fine to continue. If you take any medication regularly, mention it when you schedule the appointment so your doctor can tell you exactly what to stop and what to keep taking.
Beyond medications, there’s not much else to do. Eat normally, stay hydrated, and wear a short-sleeved shirt if the test will be done on your forearms.
The Skin Prick Test
This is the standard allergy test and what most people picture when they think of allergy testing. A nurse or technician will clean the test area (usually the inside of your forearm or your upper back) with alcohol, then draw small marks on the skin in a grid pattern. Each mark corresponds to a different allergen. A tiny drop of allergen extract is placed next to each mark, and a small plastic lancet is used to lightly prick the extract into the surface of your skin. The lancet barely penetrates; it doesn’t draw blood or go deep enough to cause real pain.
Most people describe the sensation as mild discomfort, similar to a light scratch. The pricks happen quickly. If you’re being tested for 30 or 40 allergens, the whole pricking process still only takes a few minutes. Then you wait. You’ll sit still for about 15 to 20 minutes while your skin reacts. During that waiting period, any allergens you’re sensitive to will produce a small, raised, itchy bump that looks and feels like a mosquito bite. These bumps are called wheals.
Two control pricks are always included. One uses histamine, which should produce a bump in everyone (confirming your skin can react). The other uses a saline solution, which shouldn’t produce any reaction. These controls help your doctor verify that the results are reliable.
Intradermal Testing
If the skin prick test comes back negative but your doctor still suspects an allergy based on your symptoms, they may follow up with an intradermal test. This involves injecting a tiny amount of allergen solution just beneath the surface of the skin using a thin needle, similar to a tuberculosis skin test. It’s more sensitive than the prick method and produces more consistent results, which makes it useful for detecting allergies that the surface prick might miss. It’s commonly used for drug allergies and insect venom allergies. The injection stings slightly more than a prick, but the discomfort is still brief.
Patch Testing for Skin Reactions
Patch testing is a completely different process, used when your doctor suspects contact dermatitis (a rash triggered by something touching your skin, like nickel, fragrances, or preservatives). Instead of pricks or needles, small adhesive patches containing suspect allergens are applied to your back. You then go home with the patches on.
The timeline spans five days. After 48 hours, you return to have the patches removed for a first reading. Then you come back again 48 hours later for a final reading. During the entire five-day window, you cannot shower or get the test site wet, and you need to avoid heavy exercise or anything that causes excessive sweating. You also can’t apply steroid creams to your back or expose the area to direct sunlight until after the final reading. The patches can feel mildly itchy or irritating, but that discomfort goes away once they’re removed.
Blood Tests as an Alternative
Some people aren’t good candidates for skin testing. If you have a skin condition like severe eczema that covers large areas, or if you can’t safely stop your antihistamines, your doctor may order a blood test instead. A blood draw measures the level of allergy-related antibodies your immune system produces in response to specific allergens. The sample goes to a lab, so you won’t get same-day results; it typically takes a few days to a week. Blood testing is also sometimes preferred for young children or anyone at higher risk of a severe reaction during skin testing.
Reading Your Results
For skin prick tests, the doctor measures each wheal that develops. A bump with a diameter of 3 millimeters or more (roughly the size of a pencil eraser) is considered a positive result. Larger bumps generally suggest stronger sensitivity, though size alone doesn’t tell the whole story. A positive skin test means your immune system recognizes that allergen and mounts a response, but it doesn’t always mean that allergen causes your real-world symptoms.
Research looking at the connection between positive skin tests and actual symptoms found that the correlation varies by allergen. For grass pollen and dust mites, 87 to 89 percent of positive skin reactions matched up with symptoms patients reported during real exposure. For other allergens, like cockroach, the match rate dropped to around 40 percent. This is why your doctor will interpret the results alongside your symptom history rather than treating the skin test as a standalone diagnosis. You might test positive for something that has never bothered you, or your strongest reaction might confirm exactly what you suspected.
How It Feels During and After
The pricks themselves are the easy part. The waiting period is what most people find annoying: positive reactions get itchy, red, and swollen, and you’re asked not to scratch. Some people also experience mild stuffiness, watery eyes, or a runny nose during the test if they react to airborne allergens. These symptoms are temporary.
After the test, the nurse will clean the marks from your skin. The itchy bumps from positive reactions typically fade within an hour or two, though some people notice mild redness or slight swelling at the test sites for the rest of the day. Applying a cold compress or a mild hydrocortisone cream to the area can help if the itching lingers.
Safety During Testing
Allergy testing is done in a medical office for a reason: on rare occasions, a person can have a more significant allergic reaction. A large study tracking over 30,000 skin prick test visits found that only 0.02 percent (20 out of every 100,000 visits) required epinephrine for a systemic reaction. That makes serious reactions genuinely uncommon, but allergists keep emergency medications on hand as a standard precaution. This is also why you’re asked to stay in the office for the full observation period rather than having the pricks done and heading out.
What Happens After Testing
Once your results are in, your allergist will walk you through which allergens triggered reactions and how those results connect to your symptoms. From there, the conversation shifts to a management plan. That might include avoiding specific triggers, starting allergy medications tailored to your results, or discussing immunotherapy (allergy shots or sublingual tablets) if your allergies are significant enough to warrant long-term treatment. The test itself is just the diagnostic step. The real value comes from what you and your doctor decide to do with the information.