Allergy testing is a straightforward office visit that typically takes about an hour, with most of that time spent waiting for your skin to react. The most common version, the skin prick test, uses tiny lancets that barely break the surface of your skin to introduce small amounts of allergen extracts, then your doctor watches for raised bumps that signal a reaction. The process isn’t painful, though it can get itchy.
How to Prepare Before Your Appointment
The most important preparation step is stopping certain medications ahead of time, because antihistamines and some other drugs suppress the very immune response the test is trying to measure. Over-the-counter allergy medications like cetirizine, loratadine, and diphenhydramine (found in many cold, cough, and flu products) need to be stopped several days before testing. Your allergist’s office will give you a specific timeline, but plan on at least three to seven days without these medications.
Certain antidepressants also interfere with results and should be stopped about a week before testing. These include older tricyclic antidepressants like amitriptyline, nortriptyline, and doxepin, along with some newer medications like mirtazapine and trazodone. Don’t stop any prescribed medication without checking with the doctor who prescribed it first. Herbal supplements like nettle and quercetin, both popular as natural allergy remedies, can also blunt test results and should be paused beforehand.
Beyond medications, there’s not much else to do. Wear a short-sleeved shirt or something loose, since the test is usually done on your inner forearm or back.
What Happens During a Skin Prick Test
The skin prick test is the standard first step for identifying allergies to things like pollen, pet dander, dust mites, mold, and certain foods. It can screen for up to 50 allergens in a single session.
A nurse or technician will clean the test area with alcohol, then draw small marks on your 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 top layer of skin. A fresh lancet is used for each allergen, so there’s no cross-contamination. You won’t bleed. The sensation is more like a light scratch than a needle stick.
Two control substances are always included. One is histamine, which should produce a small bump in everyone (confirming your skin is capable of reacting). The other is a saline or glycerin solution that shouldn’t cause any reaction (confirming your skin isn’t overly sensitive to begin with). These controls help your doctor interpret the results accurately.
After the pricks are done, you wait about 15 to 20 minutes. This is the least comfortable part. If you’re allergic to any of the tested substances, those spots will start to swell into raised, itchy bumps called wheals, surrounded by a red flare. The itching can range from mild to genuinely annoying, but you need to avoid scratching to keep the results readable. A bump that measures at least 3 millimeters larger than the negative control is considered a positive result.
Intradermal Testing
If the skin prick test comes back negative for a substance your doctor still suspects is a problem, the next step is often an intradermal test. Instead of pricking the surface, a very small amount of allergen extract is injected just beneath the top layer of skin using a thin needle, similar to a tuberculosis test. This creates a small bubble at the injection site.
Intradermal testing is more sensitive than the skin prick method and produces more consistent results. It’s commonly used for drug allergies and insect venom allergies, where missing a true allergy could be dangerous. A wheal of at least 7 millimeters, and at least 2 millimeters wider than the control, counts as positive. The wait time and general experience are similar to the prick test, though you may feel slightly more discomfort from the injection itself.
Patch Testing for Contact Allergies
Patch testing is a completely different process, used when your doctor suspects contact dermatitis from substances like nickel, fragrances, preservatives, or latex. It requires multiple office visits spread over about five days.
On the first visit, small panels containing suspected allergens are taped to your back. You go home with the patches on and return 48 hours later (day 3) for the first reading, when the patches are removed and your doctor checks for any skin reactions underneath. You then return again 48 hours after that (day 5) for a final reading, since some contact allergies take longer to show up.
During the 48 hours the patches are on, you can’t shower, exercise heavily, or do anything that would get your back wet or cause heavy sweating. After removal, you need to avoid rubbing the area with a towel or scratching, and the markings on your back must stay intact so the doctor can match reactions to specific allergens. If new reactions appear between day 5 and day 7, you should contact your allergist to have them evaluated.
When Blood Tests Are Used Instead
Sometimes skin testing isn’t practical, and your doctor will order a blood test that measures allergen-specific antibodies instead. Blood testing is preferred when you have widespread eczema or a skin condition called dermographism (where even light pressure causes raised marks), since either of these can produce false positives on skin tests. It’s also the better option if you can’t safely stop antihistamines, if you have a history of severe anaphylaxis to a suspected allergen, or if skin prick testing came back negative but your symptoms strongly suggest an allergy.
A blood test involves a standard blood draw from your arm. Results take a few days to come back from the lab rather than being available in the same visit. Blood tests are somewhat less sensitive than skin testing, so a negative blood result doesn’t always rule out an allergy completely.
How Results Are Interpreted
A positive skin test means your body produces an immune response to that allergen, but it doesn’t automatically mean that allergen is causing your symptoms. Your allergist interprets results in the context of your medical history, the timing of your symptoms, and what you’re exposed to in daily life. It’s possible to test positive for something that doesn’t bother you clinically, and it’s possible to have real allergies that don’t show up on a test.
For skin prick tests, the size of the wheal gives a rough sense of sensitivity, but bigger doesn’t always mean worse symptoms. A 3-millimeter wheal and a 10-millimeter wheal both count as positive. Your allergist will walk you through which results are clinically significant for your situation and discuss next steps, which might include allergen avoidance strategies, medications, or immunotherapy (allergy shots or sublingual tablets).
Safety and Side Effects
Skin prick testing is very safe. The most common side effect is localized itching and redness at the test sites, which typically fades within a few hours. Applying a cool cloth or over-the-counter hydrocortisone cream after the test can help with lingering discomfort.
Serious systemic reactions are rare. In a study of pediatric patients tested with food allergens, systemic reactions occurred in only 0.16% of patients, and anaphylaxis happened in just 0.05%. The risk was higher in patients who had a prior history of anaphylaxis and in those who underwent a more intensive form of prick testing. This is one reason allergy testing is always done in a medical setting with emergency equipment on hand. Standard skin prick testing with environmental allergens (pollen, dust, mold) carries an even lower risk profile than food allergen testing.
What the Visit Looks Like Start to Finish
For a standard skin prick test, expect to be in the office for about 60 to 90 minutes total. The first portion involves reviewing your symptoms and medical history with the allergist. The actual pricking takes only a few minutes, followed by the 15-to-20-minute waiting period. After the results are read, you’ll spend time discussing findings and a treatment plan.
For patch testing, you’ll need three separate visits over five days, each lasting 15 to 30 minutes. Intradermal testing adds perhaps 20 to 30 minutes to a visit if it’s done as a follow-up to skin prick results on the same day. Some offices schedule it as a separate appointment.
After a skin prick or intradermal test, you can drive yourself home and resume normal activities immediately. The small marks on your skin will fade over a day or two. For patch testing, normal activity resumes after the final reading on day 5, though you should watch for delayed reactions through day 7.