What Allergy Testing Does and How It Identifies Your Triggers

Allergy testing identifies which substances cause your immune system to react. A doctor or allergist exposes a small amount of a suspected allergen to your skin or blood, then watches for a reaction. The test does not treat allergies — it only tells you what is causing your symptoms so you and your doctor can decide on a management plan.

Three main types of allergy tests exist: skin prick tests, intradermal tests, and blood tests. Skin prick tests are the fastest and cheapest. Intradermal tests go deeper into the skin and catch milder allergies that skin prick tests might miss. Blood tests measure antibodies in your bloodstream and work when skin tests are not possible — for example, if you have severe eczema or cannot stop taking certain medications.

Most people get results within 15 to 20 minutes for skin tests, or within a few days to a week for blood tests. The test itself is not painful, though skin prick tests can itch for a few minutes and intradermal tests may leave small welts that fade within hours.

Key Takeaways

  • Skin prick tests are the fastest and least expensive option, with results in 15 to 20 minutes, but blood tests are necessary if you take certain medications or have active skin conditions.
  • You will need to stop antihistamines (like Benadryl or Claritin) for three to seven days before a skin test, because they block the reaction the test is trying to measure.
  • A positive result means your body reacted to that substance, but does not mean you have a severe allergy — your doctor interprets the size of the reaction alongside your symptoms.
  • Testing costs vary widely depending on your insurance and which allergens are tested, but skin prick tests typically cost less than blood tests when paying out of pocket.

Preparing for Your Allergy Test

Stop taking antihistamines three to seven days before your test. This includes over-the-counter medications like Benadryl, Claritin, Allegra, and Zyrtec, as well as prescription antihistamines. These drugs suppress the reaction your test is designed to measure, so taking them will give you a false negative result. Ask your doctor which medications are safe to stop and which ones you should keep taking.

Some other drugs can also interfere with skin testing. Tricyclic antidepressants, certain blood pressure medications, and some asthma drugs may need to be paused. Your doctor will tell you which ones during your appointment scheduling call. Do not stop any medication without asking first.

Wear loose, short-sleeved clothing on the day of your test if you are having a skin prick test, since the doctor will need clear access to your forearm or back. Eat a light meal beforehand — you do not need to fast, but some people feel lightheaded if they are hungry during the test. Bring a list of your symptoms and when they started, because this helps your doctor interpret the results.

What Happens During a Skin Prick Test

The doctor or technician will clean a section of your forearm or back with alcohol and mark it with a pen, creating a grid of small numbered squares. Each square corresponds to one allergen. The technician places a tiny drop of allergen extract on each square, then uses a small plastic or metal tool to prick the skin just enough to let the allergen in. You will feel a light scratch but not pain.

You then wait 15 to 20 minutes. During this time, if you are allergic to any of the substances, your skin will develop a raised, itchy bump called a wheal at that spot. The technician measures each wheal and compares it to a control — a bump from histamine (which everyone reacts to) and a bump from saline (which no one reacts to). A wheal larger than the saline control is considered a positive result.

Itching is normal and usually mild. You can ask for a cool compress or a topical steroid cream to ease it. The wheals fade within a few hours, though the itching may last longer. Some people develop a larger reaction 4 to 12 hours later called a delayed reaction, which is still normal and not dangerous.

What Happens During a Blood Test for Allergies

A blood test, also called a specific IgE test or RAST test, measures antibodies your immune system makes in response to allergens. The technician draws a small sample of blood, usually from your arm, and sends it to a lab. The lab exposes your blood to different allergen extracts and measures how much antibody your blood produces for each one.

Blood tests take longer to get results — usually three to seven business days — but they have advantages. You do not need to stop antihistamines beforehand. They work even if you have active eczema, psoriasis, or other skin conditions. They also measure the strength of your reaction on a numerical scale, which can help your doctor understand how severe your allergy is.

Blood tests cost more than skin prick tests when paying out of pocket, but some insurance plans cover them at the same rate. Ask your doctor's office what your insurance will cover before the test, because costs vary widely by plan and by which allergens are tested.

Understanding Your Test Results

A positive result means your skin or blood reacted to that allergen, but it does not automatically mean you have a clinical allergy. Your doctor will compare your test results to your symptoms and medical history. For example, if you tested positive for cat allergen but have never owned a cat and have no symptoms around cats, that positive result may not mean anything for your daily life.

The size of the reaction matters. A large wheal or a high antibody number usually means a stronger reaction, but even a small positive result can cause real symptoms in some people. Your doctor will help you decide which positive results are worth avoiding and which ones you can manage with medication.

A negative result means you did not react to that allergen during the test. However, a negative result does not rule out an allergy completely — some allergies are too mild to show up on standard tests, or you may have a reaction only under certain conditions (like exercise-induced allergies). If your symptoms do not match your test results, tell your doctor, because you may need additional testing or a different type of test.

What to Do After Your Test Results Come Back

Schedule a follow-up appointment with your doctor to discuss what the results mean for you. Bring a list of your symptoms and when they happen, because this conversation is where your doctor decides which allergens matter and which ones you can ignore. Your doctor may recommend avoidance, medication, or immunotherapy (allergy shots or sublingual tablets) depending on your results and how much your allergies affect your daily life.

If you tested positive for environmental allergens like pollen or dust mites, your doctor may suggest practical steps like using air filters, washing bedding in hot water, or keeping windows closed during high pollen season. If you tested positive for food allergens, your doctor will discuss how to read food labels and what to do if you accidentally eat that food.

Keep a copy of your test results. If you see a different doctor later, they will want to know what you tested positive for. Some people need repeat testing years later if their symptoms change or if they want to know whether their allergies have gotten worse or better.

Costs and Insurance Coverage

Skin prick tests typically cost between $60 and $300 out of pocket, depending on how many allergens are tested and where you live. Blood tests usually cost between $200 and $1,000 out of pocket, though the price varies widely. If you have insurance, your plan may cover part or all of the cost, especially if your doctor documents that you have allergy symptoms that need diagnosis.

Call your insurance company before your test to find out what your plan covers. Ask whether you need a referral from your primary care doctor, whether the allergist is in-network, and what your copay or coinsurance will be. Some plans cover skin prick tests fully but require you to pay more for blood tests, or vice versa.

If you do not have insurance or your plan does not cover allergy testing, ask your doctor's office whether they offer a cash discount or a payment plan. Some allergists offer reduced rates for uninsured patients, and some clinics run sliding-scale fees based on income.

Frequently Asked Questions

Can I eat before my allergy test?

Yes, you can eat normally before your test. You do not need to fast. However, eat a light meal rather than a heavy one, because some people feel lightheaded during skin prick testing if they are very hungry or have low blood sugar.

What if I am allergic to the test itself?

Severe allergic reactions during allergy testing are rare, but your doctor's office is equipped to handle them. The office has epinephrine on hand and staff trained to treat anaphylaxis. Tell your doctor when ready if you feel dizzy, have trouble breathing, or develop hives beyond the test area.

Can I get allergy testing while pregnant?

Skin prick tests are generally considered safe during pregnancy because the allergen stays on the skin surface and very little enters your bloodstream. Blood tests are also safe. Talk to your doctor about whether testing is necessary right now or whether it can wait until after delivery, since pregnancy hormones can change allergy symptoms temporarily.

Do I need a referral from my primary care doctor to see an allergist?

It depends on your insurance plan. Some plans require a referral, while others let you see a specialist without one. Call your insurance company or check your plan documents before scheduling. Your primary care doctor can also refer you if you ask.

What is the difference between an allergy and an intolerance?

Allergy tests measure immune system reactions. Food intolerances like lactose intolerance do not involve the immune system and will not show up on an allergy test. If you suspect a food intolerance, your doctor may recommend an elimination diet or other testing instead of allergy testing.