What an allergy test does and why your doctor might order one
An allergy test identifies which substances trigger an allergic reaction in your body. Your doctor orders one when you have symptoms — itching, swelling, hives, breathing trouble, or digestive problems — that happen after exposure to something specific, but the cause is not clear. The test does not treat the allergy; it names what causes it so you can avoid it or prepare for exposure.
Allergy testing is most useful when symptoms are frequent or severe enough to change how you live. If you break out in hives every spring but do not know why, or if you swell up after eating certain foods, a test gives you a concrete answer instead of guessing. Some people need testing before starting immunotherapy (allergy shots or tablets), which requires knowing exactly what you are allergic to.
Testing is also ordered when symptoms could have multiple causes. A runny nose might be allergies, a cold, or a sinus infection. A test can rule allergies in or out, which changes what treatment makes sense.
Key Takeaways
- Three main types of allergy tests exist: skin prick tests (fastest, done in the office), blood tests (ordered when skin tests are not possible), and elimination diets (for food allergies when other tests are unclear).
- Skin prick tests take 15 to 20 minutes and show results within 15 minutes, while blood tests take several days to a week for results to come back from the lab.
- You must stop taking antihistamines and some other medications three to seven days before a skin test, or the test will not work correctly.
- Allergy tests are usually covered by insurance when ordered by a doctor, though your out-of-pocket cost depends on your plan and deductible.
- A positive test result means you have an antibody to that substance, but does not always mean you will have symptoms every time you encounter it.
Skin prick tests: the fastest and most common option
A skin prick test is done in your doctor's office and takes about 20 minutes total. The doctor or nurse marks your forearm or back with a grid, then places a tiny drop of allergen extract on each marked spot. They use a small plastic tool to prick the skin under each drop — the prick itself is shallow and usually painless. You wait 15 minutes while your skin reacts.
If you are allergic to a substance, a raised bump called a wheal appears at that spot within minutes. The doctor measures the bumps and compares them to a control bump (made with histamine, which always causes a reaction in non-allergic people). A bump larger than the control means you are likely allergic to that substance.
Skin prick tests can check 20 to 40 different allergens in one visit. They work best for environmental allergies (pollen, pet dander, dust mites, mold) and some food allergies. The main drawback is that you must stop taking antihistamines, some decongestants, and certain other medications for three to seven days before the test, or the test will not show a reaction even if you are allergic.
Blood tests: when skin tests are not an option
A blood test (called a specific IgE test or RAST test) measures antibodies to allergens in your blood. Your doctor orders it when you cannot stop medications before a skin test, have severe eczema or other skin conditions that make skin testing unreliable, are pregnant, or have had a severe allergic reaction and the doctor wants to avoid any risk during testing.
The test is straightforward: a nurse draws blood, and the sample goes to a lab. Results come back in three to seven days. The lab reports how much of each allergen-specific antibody is in your blood — higher levels suggest a stronger allergic response, though the exact number does not always predict how severe your symptoms will be.
Blood tests cost more than skin tests and take longer for results, but they work when skin testing is not safe or practical. Insurance usually covers them when ordered by a doctor, though some plans require prior approval.
Food allergy testing and elimination diets
Food allergies are harder to test than environmental allergies because skin and blood tests are less reliable for foods. A skin prick test or blood test can show you have an antibody to peanuts, for example, but many people with that antibody never have symptoms when they eat peanuts. This is called a false positive.
If your doctor suspects a food allergy, they may recommend an elimination diet: you stop eating the suspected food for two to four weeks, then reintroduce it and watch for symptoms. This is slower than a test but more accurate for foods. Your doctor or a dietitian can guide you through it safely, because eliminating whole food groups without planning can leave you short on nutrients.
Oral food challenges — where you eat small amounts of the suspected food under medical supervision — are the gold standard for food allergies but are only done in a clinic with emergency equipment nearby. They are used when the diagnosis is unclear and the stakes are high.
What to do before your allergy test
For a skin prick test, stop taking antihistamines (like cetirizine, fexofenadine, and loratadine) three to seven days before your appointment. Also stop some decongestants and tricyclic antidepressants, which can interfere with the test. Ask your doctor which medications to pause — do not stop prescription medications on your own without permission.
Wear loose, short-sleeved clothing so the doctor can access your forearm or back easily. Eat a normal breakfast and bring water; some people feel lightheaded after testing, though this is rare.
For a blood test, no special preparation is needed. You can take all your medications normally. Eat and drink as usual.
For an elimination diet, write down everything you eat and any symptoms you notice for a week before you start, so you have a baseline to compare against.
Understanding your test results
A positive result means you have an antibody to that substance, but it does not automatically mean you are allergic in the way that matters. Some people test positive to pollen but have no symptoms. Others test positive and have severe reactions. The test shows sensitization, not necessarily clinical allergy.
Your doctor interprets the results alongside your history. If you test positive to cat dander and you sneeze and itch every time you visit a friend with cats, that is a clear match. If you test positive to something you have never noticed bothering you, your doctor may ask more questions before concluding you are allergic.
A negative result means the test did not find an antibody to that substance. If you took antihistamines before a skin test, a negative result may be false — the test did not work. If you took a blood test or followed the preparation instructions correctly, a negative result is reliable.
Cost and insurance coverage
Allergy testing ordered by a doctor is usually covered by insurance. Your out-of-pocket cost depends on your plan — some cover it fully after you meet your deductible, others require a copay, and a few require coinsurance (you pay a percentage). Call your insurance company before your appointment to find out what you will owe.
If you do not have insurance, skin prick tests typically cost $100 to $300 out of pocket, and blood tests cost $200 to $500, depending on how many allergens are tested. Some community health centers offer testing on a sliding fee scale based on income.
What happens after testing
Once you know what you are allergic to, your doctor will discuss ways to manage it. For environmental allergies, this usually means avoidance (keeping windows closed during pollen season, using air filters, washing bedding often) and over-the-counter or prescription medications. For food allergies, it means reading labels and avoiding the food, and carrying an epinephrine auto-injector if the allergy is severe.
Your doctor may also discuss immunotherapy — allergy shots or sublingual tablets that gradually reduce your reaction to the allergen over months or years. This is most common for environmental allergies and is not an option for severe food allergies.
Keep a record of your test results. You may need them if you see a new doctor, have a reaction you are unsure about, or want to retest years later to see if your allergies have changed.
Frequently Asked Questions
Can I eat before an allergy test?
For a skin prick test, eat a normal breakfast. For a blood test, you can eat and drink normally unless your doctor tells you otherwise. Some blood tests require fasting, but allergy tests do not.
How long do allergy test results last?
Allergies can change over time. A test is accurate for the time you take it, but you may develop new allergies or outgrow old ones over years. If your symptoms change significantly, your doctor may recommend retesting.
What if I have a reaction during the skin prick test?
Severe reactions during skin prick testing are rare because the amount of allergen is tiny. If you have itching or swelling that bothers you, the doctor can wipe off the allergen and give you an antihistamine. The office has emergency equipment if a serious reaction occurs.
Can I test for allergies at home?
Home allergy test kits exist, but they are blood tests you order online and mail to a lab. Results are less reliable than tests ordered through a doctor because there is no medical history to interpret them against. A doctor-ordered test is more accurate.
Do I need a referral to get an allergy test?
This depends on your insurance plan. Some plans require a referral from your primary care doctor; others do not. Call your insurance company or your doctor's office to find out before you schedule.