Allergy Testing Before Immunotherapy: Skin Test vs. Blood Test
- Skin tests and blood tests both look for IgE, the antibody your immune system makes against a specific allergen.
- A skin prick test gives results in the same visit. A blood test goes to a lab and takes longer.
- A blood test is the better fit if you can't stop certain medicines, or if a skin condition like eczema makes skin testing hard to read.
- Antihistamines can hide a skin test result, so most need to be stopped days beforehand.
- A positive result isn't a diagnosis on its own. Your allergist reads it alongside your history.
If you're weighing an allergy skin test vs. blood test, start here: both look for the same thing. IgE is the antibody your immune system makes against a specific allergen, like grass pollen or cat dander. A skin test finds it by watching how your skin reacts, and a blood test measures it in a lab.
Skin testing is usually where allergists start. It's fast, and the American College of Allergy, Asthma and Immunology (ACAAI) notes that blood tests take longer and cost more. Some people are better off with a blood test, though, and the reasons are specific.
Allergy testing also comes before any kind of immunotherapy. Allergy shots, allergy drops, and ILIT all target specific allergens, so your results tell your doctor which allergens to include in the extract for ILIT or allergy shots, or which drops or tablets to prescribe for treatment at home.
How a skin prick allergy test works
A skin prick test, also called a scratch test, places a tiny drop of each possible allergen on your skin and pricks or scratches it into the surface. The American Academy of Allergy, Asthma & Immunology (AAAAI) calls skin testing the most common, reliable and relatively painless form of allergy testing.
Back or arm?
Cleveland Clinic says the test is usually done on the inner forearm for adults or the upper back for children, but adults can be tested on the back too, and it has room for a large panel. A 2005 comparison of skin test devices by an AAAAI committee found that with devices that prick several spots at once, the small bumps that show a positive result came up a bit larger on the back than on the arm.
Many allergens at once
One session can check dozens of allergens, and the exact panel varies by practice. Instead of one prick at a time, allergists can use multi head devices that place a row of allergens in a single press, a trend the same 2005 study described. That's a big part of why a long panel goes quickly.
How long it takes and how it feels
According to the ACAAI, placing the allergens takes about 5 to 10 minutes. Then you wait about 15 minutes to see whether a small, itchy red bump appears where an allergen went in. The itch doesn't last long. Cleveland Clinic says most symptoms clear up within one to two hours, though some redness or bumps can stay for several hours. So the whole test usually takes about 20 to 25 minutes, and you get results before you leave.
The ACAAI says the pricks may hurt slightly, and any spot that reacts will itch. In the 2005 study, multi head devices were somewhat more painful than single pricks, so if you're nervous, ask which device the clinic uses.
If a skin prick allergy test comes back negative but your symptoms still point to a particular allergen, your allergist may try an intradermal test, where a small amount of the allergen goes just under the skin with a thin needle.
How an allergy blood test works
An allergy blood test, sometimes listed on a lab order as a specific IgE or RAST test, takes one standard blood draw. MedlinePlus, from the National Library of Medicine, says drawing the sample usually takes less than five minutes, and the needle is in your arm for far less time than that. The lab measures IgE to each allergen your allergist ordered, so results come back later rather than during your visit.
When is a blood test better than a skin test?
Medication is the most common reason. If you take something that interferes with skin testing and can't safely stop it, a blood test sidesteps the problem. Skin conditions are the other big one, and the ACAAI lists eczema and psoriasis as reasons skin can be hard to test or read.
There's also a small group of people so sensitive to an allergen that even the tiny exposure in an allergy skin test could be risky. For them, an allergy blood test is the safer route.
How to prepare for an allergy skin test
Antihistamines block the reaction an allergy skin test looks for, so they can make a real allergy look negative. The ACAAI advises stopping most antihistamines about a week before testing, with some variation by medicine. Hydroxyzine needs 10 days, while diphenhydramine (Benadryl) only needs to be stopped 48 hours ahead. Some psychiatric medicines can interfere too.
Don't stop a prescription on your own to get ready. Your allergy doctor can tell you which of your medicines matter, and whether a blood test would be simpler.
Allergy skin test vs. blood test: which is right for you?
For most people, a skin prick test is the natural first step. A blood test is the backup when skin testing can't give a clear or safe answer.
Either way, a positive result isn't a diagnosis on its own. It means your immune system has made IgE to that allergen, but the ACAAI says you may not react to it at all, and blood tests can show false positives. That's why your allergist reads your results alongside your history, such as when your symptoms flare and what seems to set them off. Two people with the same positive test can need very different treatment plans.
If you're considering ILIT, find an ILIT provider near you to talk through allergy testing and whether ILIT fits your allergens.
Frequently Asked Questions
No. MedlinePlus, from the National Library of Medicine, notes that the amount of IgE measured doesn't predict how serious an allergy will be. Levels can also run higher for reasons that have nothing to do with allergies, such as smoking.
Possibly, even if you've been tested before. The ACAAI says an allergist may want to retest you to confirm your allergens before starting a multi-year immunotherapy plan. Results from years ago, or from a different clinic's panel, may not match what's driving your symptoms now.
Usually, yes. The ACAAI says nasal steroid sprays and asthma medicines don't interfere with skin testing. Antihistamines are the main exception, and some antihistamine eye drops and nasal sprays count, so check the label and ask the clinic.
Usually not much. The AAAAI calls skin testing relatively painless, and the ACAAI says the pricks may hurt slightly, with any allergen you react to leaving an itchy bump. A 2005 device study found pricks from multi head devices somewhat more painful than single ones, so ask which kind your clinic uses if it matters to you.
Plan on about 20 to 25 minutes for the test itself. The ACAAI says placing the allergens takes about 5 to 10 minutes and reading them takes about 15 more, and results come back at the same visit, unlike a blood test.
Both work. Cleveland Clinic says adults are usually tested on the inner forearm and children on the upper back, but adults can be tested on the back too, which fits a larger panel. A 2005 comparison found that the small bumps showing a positive result came up a bit larger on the back with multi head devices, and a bit larger on the arm with single prick devices.
