Before your blood test
Do antihistamines affect allergy blood test results?
Short answer
Not the blood tests. Specific IgE blood tests — the kind TestWell's allergy panels use — measure antibodies in serum, and antihistamines do not change them, so you can keep taking Zyrtec, Claritin, Allegra or Benadryl right up to the draw. The medicines that must be stopped are for allergy SKIN testing, where an antihistamine blocks the wheal the test depends on; allergists typically ask for 3–7 days off oral antihistamines (longer for some older ones) before skin prick or intradermal tests. Nasal steroid sprays, asthma inhalers and leukotriene blockers (montelukast) do not affect either kind of test. Oral steroids do not change IgE levels meaningfully either, though a long course can blunt skin tests.
Medically reviewed by Youmna DiStefano, MD · Last reviewed October 2026
Which results does it affect?
| Marker | Direction | How long | What to do |
|---|---|---|---|
| Specific IgE (allergy blood tests: food, pollen, pet, insect) | interferes | Not affected by antihistamines | No need to stop; take your usual dose [source] |
| Total IgE | interferes | Not affected by antihistamines | No need to stop [source] |
| Allergy skin prick / intradermal tests | falsely lowers | While taking and for 3–7 days after oral antihistamines (longer for hydroxyzine and some tricyclics) | Stop per the allergist's instructions before skin testing — the blood test is the alternative if you cannot [source] |
| Tryptase | interferes | Not affected by antihistamines | Timing after a reaction matters, the medicine does not [source] |
| Eosinophils (CBC) | can lower | Mainly with oral steroids, not antihistamines | Note steroid courses when eosinophils are read [source] |
Why blood tests and skin tests follow different rules
A skin test puts a drop of allergen into the skin and waits for histamine to raise a wheal; an antihistamine blocks that histamine, so the test can read negative even in a real allergy. A blood test measures the IgE antibodies themselves, circulating in serum whether or not histamine is blocked — the antihistamine is irrelevant to the measurement. That is why allergists ask you to stop antihistamines for several days before skin testing but not before a blood draw, and why the blood test is the practical choice when stopping medication is not possible: severe hives, a child who cannot pause treatment, or someone on an antidepressant with antihistamine effects.
What does and does not need to stop
Before a specific-IgE blood test: nothing. Keep taking antihistamines, nasal steroids, inhalers and montelukast as usual. Before skin testing: oral antihistamines (cetirizine, loratadine, fexofenadine, diphenhydramine, chlorpheniramine) typically for 3–7 days, hydroxyzine and some tricyclic antidepressants and H2 blockers for longer per the allergist; nasal steroids and inhalers can continue. Neither test is a diagnosis on its own — a positive IgE means sensitization, and whether it causes symptoms is a clinical judgment — so bring the result to a clinician who knows your history rather than cutting foods on a number.
The tests this affects — order without a doctor's visit
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Total IgE (Immunoglobulin E) | Test | $29.99 | No | 1–3 business days |
| Dog & Cat Allergy Test | Test · 2 biomarkers | $89.99 | No | 3–5 business days |
| Tryptase | Test | $71.99 | No | 3–5 business days |
Common questions
Do I need to stop antihistamines before an allergy blood test?
No. Specific IgE blood tests are not affected by antihistamines, nasal steroids, inhalers or montelukast — take them as usual.
How long should I stop antihistamines before an allergy skin test?
Typically 3–7 days for oral antihistamines such as cetirizine, loratadine, fexofenadine and diphenhydramine, longer for hydroxyzine and some antidepressants — follow your allergist's instructions. If you cannot stop, a blood test is the alternative.
Does Benadryl affect blood work?
Not allergy blood tests or routine chemistry. Its drowsiness is the main practical effect; it does not change IgE, blood counts or metabolic results.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.