--- title: "Do antihistamines affect allergy blood test results?" type: prep url: "https://www.test-well.com/prep/antihistamines" factor: Antihistamines and allergy medicines aliases: - stop antihistamines before allergy test - zyrtec before allergy blood test - do antihistamines affect ige test - claritin before skin test reviewed_at: 2026-10-07 review_status: reviewed date_published: 2026-10-07 --- # Do antihistamines affect allergy blood test results? > 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. ## Which results does it affect? | Marker | Direction | How long the effect lasts | What to do | Source | | --- | --- | --- | --- | --- | | Specific IgE (allergy blood tests: food, pollen, pet, insect) | interferes | Not affected by antihistamines | No need to stop; take your usual dose | [Testing.com (AACC) — Allergy blood testing (specific IgE)](https://www.testing.com/tests/allergy-blood-testing/) | | Total IgE | interferes | Not affected by antihistamines | No need to stop | [Testing.com (AACC) — Allergy blood testing (specific IgE)](https://www.testing.com/tests/allergy-blood-testing/) | | 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 | [AAAAI — Allergy testing: stopping antihistamines before skin tests](https://www.aaaai.org/tools-for-the-public/conditions-library/allergies/allergy-testing) | | Tryptase | interferes | Not affected by antihistamines | Timing after a reaction matters, the medicine does not | [Testing.com (AACC) — Allergy blood testing (specific IgE)](https://www.testing.com/tests/allergy-blood-testing/) | | Eosinophils (CBC) | can lower | Mainly with oral steroids, not antihistamines | Note steroid courses when eosinophils are read | [Testing.com (AACC) — Complete blood count (CBC)](https://www.testing.com/tests/complete-blood-count-cbc/) | ## 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. ## 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. ## Related - [Do you need to fast before a blood test — and which tests does eating affect?](https://www.test-well.com/prep/fasting) - [Does biotin affect blood test results?](https://www.test-well.com/prep/biotin) - [How does alcohol affect blood test results?](https://www.test-well.com/prep/alcohol) - [Should you exercise before a blood test?](https://www.test-well.com/prep/exercise) - [All preparation guides](https://www.test-well.com/prep) - [Lab visit guide](https://www.test-well.com/at-the-lab) ## Sources - [Testing.com (AACC) — Allergy blood testing (specific IgE)](https://www.testing.com/tests/allergy-blood-testing/) - [AAAAI — Allergy testing: stopping antihistamines before skin tests](https://www.aaaai.org/tools-for-the-public/conditions-library/allergies/allergy-testing) - [Testing.com (AACC) — Complete blood count (CBC)](https://www.testing.com/tests/complete-blood-count-cbc/) --- Medically reviewed by Youmna DiStefano, MD (Medical Reviewer, TestWell); last reviewed 2026-10-07. 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