Allergy testing, often performed using the skin prick method, identifies specific substances, or allergens, that trigger an allergic reaction. During this test, small amounts of allergen extracts are introduced beneath the skin’s surface. A positive result is indicated by the formation of a wheal, a raised, red, and itchy bump, which occurs when immune cells release histamine.
Antihistamine medications directly interfere with this process by blocking the action of histamine, the chemical signal the test is designed to measure. Taking these medications before the test can suppress the natural reaction, leading to a false negative result. Stopping the medication is necessary to ensure the integrity and accuracy of the test results.
Standard Withdrawal Periods for Common Medications
The most common medications that must be temporarily discontinued are the second-generation, over-the-counter (OTC) oral antihistamines. These long-acting drugs include widely used agents such as cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra). For these medications, the standard withdrawal period recommended by most allergists is five to seven days prior to the skin test.
This timeframe is based on the drug’s half-life. While these second-generation antihistamines are effective for 24 hours, their active metabolites can linger in the system, suppressing the histamine response in the skin. A full five to seven days allows the body to clear the medication sufficiently, ensuring the skin’s natural response is fully restored. Levocetirizine (Xyzal) and desloratadine (Clarinex) also fall within this standard five-to-seven-day withdrawal window.
Medications Requiring Extended Withdrawal Periods
Certain medications require a longer withdrawal period, often extending to 10 to 14 days, because they have a greater potential for prolonged suppression of the skin reaction. This group includes older, first-generation antihistamines, such as diphenhydramine (Benadryl) and hydroxyzine (Atarax, Vistaril). Although some studies suggest a shorter 48-hour period is adequate for diphenhydramine, many allergists recommend a longer period, or group it with the standard five to seven days, to ensure maximum reliability.
A two-week withdrawal period is often mandatory for tricyclic antidepressants (TCAs), which are sometimes prescribed for chronic pain or sleep disorders. Medications like doxepin, amitriptyline, and imipramine have potent antihistamine properties that can suppress skin test results for up to two weeks. Patients must consult with the prescribing physician before discontinuing any psychiatric or long-term medication to prevent serious withdrawal symptoms or a relapse of the underlying condition.
Identifying Other Interferences
The interference with allergy skin testing is not limited to oral antihistamines, as several other common medications can affect the results and require temporary discontinuation.
Topical Antihistamines
Antihistamine eye drops and nasal sprays, such as those containing azelastine or olopatadine, need to be stopped because they contain localized antihistamine agents. The general recommendation for these topical products is to stop their use two to seven days before the test, though some clinics may advise as little as 48 hours.
H2 Blockers
Certain stomach acid blockers, known as H2 blockers, can also interfere with the skin test. Their effect can diminish the overall skin reaction, and they are typically advised to be held for 24 to 48 hours before testing. Examples include famotidine (Pepcid), ranitidine (Zantac), and cimetidine (Tagamet).
Patients should be aware that many multi-symptom cold, flu, and sleep aids contain hidden antihistamines, making it necessary to carefully read all product labels and discuss every medication with the allergist. It is generally safe to continue using inhaled corticosteroids and nasal corticosteroid sprays, as these typically do not interfere with skin test results.
Managing Symptoms While Off Medication
The period spent off antihistamines can be uncomfortable, but several non-pharmacological methods can help manage symptoms without compromising the test results.
Environmental Control
A primary strategy involves rigorous environmental control, which means actively reducing exposure to known or suspected allergy triggers in the days leading up to the appointment. This includes using high-efficiency particulate air (HEPA) filters in bedrooms, keeping windows closed to prevent pollen entry, and frequently washing bedding in hot water to minimize dust mites.
Symptom Relief
Saline nasal rinses and irrigation systems, such as a neti pot, are safe and effective alternatives for clearing congestion and rinsing allergens from the nasal passages. For localized relief of itchy eyes or skin, applying a cold compress can soothe irritation without medication.
Patients should prioritize continuing all medications for non-allergy conditions, especially those for serious issues like heart disease, high blood pressure, or depression, unless explicitly instructed by both the prescribing physician and the allergist to stop.