Identifying specific allergy triggers often requires diagnostic tools like skin testing, which involves introducing small amounts of allergens into the skin to observe a reaction. Accurate results depend heavily on careful preparation, particularly the temporary suspension of antihistamines. These drugs directly interfere with the biological response the test measures. This necessary “washout” period ensures the body’s natural reaction is not artificially suppressed, which could otherwise lead to misleading test outcomes.
Why Antihistamines Affect Test Accuracy
Allergy skin testing relies on the body’s natural immune response to determine sensitivity. When an allergen is introduced, it triggers mast cells, specialized immune cells, to quickly release chemical mediators, most notably histamine. Histamine then binds to specific histamine 1 (H1) receptors, causing the characteristic allergic signs: localized swelling, redness, and itching (the wheal-and-flare reaction).
Antihistamines block this process by binding directly to the H1 receptors. By occupying these receptors, the medication prevents released histamine from attaching and initiating the visible reaction. If a patient takes an antihistamine too close to testing, the drug suppresses the wheal-and-flare response, even if the person is truly allergic. This interference results in a false-negative result, preventing an accurate diagnosis.
Standard Washout Guidelines
Allergists generally recommend discontinuing oral antihistamines before skin testing for between three and seven days. This range is necessary because drugs have different half-lives, determining how long they remain active. A longer washout period is preferred to fully clear the drug and its active metabolites, ensuring reliable test results. Some clinics advise a full seven days of abstinence to account for individual variations in drug metabolism and minimize suppression risk.
This precaution applies specifically to skin testing methods (prick or intradermal tests) where a physical skin reaction is necessary for diagnosis. Blood-based allergy tests, however, measure allergen-specific Immunoglobulin E (IgE) antibodies in the bloodstream. Since these tests do not rely on a histamine-driven skin reaction, they can be performed without stopping any antihistamine medication. If a patient cannot stop medication due to severe symptoms, a blood test offers an alternative path to diagnosis.
Specific Antihistamine Classes and Required Wait Times
The required discontinuation period is directly linked to the drug’s classification and clearance time. Older, first-generation antihistamines, such as diphenhydramine, require a shorter washout period of 48 to 72 hours. Despite their shorter half-lives, some clinicians recommend a full three days to ensure complete clearance.
Second-generation antihistamines (e.g., loratadine, cetirizine, and fexofenadine) are longer-acting and require a cessation period of five to seven days. They are formulated for once-daily dosing, reflecting their longer duration of action. For a potent drug like hydroxyzine, the required wait time may be extended to ten days due to its significantly longer effect on histamine receptors.
Other Medications with Antihistamine Properties
Other medications containing antihistamine properties must also be discontinued. Many over-the-counter cold, flu, and sleep aids contain first-generation antihistamines and should be stopped for at least three days. Tricyclic antidepressants, such as doxepin and amitriptyline, possess potent antihistamine effects and can suppress test results for up to two weeks, requiring a 14-day hold time.
Topical and Nasal Antihistamines
Antihistamine nasal sprays and eye drops often have a localized effect and clear more quickly. Most require a washout of one to three days, though some doctors recommend five to seven days for caution.
Strategies for Managing Symptoms During Preparation
Temporarily stopping regular allergy medication leads to a return of uncomfortable symptoms, requiring non-pharmacological management. Patients can use non-medicated saline nasal sprays or rinses to flush allergens from the nasal passages. Strict trigger avoidance is also necessary, such as staying indoors when pollen counts are high or using high-efficiency particulate air (HEPA) filters at home. Applying a cold compress or topical cooling gels can help soothe localized skin itching without interfering with the internal histamine response.
If symptoms become difficult to tolerate, the patient should contact the allergist’s office for guidance. The physician may recommend temporary use of non-antihistamine medications that do not interfere with the test, such as a steroid nasal spray, a leukotriene modifier like montelukast, or a bronchodilator for asthma symptoms. If symptoms are severe enough to risk the patient’s well-being, the allergist may suggest postponing the skin test and proceeding with a blood test instead.