Preparing for an allergy test, such as a skin prick test, involves more than just scheduling an appointment. This diagnostic procedure relies on your body’s ability to mount a visible immune response to small amounts of allergens. Certain substances, particularly medications, can suppress this natural reaction, leading to a false-negative result, where a true allergy is missed. Conversely, some foods or supplements might cause non-allergic irritation, potentially leading to inaccurate positive readings. Understanding what to temporarily avoid is essential for an accurate diagnosis and effective treatment plan.
Essential Medications to Discontinue
Antihistamines are the most common interference with skin testing because they block the histamine response that the test aims to measure. Oral antihistamines like cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra) typically need to be discontinued for at least five to seven days before the test to clear them from your system. First-generation antihistamines, such as diphenhydramine (Benadryl), often require a shorter withdrawal period of about three days, but this can vary depending on the specific drug and the clinician’s protocol. Nasal sprays and eye drops that contain antihistamines, such as azelastine or olopatadine, also need to be stopped, generally for 24 to 48 hours prior to the appointment.
Certain medications prescribed for other conditions can interfere with test results. Tricyclic antidepressants, including amitriptyline and doxepin, block the histamine response and often require a longer withdrawal period, potentially seven to 14 days, due to their mechanism of action and half-life. Abruptly stopping any psychiatric medication can be dangerous, so patients must consult with their prescribing physician before making any changes to their regimen. Similarly, some heartburn medications, specifically the Histamine-2 (H2) blockers like famotidine (Pepcid) and ranitidine (Zantac), can suppress the skin’s reaction, and most guidelines recommend stopping them about 24 to 48 hours before testing.
Many over-the-counter cold, cough, and sleep-aid products contain antihistamines. Any medication labeled with “PM” or containing ingredients like diphenhydramine should be assumed to interfere and must be discontinued for the appropriate duration. Beta-blockers, used to treat conditions like high blood pressure or glaucoma, do not interfere with the test mechanism itself. However, they can complicate the management of a severe allergic reaction, making it important to discuss their use with your allergist before testing.
Dietary Supplements and Herbal Products That Interfere
Beyond prescription drugs, dietary supplements can impact the skin’s sensitivity to allergens. Supplements with anti-inflammatory or antioxidant properties, such as high-dose Vitamin C, may dampen the immune response and lead to a false negative result. Some clinics recommend avoiding high-dose Vitamin C (over 600 mg) for up to seven days before the test.
Herbal supplements interfere with the body’s reaction to allergens. Products like Quercetin, which is sometimes marketed for its antihistamine effects, and St. John’s Wort, an herbal remedy, are advised to be stopped. Although some studies suggest common herbal products may not significantly affect the histamine response, protocols generally advise discontinuation for several days, typically between 24 hours and a week, to prevent any confounding results. Reporting all ingested products to the allergist is crucial, as the consistency and quality of these supplements can vary.
Standard Food Consumption and Test Results
For a standard allergy skin prick test or a blood test, eating normal meals beforehand is acceptable, and fasting is not required. These tests measure the body’s pre-existing immune response to a specific allergen. Maintaining hydration and comfort by eating a meal is encouraged, as the testing process can take time.
An exception involves avoiding caffeine and alcohol on the day of the test, as these substances can affect the body’s physiological responses. Some patients may be advised to avoid foods high in histamine, such as aged cheese or certain fermented products, or those that exhibit cross-reactivity with common allergens, like some raw fruits, which might cause a non-allergic skin irritation that confuses the reading. If the procedure is an oral food challenge, which involves consuming the suspected allergen under medical supervision, extensive and strict dietary restrictions are required for days leading up to the test.