Allergy testing, such as a skin prick test or a blood test, identifies specific substances that trigger an immune response. Accurate results are fundamental for developing an effective treatment plan, requiring proper preparation. The test measures the body’s reaction to allergens through the release of chemicals like histamine. Anything that interferes with this process can lead to inaccurate readings. This article clarifies the substances and foods that must be avoided before testing to ensure results accurately reflect your sensitivities.
Substances That Directly Impact Test Results
The primary concern for pre-test preparation involves any substance that suppresses the body’s natural histamine response, potentially causing a false negative result. Oral antihistamines are the most common culprits. They are designed to block histamine receptors, preventing the inflammatory reaction the test relies upon. This category includes first-generation sedating medications, such as diphenhydramine, and second-generation non-drowsy options like cetirizine, loratadine, and fexofenadine.
Certain acid blockers, known as H2 receptor antagonists, must also be discontinued because they function as histamine blockers. These medications, including cimetidine (Tagamet) and famotidine (Pepcid), are used to treat acid reflux and heartburn. Their mechanism of action can interfere with the skin test’s ability to produce a measurable reaction. Certain psychiatric medications, specifically tricyclic antidepressants like doxepin, also possess potent antihistamine properties that suppress skin reactivity.
Certain dietary supplements can also influence test outcomes. High-dose Vitamin C may exert an antihistamine-like effect that reduces the size of the skin reaction. Some herbal remedies, such as licorice, green tea, and St. John’s Wort, are advised to be stopped, as they may contain compounds that interfere with inflammatory pathways.
Standard Foods That Do Not Require Avoidance
A common misconception is that patients must fast or avoid eating suspected allergens before a standard skin prick or blood test. For a typical skin test, which measures immediate IgE-mediated sensitivity, standard meals do not need to be restricted. The test measures baseline sensitivity to allergen extracts introduced directly into the skin, and a recent meal does not interfere with this process.
Avoiding a known allergen before the test is often counterproductive because the physician needs to observe the body’s true reaction to confirm the diagnosis. The only exception is if the testing procedure includes a specific oral food challenge. This involves the controlled, in-office consumption of a food to observe a reaction, and strict fasting instructions are provided by the specialist in that scenario.
While most foods are allowed, it is recommended to avoid consuming large amounts of alcohol or caffeine in the 24 hours before the test. Alcohol can affect the immune response and potentially skew results. Caffeine may increase heart rate and anxiety, which could complicate the observation of systemic reactions. This is a minor restriction compared to medication cessation, and a light meal before the appointment is encouraged to prevent lightheadedness.
Timeline for Cessation and Preparation
The duration for which substances must be stopped relates directly to how long the medication remains active in the body. Long-acting, second-generation oral antihistamines (loratadine, cetirizine, and fexofenadine) require cessation for five to seven days before testing. This extended period ensures the drug is cleared from the system, allowing the histamine response to fully recover.
Short-acting, first-generation antihistamines like diphenhydramine, often found in sleep and cold remedies, have a shorter half-life and require being stopped for 48 to 72 hours. H2 blockers (famotidine and cimetidine) require a 24- to 72-hour window of abstinence before the skin test. For tricyclic antidepressants, the required cessation time is significantly longer, often up to 14 days. This must only be done under the supervision of the prescribing physician due to withdrawal risks.
Topical medications applied directly to the testing area, typically the back or forearms, also require discontinuation. Topical steroids must not be applied to the testing site for five to seven days, as they suppress the localized inflammatory reaction necessary for a positive result. Patients should continue all prescribed asthma inhalers and nasal steroid sprays, as these do not interfere with the skin test results and discontinuing them could be unsafe.