Prior to the scheduling of allergy testing, the patient will undergo a full physical examination. As part of this process, the physician will take a complete medical history and ask the patient for specific details about the patient’s past allergic reactions. It is crucial that patients explain in as much detail as possible the particulars of their symptoms – when they occur and under what circumstances – as well as identify possible allergens that may have been encountered at the time of the reactions.  

Patients scheduled to undergo allergy skin testing need minimal preparation before the test. However, it is crucial that patients inform a physician of any medications they are taking. Medications that can interfere with test results include:

• Antihistamines. Antihistamines can prevent allergic reactions, thus potentially skewing test results. Antihistamines block the work of histamine, a chemical released during allergic reactions. Histamine contributes to symptoms such as hives, sneezing, runny nose and itchy eyes. Antihistamines attach to cell receptors – known as H1, H2 and H3 – and prevent histamine from doing the same. The histamine (which primarily does its work at the H1 receptor) is blocked, short-circuiting the allergic process. All over-the-counter antihistamines should be avoided for at least 48 hours before the test, while prescription antihistamines should be discontinued for five to seven days before the test (or as directed by the physician).
  
  
• Tricyclic antidepressants. These can cause the skin to become more sensitive to light than normal and may therefore impact the accuracy of allergy skin test results. They also may have strong anti-H1 and H2 activity. Tricyclic antidepressants are used in the treatment of depression, obsessive compulsive disorder, bedwetting and other conditions.
  
  
• H2 antagonists. These also block the release of histamine and can therefore skew test results.
  
  
• Corticosteroid creams. These can suppress reactions in the areas they are applied.
  
  
• Tranquilizers (including sleep medications).
  
  
• Anti-emetics (used to treat nausea and vomiting).

Patients should not stop taking any medication without first consulting a physician. In some cases, it is not prudent to stop taking the medication. A physician is then likely to perform a control test that will indicate if the particular drug interferes with a skin test. Two control substances – histamine (used in a positive control test) and saline (used in a negative control test) – are tested on the patient. The control substances help a physician to determine how valid the skin test is likely to be.