afternoon and early evening. The age of the individual undergoing testing also influences the size of the skin test response. Young infants have wheals and flares that are smaller in diameter than those in adults. In addition, skin test reactivity declines after age 60.
The reproducibility of epicutaneous skin testing, particularly that using a skin prick test, depends on the degree of pressure applied to the skin. Medications can significantly affect the skin test response. For example, antihistamines and tricyclic antidepressants must be discontinued for at least 4 to 5 days before testing occurs (I. L. Bernstein, 1988). Some long-acting antihistamines such as astemizole (Hismanal®) may interfere with skin test results for up to 6 weeks. Long-term use of high-potency topical steroids also decreases skin test reactivity. Oral corticosteroids at doses of up to 30 mg a day for 1 week do not suppress the allergic skin test response.
Because skin tests may be affected by a number of factors, positive and negative controls should be applied. Some general recommendations for skin testing procedures are summarized in Box 5-1.
One of the major difficulties with reproducibility of skin test responsiveness is the lack of standardized reagents. Certain indoor allergenic extracts (e.g., dust mite and cat) now contain standardized amounts of major allergens, and further efforts at standardization are under way for other allergens. Because extracts often become less potent over time when stored under warm conditions, refrigeration is important. The addition of glycerine to skin prick test reagents enhances their stability, as does the use of
BOX 5-1 Allergy Skin Test Procedures