between each test. This problem can be avoided by using a fresh needle for each test site. Another potential area of difficulty involves placement of the allergens: if they are too close, overlapping reactions cannot be separated. It is therefore recommended that the extracts be placed at least 2 cm apart.

Infection and bleeding can lead to false-positive results. Likewise, insufficient penetration of the skin by the puncture instrument may lead to false-negative results. (This problem is more likely to occur with plastic devices [Bousquet, 1988].) Skin prick tests are applicable to children as young as 1 month of age if clinical indications are present. Use of the test usually requires allergen concentrations of 1:10 to 1:20, weight/volume. Skin prick tests pose an extremely low risk for the development of systemic anaphylactic reactions or fatalities.

In intradermal testing, allergenic extracts are diluted in a buffered saline solution containing 0.3 percent human serum albumin as a stabilizer. Volumes of 0.01 to 0.05 ml are injected intradermally. As noted above, individual, unitized syringes should be employed for each skin test to avoid any risk of contamination from syringes with removable needles that have been used repeatedly (Shulan et al., 1985). Solutions containing less than 1 percent glycerine may be used for skin testing; concentrations greater than this amount may induce nonspecific reactions.

Because a small but definite risk of anaphylaxis and fatalities exists with intradermal testing (Lockey et al., 1987), skin prick tests should be conducted prior to any such testing. In addition, patients who are suspected of having allergic disease but who have a negative skin prick test may be candidates for intradermal testing because intradermal testing is more sensitive. Properly conducted negative intradermal skin tests virtually exclude the presence of specific IgE antibody.

Scratch or abrasion methods should probably be abandoned (Van Arsdel and Larson, 1989). These tests have poor reproducibility because of the variable amount of allergen introduced into the skin. False-positive reactions may occur if bleeding is induced; there is also a risk of systemic allergic reactions (Guerin and Watson, 1988).

Variables and Controls

Several variables can affect the size of the cutaneous reaction. The magnitudes of both allergen and histamine reactions vary over different parts of the body. The upper and midback are more reactive than the lower back. The back is more reactive than the forearm. The ulnar side of the arm is more reactive than the radial. The wrist area is less reactive than the space in front of the elbow (H. S. Nelson, 1983). There is also some variability in skin test responsiveness at different times of the day. At 7 a.m., the reaction to allergen and histamine is less than that in the late



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