Method: skin prick test
Solutions:
(1) histamine as positive control (10 mg/mL)
(2) sterile saline as negative control
(3) test allergen (extract diluted 1:20 w/v in 50% glycerin)
Measurements:
(1) diameter of wheal for histamine in mm
(2) diameter of wheal for saline in mm
(3) diameter of allergen in mm
skin index (SI) =
= (diameter of test allergen wheal) / (diameter of histamine wheal)
Criteria of an invalid test - one or both of the following:
(1) histamine wheal < 3 mm in diameter
(2) saline wheal > 2 mm in diameter
Interpretation:
• The SI ranged in the study from 0.2 to 8.3.
• A high SI correlated with a positive food challenge.
• A patient with a negative food challenge test usually had a SI <= 1 and none were above 2.1.
• A patient with a positive food challenge showed overlap with those patients having negative food challenges, but some showed an SI > 2.1
