Interpretation of skin tests

The size of the weal-and-flare should be read at the peak of its reaction, after approximately 10-20 minutes. A copy of the weal-and-flare reaction should be transferred using pen and clear tape, to ensure a permanent record is kept. The mean of the longest and midpoint orthogonal diameter of the weal has been shown to correlate well with more precise planimetry methods, despite the weal often having a rather irregular shape.

The 'cutoff' at which one declares a test positive will influence the test's sensitivity and specificity. A 3 mm mean weal diameter is the common definition of a clinically significant positive reaction, corresponding to a 10 mm mean flare diameter.

Researchers or clinicians may wish to quantify the reaction to assess how much specific IgE the patient has. There is no good correlation between the size of the weal and the amount of specific IgE. In Scandinavia workers compare the size of the weal-and-flare reaction with the size of the reaction produced by the positive histamine control, but this too is inaccurate.

The operator plays a significant role in the reproducibility of skin prick tests, and it has been suggested that duplicate skin tests should be performed. A rate of 5% single-negative tests in clearly sensitised subjects is to be expected, even in the most experienced operator's hands.

The type and quality of the allergen clearly affects the diagnostic efficacy of all assessments of specific IgE, including skin tests. Attempts to typify the specific proteins from each foodstuff that are most commonly responsible for clinical symptoms are ongoing. As these are described they will enable skin prick tests with these recombinant allergens, possibly improving standardisation. Fruit and vegetables seem particularly difficult substances from which to produce reliable allergen extracts. Some workers advocate the 'prick-prick' method, first pricking the relevant fruit or vegetable, and then the patient's skin. The concerns about this method include lack of standardisation of the allergen.

Other factors which may influence the size of the reaction include age (skin test weals increase from infancy to adulthood and then often decline after the age of 50), race (dark skin pigmentation elicits a greater weal response from histamine), season, pathological conditions, and drugs.

Those without symptoms but with positive skin prick tests may lie in one of two groups. They may indeed be false positives, and the positive reaction may be due to irritants or other mast cell secretagogues and not an indication of specific IgE. The other group includes the asymptomatic but skin prick test positive people who are at greater risk of developing allergic symptoms, but not necessarily food allergies, later in life. This is termed 'latent allergy'.27

The negative skin prick tests of those with symptoms may be explained by poor technique, drugs or disease attenuating the skin's reaction, poor quality extracts and decreased reactivity of the skin of infants and the elderly.

The interpretation of positive skin prick with food extracts is even more difficult than with aeroallergens. Only a fraction of people even with positive reactions to the more specific prick-puncture tests to foods will react during a challenge.20

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