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Skin testing - what the numbers mean

June 5, 2002

How accurate are skin tests? If I tested a 2 for cat dander, What does that mean? -- J.A.C., Tennessee

Several years ago I had a "Skin Prick" test done and now, looking at the results, I can't remember how to interpret the results. Under Trees, Mulberry, the Prick Test column reads "4/15" and the I.D column reads "7/30." Does this mean I am allergic to the pollen from this tree? -- Jane, California

There are basically two different ways to report skin testing results.

One method uses 1+, 2+, 3+, and 4+, with 1+ being a very small reaction, and 4+ being the largest. One of the physicians I worked with said 1+ and 2+ are too small to be a true positive result. Thus we only reported 3+ and 4+ results. Other physicians sometimes report 1+ and 2+ as well.

I've not seen any standardized guidelines as to exact size of wheal that should fit each score. When I was taught this method, it was based, in part, on a comparison to the sizes of the saline (negative control) and histamine (positive control) tests. The AAAAI says that the key to scoring should be included with the test results, and must be based on measurement of wheal and flare.

Another way to measure skin test results is to measure the actual wheal size in millimeters (mm.) The AAOA courses teach that a wheal of 7 millimeters or greater wheal is positive.

A wheal without at least some degree of redness around it (also called erythema or flare) is usually considered not a result of a true allergic response. Thus many physicians report both wheal and flare results.

It is impossible to interpret another physician's test without knowing his criteria for reporting. But a 7/30 test result usually means a 7 mm. wheal and a 30 mm. flare.

My experience has been that when skin tests are administered properly and interpreted correctly by one of the above methods, the results are quite accurate.

This is evidenced by the degree of relief patients report when we give them shots that include the items which tested positive. It is also shown by the consistency of results when tests have been repeated two or three years later on patients who have never taken allergy shots.

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References:

Allergy Testing, American Family Physician, August, 2002.