Doctors have at their disposal a variety of ways to test for allergy. But trying to get a consensus of opinion on which allergy tests are the best is like trying to find out which car is the ‘best’. Everybody has their own preference. For instance, some doctors say that the RAST test (a type of blood test) is the ‘best’ way to test for food allergy and that skin tests are the worst. Others say just the opposite. But whatever test your doctor uses, it’s important to realize that all allergy tests have shortcomings and that no matter what the results, you still must pay close attention to your diet and environment to help your doctor diagnose allergy correctly.
Traditionally, skin tests have been the techniques of allergy diagnosis. Here are several methods of skin testing.
1. Placing a drop of allergen extract (a diluted amount of the suspected substance) on the skin and scratching the surface of the skin (scratch test).
2. Pushing the test substance into the skin with a needlelike probe (prick or puncture test).
3. Injecting the test substance between layers of skin (intradermal or intracutaneous test).
When the allergen makes contact with the skin, mast cells release histamine and other allergy-inducing substances, which usually produce a ‘weal-and-flare’ reaction within ten to fifteen minutes if the test is positive. (If the test is negative nothing happens.) The raised weal, or welt, may vary in size from that of a mosquito bite to that of a large thumbnail; flare is simply another word for redness. To distinguish between irritation at being stuck with a sharp metal object and a genuinely allergic reaction, a separate test dose of plain salt water is applied also.
The scratch test is the least sensitive but the safest of the three skin tests, since the material can be wiped off if a severe reaction is unexpectedly triggered. And the scratch test is the least painful, although no one looks forward to any kind of skin test. (Children, especially, tend to fuss at encounters with needles.) Injected material, on the other hand, cannot be removed, but it most accurately reflects the allergic tendency, picking up levels of sensitivity so low that they often do not produce any symptoms.
The puncture test is basically a version of the intradermal test; once the fluid has been placed in the puncture hole, it essentially has been injected. And the puncture test is only slightly less sensitive than an intradermal test, without the added risk and discomfort. So unless your doctor can give you a good reason for choosing a scratch or intradermal test, the puncture test is the most preferable of the three skin tests.
None of these skin tests, however, is 100 per cent foolproof. In fact, the irony of skin tests is that they’re more accurate for people who suffer immediate, severe reactions – people who probably already know what they’re allergic to. They’re not as useful for diagnosing the cause of hives or eczema as they are for diagnosing the cause of asthma, hay fever or other classic allergy symptoms.
What’s more, skin tests can only be used to test for allergy to certain things. Except for penicillin, skin tests are not useful for diagnosing drug allergy. And even when skin tests for penicillin allergy are used, the only people to get tested are those who have a known allergy to penicillin but must take the drug for a serious infection. The only other use for a drug skin test is to establish the safety of egg-derived vaccine in children who have a history of egg, chicken or feather sensitivity.
As for detecting allergy to stinging insects, skin tests are notoriously poor. They’re also fairly unreliable when it comes to food allergy. For instance, a person may have a ‘positive’ skin test for egg, but if he can eat eggs with no problem the test is meaningless; for all intents and purposes, no allergy to egg exists. (A ‘positive’ test means a person has the allergy; a ‘negative’ test means there’s no allergy.)
In the case of pollen or dust, skin tests are not 100 per cent accurate. So the real proof is in the breathing.