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 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.

Skin Tests

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 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 (discussed in What Is an Allergy?) release histamine and other allergy inducing substance, which usually produce a ”wheal and flare” reaction within 10 to 15 minutes if the test is positive (if the test is negative nothing happens).

The raised wheal, 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 test, since the material can be wiped off if 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 are 100 percent foolproof. In fact, the irony of skin test is that they're more accurate for people who suffer immediate, severe reaction – 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 of 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 exist (A ”posititve” test means there’s no allergy). In the case of pollen or dust, skin test are not 100 percent accurate.

So the real proof is in the breathing. A skin test to ragweed may be positive. But if the individual can breathe freely from August 15 to October 1 – prime ragweed season – that person is not allergic to ragweed, no matter what the test says.

Fasting

Some of the many doctors who find skin tests to be unreliable in revealing food allergy use fasting – no eating for a few days – as a test. After all, the simplest way to find out if you’re allergic to food is to not eat any and see what happens. Usually, if symptoms are due to a food allergy of any kind they get worse during the first, second or third day without food, but disappear by fourth or fifth.

Then eating is resumed. During a fast, drinking plenty of water is essential. The body can live off it's fat reserves for several days, but it needs water daily. Distilled water is best during a fast, since a few people are sensitive to ingredients in tap water.

Still, going without food is stressful, and most physicians who use fasting as a diagnostics tool do not recommended that people try it on their own, especially if they have diabetes, hypoglycemia, are underweight or suffer any chronic illness. Ideally, a person on an allergy fast should be away from the home, school or work environment to avoid allergens which may be reinforcing (or confusing) any reactions to food.

In some cases, that calls for hospital controlled fasting. Because of both the inconvenience and stress on the body, allergy doctors fell that fasting is the best reserved for highly allergic people. As an alternative to a total fast, some doctors will allow an individual just one food at each meal during the test period (three to five days). That’s monotonous, but less grueling.

Inhalation Challenge (Nasal and Bronchial Provocation)

Because skin tests sometimes miss the mark entirely, some doctors use an inhalation challenge for allergy to inhalants. To test for mold allergy, for instance, a small amount of dried, powdered, sterilized mold is placed on the end of a toothpick and sniffed.

Symptoms are expected to appear within five minutes if the individual is allergic to mold. So far, these tests all carry an element of anxiety – the fear that you will react. No wonder – that’s the whole idea. The ideal allergy test, from the patient’s point of view, is one that’s accurate, yet doesn’t risk a reaction. Enter the RAST test.

RAST (Radiollergosorbent Test)

The RAST test measures the amount of IgE in your blood. It's certainly safe, since the test is conducted on a blood sample in the laboratory. Anaphylactic shock, which occasionally occurs with skin testing, is impossible with a RAST test (although you still have to endure the discomfort of a needle).

RAST is more sensitive than a puncture or scratch test. ”One of the criticisms of the RAST test is that it's expensive,” comments Jonathan V. Wright, M.D., from Kent, Washington, who uses the RAST test a great deal, ”Unfortunately, no other test comes as close to it accuracy.” A RAST test measures the amount of IgE (allergy provoking antibody) in the blood.

In contact, skin tests merely measure the wheal and flare – indirect evidence of IgE activity. More precise measurement of IgE activity by RAST means that, if needed allergy shots (discussed on Immunotherapy – a Matter of Choice) can be started at a customized dose, and relief can be expected in three or four months.

In contrast, allergy shots based on skin test are begun at a lower estimated dose and gradually increased until the optimal dose is reached, which sometimes take six months to a year (incidentally, IgE levels run higher in smokers than nonsmokers, for some unknown reason. Be sure to let your doctor know if you smoke so that factor can be taken into consideration then interpreting your RAST test).

For all it's advantages, the RAST test is somewhat controversial. Aside from increase cost, doctors must resist the temptation to rely on it too heavily for diagnosis. For instance, many people react positively to both skin and RAST tests for cereal grains, but eat them routinely with no ill effects.

Like any allergy test, the RAST is meant to supplement, not replace, a good, thorough medical history. Consequently, many physicians feel that the most accurate way of testing for food allergy is still elimination and re-challenge, described in Rotary Diets.