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 mould allergy, for instance, a small amount of dried, powdered, sterilized mould is placed on the end of a toothpick and sniffed. Symptoms are expected to appear within five minutes if the individual is allergic to mould.
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 (radioallergosorbent 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 Dr Jonathan V. Wright, from Kent, Washington, who uses the RAST test a great deal. ‘Unfortunately, no other test comes as close to it in accuracy.’
A RAST test measures the amount of IgE (allergy-provoking antibody) in the blood. In contrast, skin tests merely measure the weal-and-flare – indirect evidence of IgE activity. More precise measurement of IgE activity by RAST means that, if needed, allergy injections can be started at a customized dose, and relief can be expected in three or four months. In contrast, allergy injections based on skin tests are begun at a lower estimated dose and gradually increased until the optimal dose is reached, which sometimes takes six months to a year. (Incidentally, IgE levels run higher in smokers than non-smokers, for some unknown reason. Be sure to let your doctor know if you smoke so that factor can be taken into consideration when interpreting your RAST test.)
For all its advantages, the RAST test is somewhat controversial. Aside from increased 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 doctors feel that the most accurate way of testing for food allergy is still elimination and re-challenge.