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2.2 Allergy Tests


This article is from the Children Allergies and Asthma FAQ, by Eileen Kupstas Soo kupstas@cs.unc.edu with numerous contributions by others.

2.2 Allergy Tests

Blood tests can be done to look for elevated white blood cell counts,
level of particular antibodies, or for reactions with allergen
extracts. Blood tests are not 100% reliable and, to get good results,
must be done by highly trained lab technicians. Some doctors use these
tests, while others prefer not to. The patient will need to have blood
drawn for this, which may be a drawback for testing children.

"Scratch" test involves scratching the skin, then dropping liquid
allergen on the scratch. It's done on the arm or (for very small
children) the back. It seems to hurt a little, but may be scary to
little ones. [Ed. - it doesn't hurt much; it's just annoying.] Each
slate has up to 6 tests, plus positive (histamine) and negative
controls. Bumps/weals for a reaction appear immediately or several
minutes later. The patient must remain in the office in case of severe
reaction (rare).

A positive reaction is reliable, but a negative reaction may not be;
that is, you may be allergic but not react. Skin tests are more
reliable for airborne allergies than for foods.

Elimination diets are the only guaranteed way to determine food
allergies. The patient goes on a *very* restricted diet, composed
only of foods that rarely cause allergy problems. A new food is added
each week. If the patient does not have any allergic symptoms to the
new food during that week, then it is not considered an allergen. A
new food can be added the next week. If the patient has a reaction to
the food, the food is considered an allergen and removed from the
diet. The patient then goes back to the previous diet until all
symptoms are gone for three days; then a new food can be added. This
is a very slow way to build up much of a varied diet, but it is
certain. In adding foods, you must be careful that it is only one food
that is being added. This means no processed foods (may have
additives), no pre-packaged foods (may have additives), no seasonings
(except salt), etc. This can be very difficult to follow if you eat
out for any meals. Generally safe, non-allergenic foods usually
include apricots, peaches, pears, beets, sweet potato, rice, distilled
or spring water, cane sugar, salt, tapioca, olive oil, lamb and
chicken. (Not very exciting..) Your doctor may give a different list,
based on your personal situation. It is fairly easy to put a young
baby on an elimination diet, but it gets harder as the child gets
older. For very young children, this should only be done under a
doctor's supervision (unless the child is exclusively breastfed) to
insure a balanced diet.

If the original allergic reaction was moderate or worse, you must
challenge test when adding a suspected new food. That is, you start
with a pea-sized piece. If no reaction, on day 2 try a 3-pea-sized
piece. No reaction, day 3 try a 9-pea-sized piece. Discuss this with
your doctor. ALWAYS get instructions beforehand (and medicine, if
necessary) on what to do for a severe reaction. If the original
reaction was severe, your doctor will want to do this at his office or
at the hospital.


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