The below tests are
useful in the differential diagnosis of
food-related allergies, but they may
quite frequently yield inaccurate
results. They are useful additions to a
careful history and observation of
food-related reactions.
The food history should include:
- Identification of suspect food
- What is the time lag between eating
and development of symptoms?
- What types of symptoms ensue?
- What amount of food is required to
cause reaction?
- Does the reaction occur with every
ingestion of the food?
- Does the reaction only occur under
certain circumstances, e.g. exercise or
stress?
- When did the last reaction occur?
- Are there other known allergies, e.g. hay fever,
asthma, eczema?
- Do other family members have asthma?
Skin prick testing
A negative test excludes an allergy with
a certainty of approximately 90%
(sensitivity). However, a positive test
confirms a specific allergy only in 50%
of cases. The quality and
standardization of tests differs widely
and these tests only apply to IgE
mediated allergies. The use of these
tests together with a careful medical
history can lead to accurate diagnosis
in approximately 70% of patients. Strong
reactions to skin testing may occur in
highly sensitive individuals.
Laboratory tests
Specific IgE blood levels can be
measured, for example using the radio-allergo-sorbent-test
(CAP RAST). However, IgE produced in the
intestines in response to ingested food
may not lead to raised blood IgE levels.
Similar to the skin tests, a negative
test excludes an allergy with a
certainty of approximately 90%
(sensitivity), but a positive test is
more difficult to interpret. However,
newer tests may allow a better
predictability of positive results in
egg, fish, milk protein, peanut and tree
nut allergies. Threshold values are being
defined for clinical use. IgE-independent measures,
such as ECP and EPX in the blood or
stool can be helpful in supporting a
suspected diagnosis of food allergy.
Basophil activation tests (BAT)
These new tests measure the expression of
particular activation markers on specific cells
involved in IgE-mediated allergies (basophils). They
have been validated in various food allergies and
complement the above conventional tests. The exciting
promise of these tests, however, is that they can
discriminate between sensitization (a laboratory phenomenon
demonstrated by IgE and skin tests) and clinical allergy. Further
testing is required to confirm the place of BAT tests in clinical
practice.
Food challenge testing
A useful but cumbersome form of testing is the
double-blind, placebo-controlled food
challenge by a specialist. Increasing
doses of the suspected food or a placebo
are given at intervals and the patient
is observed for signs of food allergy.
As these challenges carry a small risk
of severe reactions (anaphylaxis), they
should be performed under adequate
medical supervision. Food challenge
testing does not reliably distinguish
between intolerances and allergies and
tests are not well standardized. Open
food provocation tests carry similar
risks, but are less conclusive.
Elimination diets
Sequential elimination of specific food
groups from the diet may allow
identification of the offending food.
Standardized elimination diets are
available, with stepwise introduction of
new foods every 2-3 days and
documentation of food reactions and
symptoms using a food diary (See
Food diary). Such diets are best
performed under the guidance of an
experienced dietician.
If the above tests do not lead to a
clear diagnosis, exclusion of other
diseases is warranted.