USE OF SIMPLE SKIN TESTS TO DETECT FOOD
ALLERGY
When applied to testing for food
allergies, the optimal type of skin test
to use depends strongly on the type of
food allergy which is being sought. When
positive, patch, scratch, prick, and
single intradermal skin tests are
helpful. But, because food allergies are
immunologically more complex than
inhalant allergies, these simple tests
markedly underestimate food induced
reactions in many patients. The reason
for this insensitivity of simple skin
tests is because there are two different
clinical types of food allergy : fixed
and cyclic. Fixed food allergies are
typical Ige mediated reactions,
identical to those seen in inhalant
allergy. Because fixed food allergies
are usually immediate and dramatic, they
are normally easily recognized, and can
be confirmed by any skin test technique.
However, due to the hazard of provoking
anaphylaxis by skin testing for fixed
food allergies, these reactions are
usually confirmed by using (I>in vitro
blood tests.
CYCLIC FOOD ALLERGIES
Cyclic food allergies are not typical
IgE mediated reactions, although small
amounts of IgE may be involved in their
pathogenesis. It is believed that cyclic
food allergies are produced by immune
reactions of Gel and Coombs type III
(immune complex) with variable
contributions from the three other types
of Gel and Coombs reactions. Unlike
fixed food allergies, cyclic food
reactions often show delayed onset of
symptoms after eating a sensitive food,
and symptom severity is related to both
the quantity of food ingested and to the
frequency with which it is eaten. This
is a very different clinical picture
from fixed food allergies, and means
that cyclic reactions are not easily
recognized, unless the physician is
familiar with the cyclic allergy concept
and knows how to evaluate the patient
for this problem. For cyclic food
allergies, simple skin tests are seldom
positive, because, in contrast to fixed
food allergies, a large amount of
antigen must be injected in order to
trigger enough mediator release to
produce a detectable skin wheal.
Herbert Rinkel was one of the first
allergists to realize that food allergy
differed significantly from inhalant
allergy, and he sought to develop better
methods to test for the more difficult
to diagnose cyclic food reactions.
During Rinkelıs lifetime, IgE had not
yet been discovered, and almost nothing
was known about the immunology of
allergy. However, he compiled careful
clinical observations of the reactions
of thousands of patients to skin tests,
oral challenge feeding tests, and
exclusion diets. By 1932, he had
developed the concepts of fixed and
cyclic food reactions, and by 1934,
Rinkel had developed the basic procedure
for the deliberate individual food test,
now usually termed the oral challenge
food test (OCFT). This unblinded test
remains the most accurate simple test
for food allergies, and is a major
advance over simple skin tests, since it
is capable of detecting both fixed and
cyclic food allergies. In practice, oral
food challenges, like skin tests, are
not normally used to confirm suspected
fixed food allergies that may cause
anaphylaxis. The OCFT also has
disadvantages: it is slow to perform,
requires considerable will power from
patients, requires a major physician
time commitment, and can be subjective,
if observable signs and symptoms are not
produced. Blinded oral challenges are
not as subjective, but are even more
difficult to perform, and may not test
with sufficiently high doses of food to
cause symptoms. Therefore, a good
diagnostic skin test for cyclic food
allergies was sought.
USE OF SET TO DETECT CYCLIC FOOD ALLERGY
Skin endpoint titration (SET), also
known as serial dilution endpoint
titration, is a quantitative
modification of intradermal testing
which tests multiple antigen
concentrations to establish the precise
amount of antigen required to produce a
definite skin reaction. SET has been
approved as both useful and effective by
the Panel on Allergy of the AMA Council
on Scientific Affairs. About 1958,
Carleton Lee studied SET responses to
food antigens in an attempt to find ways
to desensitize cyclic food allergy
patients. He knew, from Rinkelıs work,
that large intradermal doses of food
antigen could produce both symptoms and
skin wheals in sensitive (but
non-anaphylactic) food allergy patients,
and that these symptoms could be
replicated by oral challenge feedings.
When he adapted Rinkel's technique for
treatment of pathogenic fungal allergy
by using intradermal doses weaker than
the SET endpoint, Lee discovered that if
symptoms were produced by a strong food
extract injection, subsequent injection
of a weaker dilution could neutralize,
or extinguish, those symptoms (Figure
1). Lee then learned that still weaker
dilutions below the neutralizing dose
could also provoke symptoms (underdosing),
and that again injecting the
neutralizing dose could relieve them. He
began to apply this knowledge to
treatment, and found that food antigen
injections, based on SET diagnosis, were
frequently helpful to food allergy
patients. Rinkel, and subsequently his
colleagues and students, confirmed, and
then developed variations of Lee's
procedure.
Figure 1. Carleton Leeıs
Observations of Cyclic Food Skin Test
Titrations
****
THE INTRACUTANEOUS PROGRESSIVE DILUTION
FOOD TEST (IPDFT)
Rinkel was primarily interested in
diagnostic food allergy skin testing,
since he believed strongly that diet
manipulation was the best treatment for
food allergy. However, since his death,
both diagnosis and treatment of food
allergies based on SET have become
popular, and many test variations have
been developed. These tests differ
mainly in technical details of initial
test dose strength, wheal size, and in
the route of food extract administration
(sublingual, intradermal, or
subcutaneous). The most substantial
difference lies in whether or not skin
whealing is evaluated, or whether just
symptom production is analyzed. The AAOA
addressed this issue, and also evaluated
the accuracy, safety, and usefulness of
a standardized SET skin test technique
for diagnosis and treatment of food
allergy. The 1988 multi-center
comparison study of food testing
techniques lead to a consensus protocol,
the IPDFT, that has been adopted by the
AAOA for purposes of uniform teaching of
in vivo food testing. During the study,
both skin whealing and symptom
production were recorded and analyzed.
Both were found to be valid measures of
the presence of cyclic food allergy.
When compared to the results of oral
feeding challenges, symptom production
was found to be very specific, but only
moderately sensitive. On the other hand,
skin whealing responses were found to be
nearly as specific, and significantly
more sensitive, than symptom production.
For this reason, when using the IPDFT,
symptom production is not required,
since a positive result may be
determined from the skin whealing
response alone. The IPDFT was compared
with both skin prick tests and in
vitro tests, and was found to be the
most sensitive and specific diagnostic
test for cyclic food allergy. While the
IPDFT did not always correlate with the
OCFT, it gave identical results in about
80% of cases, and was far better than
any other test. Note that although
conceptually identical to inhalant
allergy SET testing, there are
substantial technical differences when
testing for food allergies, and certain
added safety precautions are necessary
when performing these tests. Therefore,
the IPDFT technique should not be used
without first attending an accredited
instructional course, and developing a
clear understanding of how to suspect,
diagnose, and treat all types of food
allergies.
PRINCIPLES OF SET APPLIED TO IPDFT
TESTING
In order to understand the IPDFT, first,
inhalant SET testing must be understood.
A standardized protocol for performing
SET testing has been published by the
American Academy of Otolaryngic Allergy
(AAOA). In SET testing, successively
more concentrated inhalant antigen
solutions are injected, usually
beginning with a dilution of 1: 312,500
weight/volume (W/V). This starting
concentration introduces into the skin
about half the antigen as does a
standard prick test using 1: 50 W/V
antigen, and because it uses such a
small initial antigen dose, is a safe
method for testing for IgE mediated
inhalant allergy. The sequence of
negative, positive, and then larger
positive wheals as antigen concentration
is increased, confirms that the antigen
titration is on the ascending portion of
the dose-response curve, where wheal
size is proportional to antigen dose
injected. The endpoint is defined as the
first reactive wheal, at least 2 mm
greater than the negative control, that
initiates progressively greater
reactions with further increases in
antigen concentration. It has been found
experimentally that skin wheals in the 4
mm to 15 mm size range lie within the
linear portion of the dose-response
curve, where wheal size and antigen dose
can be easily correlated. Because of
this, only injection volumes of between
0.01 cc and 0.05 cc (yielding initial
wheal sizes from about 4 mm to 7 mm) are
used with SET.
For cyclic food testing using the
IPDFT, test dilutions, numbered from
strongest (#1) to weakest (#6), are
prepared using glycerin free phenolated
saline diluent (see Table 1). Fivefold
(1: 5) dilutions are used because they
are the best compromise between accuracy
and efficiency for clinical use.
Normally, 1: 10 W/V food antigen
concentrates, preserved with 50 gm/100
cc (%) glycerin, are used. Larger wheals
(0.05 cc, 7 mm) are used than in
inhalant testing, because sizable
amounts of antigen must be used in order
to detect mediator release from cyclic
food induced immunologic skin reactions.
In IPDFT, the endpoint of the titration
is defined as the strongest dilution
(maximum concentration of antigen) which
is non-reactive. A positive IPDFT
reaction is determined when intradermal
injection of 0.05 cc of an antigen
dilution produces, after ten minutes, a
wheal which is 2 mm, or more, larger
than the negative control. A negative
IPDFT reaction occurs when an antigen
injection produces, after ten minutes, a
wheal which is less than 2 mm larger
than the negative control. As in
inhalant SET testing, only wheal size is
measured, and flares are not recorded.
This definition differs from the
inhalant SET endpoint definition, but
because of the difference in wheal
injection volumes, the endpoint
concentration of antigen determined by
both methods is identical. IPDFT testing
is then seen to be essentially reverse
SET, with progression from a known
positive test (stronger dilution) to a
negative test (weaker dilution). The
reverse titration is only possible
because of the unique nature of cyclic
food allergies which allows safe use of
strong food antigen concentrations for
testing. NEVER use the IPDFT
technique to test for inhalant allergy
or for fixed, anaphylactic food allergy,
because it is not safe to test for these
IgE mediated allergies with initial
strong antigen concentrations.
In IPDFT, since strong antigen
concentrations are being tested, and
since these contain significant amounts
of glycerin that may cause skin
irritation, it is critically important
that antigen wheals be compared with
negative control wheals that contain the
same concentration of glycerin.
Therefore, glycerin controls are also
prepared as 1: 5 dilutions with
phenolated saline from a 50 % glycerin
concentrate.
|
Table 1. |
|
IPDFT Skin Endpoint Titration
Dilutions
|
|
Dilution |
Antigen
Concentration (W/V) |
Glycerin
Concentration (%) |
|
Concentrate |
1:
10* |
50* |
| #1
|
1:
50 |
10
|
| #2
|
1:
250 |
2
|
| #3
|
1:
1,250 |
0.4 |
| #4
|
1:
6,250 |
0.08 |
| #5
|
1:
31,250 |
0.016 |
| #6
|
1:
156,250 |
0.003 |
* Concentrates are not normally
used for intradermal testing.
|
IPDFT
TESTING TECHNIQUE PRECAUTIONS
Because cyclic food allergies involve
frequent eating of large amounts of the
sensitizing foods, and thus, significant
quantities of these foods are in the
body at the time of testing, IPDFT tests
do not usually increase the antigen
level enough to trigger dangerous
reactions. Ideally, each tested food
should have been eaten within 24
hours of testing to minimize the
risk of producing symptoms. Testing a
food that a patient has been avoiding is
hazardous, since it may be an
anaphylactic food. Never test a
known or suspected anaphylactic food,
unless you are prepared to treat
anaphylactic shock. Deaths have
occurred from attempting to skin test
patients with anaphylactic, fixed food
allergies. Also, patients should be
carefully questioned for any history of
a past serious allergic reaction or
severe asthma, and asked about
medications, such as beta blockers,
which may increase the risk of testing.
These brittle patients are best tested
by experienced personnel, or else in
vitro testing (with it's
limitations) should be used. With these
precautions, skin testing for cyclic
food allergies is a safe procedure.
PERFORMING IPDFT TESTS
Beginners should test one food at a
time, individually. Starting with
individual food tests allows one to
easily learn to apply and read the
test wheals, gives a chance to
observe some simple symptom
production, and demonstrates the
concept of neutralization in an
uncomplicated, easily interpreted
situation. Clinical experience has
shown that it is safe to begin
testing for cyclic food allergies
with much larger quantities of
antigen than can be used when
testing for fixed, anaphylactic food
allergies, or for inhalant
allergies. In most cases, in non-
brittle patients, IPDFT testing may
begin with a # 1 dilution of food
extract ( 1: 50 W/V). However, when
first beginning to use the IPDFT, a
# 3 starting dilution ( 1: 1,250
W/V) is recommended. Rarely, weaker
dilutions may be appropriate for
initiating testing. Consider, also,
that initiating testing at too great
a dilution may cause an underdose
reaction (see above).
-
Single IPDFT Tests
After first checking skin reactivity
with a positive histamine control,
begin the IPDFT with application of
a # 3 food antigen dilution and the
corresponding # 3 glycerin control.
Use standard food wheals of 7 mm
diameter, produced by injection of
0.05 cc of antigen solution. After
ten minutes, the wheals are measured
and the antigen wheal is compared
with the control glycerin wheal.
Since the # 3 glycerin control will
rarely show wheal growth, most # 3
antigen wheals that grow will be
true positives.
-
Positive Wheals
Any # 3 antigen wheal that is 2 mm,
or more, larger than the control is
positive, and indicates an allergic
food. All positives must be further
tested to determine the degree of
sensitivity and find the endpoint.
-
Negative Wheals
Any # 3 antigen wheal that is less
than 2 mm larger than the control is
negative, but this does not
indicate that this is not an
allergic food: these negative tests
must be completed. We know from
inhalant testing that, after a
negative test, it is safe to jump up
two dilutions, or 25 fold, for the
next test. Consequently, a negative
# 3 food test is followed by a food
dilution # 1 test, and a
corresponding # 1 glycerin control.
Subsequent test wheals are normally
placed to the right of prior wheals,
in horizontal rows, and separated
far enough (at least 1 cm between
wheal edges) that the wheals do not
contact each other.
-
Food Endpoints
Since the endpoint in food testing
is the strongest non-reacting
dilution, once a positive wheal is
produced, the titration proceeds one
dilution weaker, step by step, until
a negative wheal is produced. To
determine the endpoint of a #3
dilution positive food test, the
next most dilute antigen solution (
# 4 ) is now placed, allowed to
develop for ten minutes, and again
compared to the appropriate glycerin
control. Any # 4 antigen wheal that
is less than 2 mm larger than the #
4 control is negative, indicating a
completed test. This first negative
wheal is called the endpoint of the
food test, and can be used to
determine a treatment dose for
neutralization therapy. Successively
weaker dilutions must continue to be
applied for each positive food until
a negative wheal is finally
obtained. A food cannot be said to
be non-allergic unless it is
negative at the strongest ( # 1 )
dilution. Most endpoints in
non-brittle patients range from
dilution # 2 to dilution # 4.
-
Glycerin Controls
Matching glycerin controls must be
used for each antigen dilution. # 1
glycerin wheals frequently show
growth, and # 2 wheals often do
also. Rarely, # 3 or # 4 glycerin
will also show positive wheal
growth. Despite these irritative
reactions, if true food allergy is
present, antigen containing wheals
will normally grow 2 mm or more
larger than the corresponding
glycerin dilutions, and so will
still be detectable. In patients
that have a good history for food
allergy, but antigen and control
wheals are of similar sizes and no
positives are found, or if symptom
production occurs without a positive
wheal, then it will be necessary to
use diagnostic oral food challenges
instead of skin tests.
-
Multiple IPDFT Tests
After gaining some experience food
testing, beginners can consider
testing from one to three foods
simultaneously, each in a separate
horizontal row of wheals. Multiple
tests are exactly like individual
food IPDFT tests, except that when
multiple tests are done at the same
time, symptom production is less
likely to occur. Multiple tests have
been used by many individuals, but
Walter Ward and William King were
primarily responsible for perfecting
and standardizing the technique for
the AAOA. Multiple testing is more
time efficient, and therefore, most
experienced testing personnel
usually learn to do it. However,
whenever many allergen skin tests
are done, the total allergen load
may be large enough to produce
systemic symptoms, and,
consequently, experience is needed
to enable the tester to know when to
stop testing. As a general guide, no
more than ten cyclic foods should be
tested simultaneously, even by the
experienced. It is suggested that
prior to attempting multple testing,
the responsible physician and their
testing personnel attend the AAOA
Advanced Allergy Course.
EXAMPLES OF INDIVIDUAL IPDFT
TESTS
Example 1.
Use of an Individual Test by a
Novice Food Tester, on a
Non-Brittle Cyclic Food Allergy
Patient.
***
This test must be followed up
by testing the next most dilute
solutions of antigen and control
(see below). The second set of
wheals is now applied.
***
In this example, the test for
wheat is positive, since the # 3
antigen wheal is 2 mm larger
than it's control wheal, and the
endpoint is # 4, which is the
strongest non-reactive dilution.
End of First Example.
Example 2.
Use of an Individual Test by a
Novice Food Tester,on a
Non-Brittle Cyclic Food Allergy
Patient.
***
This test must be followed up
by testing more concentrated
solutions of antigen and
control. Since we know it is
safe to advance 25 fold, we
progress to testing the # 1
dilution (see below). The second
set of wheals is now applied.
***
Since we do not test
concentrates on the skin, this
test is finished. The control
wheal grew just as much as the
antigen wheal, therefore the
growth is due to glycerin
irritation, and not due to a
food allergy. Even with this
negative test, if the clinical
history suggests milk allergy,
you should still proceed to do
an oral food challenge ! End of
Second Example.
SOURCES OF ERROR
The IPDFT is a straightforward
test with high reliability,
provided that there is prior
good working knowledge of SET
inhalant testing and familiarity
with the concept of cyclic food
allergy. As long as wheals are
correctly made, spaced far
enough apart to not interfere
with each other, and proper
glycerin controls are employed,
valid endpoints should be
produced by all tests. However,
whenever a test yields a
questionable result, it should
be repeated. The most critical
aspect of testing is careful
patient and food selection, so
that no anaphylactic foods are
tested for, and no high risk
patients are tested without
advance knowledge and making
proper preparations for possible
emergency treatment. Finally, it
must be appreciated that the
results of this test, like all
others, are only as good as the
interpretive power of the
ordering physician. Knowing when
to use in vitro testing,
oral food challenge testing, or
a diagnostic diet instead of, or
in addition to, the IPDFT,
remains a matter of the
physicianıs knowledge and
experience.
SUGGESTED REFERENCES
1. Gordon BR: Allergy Skin
Tests for Inhalants and Foods:
Comparison of Methods in Common
Use. Otolaryngology Clinics N A
30: in press 1997
2. Gordon BR: Prevention and
Management of Office Allergy
Emergencies. Otolaryngology
Clinics N A 25: 119-134 1992
3. Gordon BR: Allergy Skin
Tests and Immunotherapy:
Comparison of Methods in Common
Use. Ear Nose Throat J 69: 47-62
1990
4 King WP, King HC: The
Evolution of Otolaryngic Allergy
Practices. Ear Nose Throat J 69:
11-26 1990
5. King WP, Rubin WA, Fadal
RG, et al: Efficacy of
Alternative Tests for Delayed
Cyclic Food Hypersensitivity.
Otolaryngol Head Neck Surg 101:
385-387 1989
6. King WP, Rubin WA, Fadal
RG , et al:
Provocation-Neutralization: A
two-part study. Otolaryngol Head
Neck Surg 99: 263-277 1988
7. Lee CH: Titration Method
of Food Testing and Treatment.
Buchanan Co Med Bull (St.
Joseph, MO) 15: 9-12, 1961
8. Mabry RL: Skin Endpoint
Titration: AAOA Monograph
Series. New York. Thieme 1992
9. Miller JB: Food Allergy,
Provocative Testing and
Injection Therapy. Springfield,
IL. Charles Thomas Co. 1972
10. Rinkel HJ, Randolph TG,
Zeller M: Food Allergy.
Springfield, IL. Charles Thomas
Co. 1951
11. Rinkel HJ, Lee CH , Brown
DW, et al: The Diagnosis of Food
Allergy. Arch Otolaryngol 79:
71-79 1964
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