A. Case study
B. More information
C. Editors' comments
E. CPD questions (South Africa, Australia)
A. Case study
A 5-month-old girl was taken to the doctor because she had developed
generalized eczema. She was on a cow's milk-based formula exclusively.
After taking a brief history, the doctor performed a skin prick test
for cow's milk on the girl. It was positive (10mm). The mother was asked
to change the formula to soy formula. The child's symptoms improved
dramatically. She still had very slight eczema at times, but it was
manageable with steroid creams. At the age of 13 months the eczema relapsed.
It seemed to be a constant rash (with no specific flare-ups). By this
age she was already eating many different foods, but she was still on
a soy formula (the follow-on formula).
What could be the cause of the reaction re-occurring?
a. She was being exposed to a hidden source of cow's milk.
b. Something had been added to her environment (e.g., exposure to house
dust mite, pets, stress, smoke, perfumes or fabric softeners) that she
could be reacting to or that could aggravate the reaction.
c. She was reacting to a specific food or foods in her diet.
a. A careful diet history was taken. No hidden sources of cow's milk
could be identified.
b. The mother said that the house was a bit dusty at times; they did
have a dog, but she had not seen the child having a reaction after contact
with the dog. The child was not under any type of stress. No one in
the house smoked. And she had changed the laundry detergent and related
items a couple of months before (to brands that would more certainly
not affect her child). House dust mite and the dog might therefore play
a role. However, there were no recent changes that would bring the child
into contact with these more regularly.
c. No one specific food could be identified as the cause, as the child
had a constant rash (i.e., there were no flare-ups that could be directly
associated with a specific food). The mother had, however, only recently
introduced fish into the child's diet.
What could be done at this point?
a. Consider other allergens as a possible cause by doing more skin prick
tests, or do serum-specific IgE tests.
b. Ask the mother to keep a food-symptom diary for the child, to determine
which food could be causing the problem.
c. Put the child on an elimination diet until the symptoms have disappeared
and then challenge the child to determine the culprit food.
a. This could be helpful.
b. The diary would probably not be helpful in the case of a child experiencing
constant eczema. One would not be able to pinpoint a specific food as
c. This is also a possibility, but only after skin prick tests are done.
A range of skin
prick tests were performed: house dust mite, dog (as she was exposed
to these in her environment), cow's milk, soya, wheat, egg and fish
(as these are 5 of the 6 main allergens; she had not been exposed to
peanut yet). The results were:
Histamine control: 8mm
House dust mite: 6mm
Cow's milk: 0mm
What do the results of the tests mean?
The results of SPTs cannot be interpreted on their own. One has to consider
the history, the symptoms, and the results of the other allergen SPTs.
a. House dust mite:
The SPT wheal was smaller than that of the histamine control, which
could have indicated a number of things (see below). The reaction could
explain the slight eczema that she continued to experience after the
exclusion of milk. She probably had been exposed to house dust mite
since birth and therefore could have been reacting to it since then.
b. Dog, soya and
Comparing these negative results with the histamine control and the
positive results of the other allergens, it is very unlikely that the
child was reacting to any of these. (SPTs have a >95% negative predictive
c. Cow's milk:
Why was the cow's milk test negative? Was the previous test done incorrectly,
or did the patient outgrow the allergy? Did this mean that milk could
be reintroduced into the child's diet?
The first test could have been done incorrectly, but the child's symptoms
did improve following the exclusion of cow's milk. A more likely reason
is that, because the child was not exposed to the allergen for the previous
8 months, the body had not produced any IgE to cow's milk during that
period and IgE would therefore not show up on the skin prick test, whether
the allergic tendency persisted or not. To outgrow a sensitivity to
an allergen requires a period of time that varies between individuals.
The longer the allergen is avoided, the more likely it is that the sensitivity
has been outgrown. This child may not have outgrown the allergy and
could still react to cow's milk if she ingested it. It was suggested
that the child be challenged with milk (to see whether she was still
sensitive to it and would react), but only once the cause of the current
eczema was established, the culprit food(s) excluded, and the symptoms
brought under control. Only then would it be possible to see the effects
of a challenge.
d. Egg and fish:
The child had started eating egg and fish only in the last couple of
months and therefore had probably developed a sensitivity to them.
To confirm egg and
fish allergy, the mother was asked to exclude all sources of these from
her daughter's diet until the symptoms disappeared. Thereafter, the
child could be challenged with each allergen to see whether she in fact
did react to each. The mother was also given guidelines on how to minimise
exposure of the child to house dust mite.
|TIP for Allergy
Information about the diagnostic tests that are useful or
applicable for assessing a specific allergen can be found
in the main menu for that allergen, under "Adverse Reactions",
"Diagnostics". This function lists the tests that
are available as well as the diagnostic value of each test.
B. More information:
primary tools to diagnose food allergy are the clinical history, physical
examination, food-symptom diaries, elimination diets, oral food challenges,
and allergy tests.1,2 Appropriately designed challenge testing remains
the gold standard, although it has many limitations.3
The clinician should
use the findings from the history and physical examination to establish
whether the patient experiences a food-induced allergic disorder and
whether an IgE-mediated or non-IgE-mediated mechanism is most likely
responsible. If an IgE-mediated mechanism is suspected, laboratory tests
can assist in the diagnosis, but there are no rapid, specific diagnostic
tests for non-IgE-mediated disorders (except in the research setting).4,5
tests have been developed to assist in the diagnosis of food sensitization.
These include multi-detection assays, immunoblotting, screening of basophil
activation (BAT or FAST), assays for leukotriene LTC4 release (CAST),
measurement of plasma histamine, serum tryptase, serum ECP (eosinophil
cationic protein), urinary EDN (eosinophil-derived neurotoxin), assay
of fecal IgEs, and RAST for specific IgG.3,6 As a group, these tests
are not very widespread or accessible. They have either been limited
to academic and research settings for various technical reasons, or
it is simply too early to evaluate their clinical usefulness.1 Most
of these are in vitro methods of diagnosis. The in vivo tests, on the
other hand, include skin testing and challenge procedures.1
The tests most often
used to diagnose IgE-mediated allergy are the skin prick test (SPT)
and the serum IgE test (total and/or serum-specific IgE). However, patch
tests have recently been used more often in the diagnosis of food allergy
as well. Numerous additional tests may be needed in various clinical
scenarios to assist in determining whether an adverse reaction to food
is the cause of a clinical problem. Among available resources are stool
culture, endoscopy with biopsy, total eosinophilic count, pH probe,
breath hydrogen, and reducing substances in the stools.1
The SPT and the
serum IgE tests indicate the presence of allergen-specific IgE, but
they do not necessarily indicate that an allergic reaction would occur
upon ingestion of the tested food.2,4 The presence of IgE antibodies
to a specific food merely indicates a certain probability of a clinical
reaction to that food; the risk levels are unique in each patient.1
This is explained in more detail below.
Review will focus on the use, benefits and limitations of skin prick
tests. Serum IgE (RAST) tests, atopy patch tests, how to take a thorough
clinical history, elimination diets, oral challenges, and controversial
allergy tests will be discussed in future issues.
THE SKIN PRICK
are used for allergy skin testing (e.g., scratch test and intradermal
test), but studies indicate that the SPT is the most useful technique
with the most predictable results.5 Variations exist on the
name and method of the SPT, including the prick-puncture skin test,
prick skin test and prick-to-prick method. This Review will, however,
focus on the most commonly used method.
the scratch test, a scratch is made on the skin and a drop of allergen
is deposited onto the site.5,7 Intradermal skin testing
(where the allergen is injected into the skin dermis from a syringe)
is not recommended for the evaluation of food allergy. This method
gives an unacceptably high false-positive rate, and has been associated
with systemic reactions, including fatal anaphylactic reactions.1,3,4,5,7,8
Skin testing is
simple, quick to perform with immediate results, relatively safe, cost-effective,
highly reproducible and, when properly performed and interpreted, very
informative as a diagnostic tool.1,9,10 Another benefit is
that the patient sees the reaction occurring on his or her arm (compared
to the blood test, during which the patient is not there); there tends
to be a better buy-in to the diagnosis. The SPT does, however, have
limitations, which will be discussed below.
The SPT involves placing a liquid extract of the allergen (1:10 or 1:20
dilution) on the skin and pricking the skin through it (with a device
such as a needle, bifurcated needle, probe, or lancet) so as to just
puncture the skin.1,11 It is unnecessary to scratch or lift the skin,
and no blood should be drawn. The test is best performed on the inner
aspect of the forearms, avoiding the flexures and the wrist area. It
may also be performed on the back.9 It must be performed on intact
skin; no active eczema may be present.
Any number from
1 to 25 allergens can be tested at a time, although it is more usual
to test only 3 to 6. Although the amount of allergen introduced into
the skin is very little, there have been reports of anaphylactic reactions
following skin prick testing, especially with allergens such as peanut
and bee sting.11
Up until recently,
it was believed that SPT could not be performed on children under the
age of 4 years, but there is no evidence to substantiate this. Although
a IgE response is not always present in young children, the test has
been anecdotally shown to be useful in detecting elevated IgE levels
to allergens such as soya and milk in breastfeeding infants. It may
therefore be of use, especially where taking blood from an infant is
skin test detects any IgE that has been produced by the immune system
through previous contact to the allergen. The allergen-specific
IgE is attached to the surface of mast cells in the skin. When the
test allergen comes into contact with the IgE, inflammatory mediators
are released into the surrounding skin, resulting in a small swollen
area at the site of application (the wheal) and the spreading of
histamine into the area surrounding it (flare; the flat, reddened
area). This is the wheal-and-flare reaction that is the cornerstone
of skin testing for allergy.7,12
The skin is also
punctured through positive (histamine) and negative (saline-glycerin,
the diluent used to preserve the allergen extract) controls.1,4,9
The test site is examined 10 to 20 minutes later.1,9,11
A positive skin test is indicated by the development of a wheal-and-flare
reaction. There are different ways to grade the extent of the reaction
that is experienced.7 The most often used is the measurement of the
wheal diameter. If the diameter is 3mm or greater than a negative control
(which should be 0), it is generally considered positive.1,4 If the
wheal is <3mm, it can be a sign of sensitisation or of the beginning
of the development of allergy. Increasing skin test wheal size correlates
directly with increasing IgE antibody to the allergen and increasing
probability of clinical reactions.1 The grading of 0 to 4+ is subjective
and no longer recommended.9
control: No response is expected to the negative control. If there
is a response, it may be that the skin is oversensitive to pressure
(as with dermatographism) and that the response to the test is a 'false
positive'.11 That is why a positive test is generally regarded as
one with a mean wheal diameter at least 3 mm greater than the positive
control: Everyone is expected to react to the positive control,
as it is a solution of histamine. If there is no response, it is possible
that something is preventing the skin from developing an allergic reaction.
A number of drugs can do this, including all antihistamine preparations,
some antidepressants, some cough mixtures and, under certain conditions,
preparations that contain steroids. These medications should, if possible,
be stopped for an appropriate period of time (e.g., 1 to 2 days with
a short-acting and up to 1 week with a long-acting antihistamine) before
the skin testing.9,11
The sensitivity and specificity of a test comprise its ability to identify
a known condition. Sensitivity is defined as the proportion of test-positive
patients in relation to the total number of patients who have the disease,
whereas specificity is the proportion of test-negative persons in relation
to the total number of patients who do not have the disease. Sensitivity
and specificity are affected by intrinsic properties of the test, but
the clinically important issue for the health professional is the probability
that a patient has food allergy if the test is positive (positive predictive
value, PPV) or does not have food allergy if the test is negative (negative
predictive value, NPV).1
of the SPT is usually high (>80%); the specificity is usually lower
than the sensitivity but usually better than 50% if good-quality food
extracts are utilized.1,10 The intrinsic properties of the test that
affect the sensitivity and specificity are factors such as the extract
concentration, the device used, the pressure applied, and the location
where the test is performed.1 More details are set out below.
Negative skin tests
for IgE-mediated allergy to the highly allergenic foods such as egg,
peanut, wheat, milk, fish, and tree nuts are considered to be accurate
about 95% of the time (i.e., NPV of >95%). However, negative reactions
to other food allergens, such as soy, have a significantly reduced rate
of accuracy, and some practitioners rate them no higher than 30 to 50%
(i.e., only 30 to 50% of the reactions are true). (Commercial extracts
from fruit and vegetables have an even lower accuracy, depending on
the speed at which the allergen denatures.) Positive skin tests have
a lower predictive value: less than 50% accuracy (i.e., PPV of <50%).5,7,11
SPT's are, therefore,
most valuable when they are negative; they are an excellent means of
excluding IgE-mediated food allergies. Positive SPT responses only "suggest"
the presence of symptomatic allergy.4,10 However, the larger the wheal,
the higher probability that it is a truly positive indicator. It follows
that a positive skin test in isolation cannot be considered proof of
clinically relevant hypersensitivity, whereas a negative test virtually
rules out IgE-mediated food allergy to the food in question. Oral food
challenge may be necessary to confirm food allergy, except where the
history is overwhelmingly convincing.13
It must be remembered
that, crucially, a negative result does not exclude the possibility
of cell-mediated allergic reactions or intolerance.1
In some patients
a delayed skin reaction can occur 3 to 5 hours after the skin test has
been performed, and it is important to remind all patients to look out
for these reactions. Delayed reactions are often associated with more
severe and significant allergies.9
selection of which skin test extract to use should be based on the
individual patient history. The indiscriminate use of large numbers
of different allergen tests often leads to more confusion than clarification.
If the history does not highlight possible culprits, but the symptoms
suggests a food sensitivity, it may be useful to perform a limited
number of skin tests to establish the probability that food allergy
is present. The major foods for which skin testing has been found
to be most helpful (as they have a high negative predictive accuracy,
as discussed above), include egg, milk, wheat, peanut, tree nuts,
fish, and shellfish.5
step in performing and interpreting skin tests may affect the final
interpretation. For example:
The commercial extracts that are currently available are not standardized
to a norm, and their potency varies from one food to another as well
as from one manufacturer to another.3
It has been suggested
that neo-allergens are created during the processing and digestion of
foods. For example, the allergenicity of peanuts is enhanced and/or
formed by the roasting process. In some cases, food allergens are destroyed
during processing, which is important in the case of patients who tolerate
the cooked but not the raw food.1,7 This variability may naturally
influence the effective diagnostic contribution of extracts, according
to their preparation. Also, the allergen stability of commercial extracts
over time varies from food to food. Extracts of the major allergens
retain their allergenicity for long periods, whereas most of the other
extracts that are commercially available, especially the fruits, denature
fairly quickly. (This may be the reason there is only a limited number
of extracts available for skin prick testing, compared to the hundreds
of RAST tests available.) The extract that is used for the SPT may therefore
no longer be in its natural form and would not elicit an immune response.7
It is evident that
the selection of source material, the extraction process, and the preparation
of stable skin test solutions are all important factors influencing
the suitability of extracts.1,3,14,15 The reliability of the extract
may, however, be enhanced by using fresh or native foods or using the
"prick-by-prick" technique (pricking the fruit and then the
patient with the same needle, thereby transferring the juice containing
the allergen).3 Reagents sourced in-house are more concentrated and
may intensify reactions (causing irritation), a possible drawback of
this method, as it may increase the risk of a false-positive reaction.1,8
The patient may be reacting to natural non-allergenic constituents (or
even the acidity) in the food, which will not be present in commercial
extracts. To obviate confusion, negative SPT responses with commercially
prepared extracts that contradict convincing histories of allergic reactions
to a particular allergen should be replicated with the fresh food before
any conclusion that the allergen-specific IgE is absent.
for pricking the skin and the technique used (the area, depth and pressure
at which the allergen is introduced) may influence the results. This
may affect the comparability of the skin prick test results from different
also vary according to their location on the body. For example, the
back is 20% more reactive than the arm.1,3,14
Below is a summary
of the imperfections of SPT, including some factors not covered, or
not covered in detail, above.
Reasons for false
positive tests include the following:
The food can cause a nonspecific ("irritant") positive skin
The allergen extract may have been contaminated with a substance
to which the person reacts.7
The test has been performed improperly.11
The skin on which the test is performed was not clean and free
of active eczema or dermatographism.9
The person has been sensitized to a cross-reactive allergen,
without having a clinical reaction to it. Other family members should
therefore be considered when evaluating test results.1,2,6,15
One always needs
to consider that a person may have specific IgE antibodies to a certain
food, and therefore be sensitized, but experience no symptoms to the
food when it is ingested.7,11
Reasons for false
negative tests include the following:
The person being tested is very young or very old (these groups
have suppressed skin reactivity).11 In children younger than 1 to
1.5 years, the NPV of the skin test is lower, probably by 80-85%.5
The person may react to the specific food, but the reaction may
not be mediated by IgE antibodies.11
The extract used is of poor quality or is out of date.7,11
The test has been performed improperly.11
The food extract used in the test is not identical to the food
in its natural state, e.g., raw rather than cooked.4,7 This would
be applicable to, for example, a patient who is sensitive to a heat-labile
allergen. Heating the extract will destroy the allergen and the person
would not react to the extract.
The skin used for the testing has been treated with topical steroids
and therefore produces smaller wheals than untreated surfaces would.4,16
Antihistamines interfere with the production of allergen-specific
wheals.4 The patient must exclude the use of antihistamines for an
appropriate length of time before the test is performed.1,16
If an SPT is done
and the serum IgE to the same allergen is measured, the results may
differ. The proposed reason for this is that IgE is attached to the
mast cells (which are evaluated with a SPT), but occurs freely in the
serum (which is evaluated when serum IgE is tested for). The concentration
of the IgE in different locations is an important issue. This is also
the reason antihistamines do not have to be avoided before the serum
IgE is measured. In patients with severe eczema, it is often very difficult
to stop using medication for a period. They are also more likely to
have active eczema at the sites where the SPTs would normally be performed.
||compiled by Karen du Plessis
Food & Allergy Consulting & Testing Services (FACTS)
PO Box 565
Comments by our editors
Janice M. Joneja Ph. D., RDN
The accurate diagnosis of food allergy is fraught with difficulties,
as this review reveals. No single test can definitively identify
the food components responsible for the clinical expression of
an immunological or non-immunological reaction to a food. In the
end, elimination and challenge must be undertaken to determine
the precise role of a food component in triggering symptoms. However,
although skin testing is still the only in vivo test that is universally
employed in clinical practice, its potential hazards have been
greatly underestimated. It is well known that many agents can
be effectively delivered to the body via the skin. Hormones, vaccines,
antitoxins and proteins are efficiently introduced into the body
via this route, circumventing the digestive tract and powerfully
targeting the effector system for which they are designed. There
is no reason to suppose that allergens delivered through the skin
by absorption (in a patch), by injection (intradermally) or by
pricking or scratching, should not induce allergen-specific IgE
in a similar manner. Many allergists will not skin test their
atopic patients with the highly allergenic foods, such as peanuts
and nuts, because they are aware that antigen delivered via this
route can trigger an anaphylactic reaction. It is only logical
to assume that primary sensitization can occur by this route also.
After all, vaccination using antigen delivered on skin patches
is proving very effective in protecting individuals from toxins
as powerful as ricin.
I have actively discouraged my patients from having skin tests
performed, especially on atopic children, because of the risk
of inducing IgE via this route. In good conscience I could never
condone any action that might result, in an extreme case, in a
life-threatening anaphylactic reaction. Even milder reactions
can result in a life-time of misery. Until I see well-conducted
scientific research that proves that there is no possibility of
immunological sensitization via the skin, I shall continue to
discourage my patients from undergoing this method of allergy
There are many alternative in vitro methods for detecting allergen-specific
IgE; RAST and ELISA tests have the potential for providing information
that in most cases is as accurate as any skin test. The cost and
need for laboratory facilities might limit their use for the present,
but refinement of the technique should make them more economical
and universally available in the near future. Awareness by clinicians
of the potential for primary sensitization to allergens through
skin testing will undoubtedly stimulate the speedy development
of safe in vitro tests.
Harris Steinman M.B.Ch.B.
circumstances skin prick tests can be very useful. For an accurate
interpretation of the test results, one has to evaluate them on
an individual basis and consider all variables involved.
For more information
on this subject and other allergy and intolerance related topics, visit:
To join a professional
food allergy discussion list where this subject can be discussed further,
go to http://groups.yahoo.com/group/AllergyDietitian
|We invite you to send us interesting
case studies. We pay US$100 for each case study we use in our newsletter.
To subscribe or
unsubscribe, send an e-mail to email@example.com
and put "subscribe Educational" or "unsubscribe Educational
" as the subject.
1. Metcalfe DD, Sampson HS, Simon RA. Food allergy: adverse reactions
to foods and food additives 3rd edition. Blackwell Publishing, 2003.
2. Sicherer SH. Beyond oral food challenges: improved modalities to
diagnose food hypersensitivity disorders. Current opinion in allergy
and clinical immunology 2003;3:185-8.
3. Bahna SL. Diagnosis of food allergy. Ann Allergy Asthma Immunol.
2003 Jun;90(6 Suppl 3):77-80.
4. Sampson HA. Food allergy. Part 2: diagnosis and management. J Allergy
Clin Immunol. 1999 Jun;103(6):981-9.
5. Bock SA. Diagnostic evaluation. Pediatrics 2003;111(6):1638-44.
6. Moneret-Vautrin DA, Kanny G, Fremont S. Laboratory tests for diagnosis
of food allergy: advantages, disadvantages and future perspectives.
Allerg Immunol (Paris). 2003 Apr;35(4):113-9.
7. Joneja JV. Dealing with Food Allergies. A practical guide to detecting
culprit foods and eating a healthy, enjoyable diet. Bull Publishing
Company, USA, 2003.
8. Williams LW. Skin testing and food challenges for the evaluation
of food allergy. Curr Allergy Rep. 2001 Jan;1(1):61-6.
9. Toerien A, Potter PC, Buys C. Appendix IX. The skin prick test. The
ALLSA handbook of practical allergy, 2nd Edition. Ince (Pty) Ltd., South
10. Sampson HA. Improving in-vitro tests for the diagnosis of food hypersensitivity.
Current opinion in allergy and clinical immunology 2002;2:257-61.
11. Wright T. Food allergies: enjoying life with a severe food allergy.
Class Publishing, London, 2001.
12. Poulsen LK. In vivo and in vitro techniques to determine the biological
activity of food allergens. J Chromatogr B 2001;756:41-55.
13. Williams LW, Bock SA. Skin testing and food challenges in allergy
and immunology practice. Clin Rev Allergy Immunol. 1999 Fall;17(3):323-38.
14. Roberts G, Lack G. Food allergy - getting more out of your skin
prick tests. Clinical and experimental allergy 2000;30:1495-8.
15. Vieth S, et al. Factors influencing the quality of food extracts
for in vitro and in vivo diagnosis. Allergy 1998;(Suppl 46)53:65-71.
16. Host A, et al. Allergy testing in children: why, who, when and how?
Questions (For South African dietitians only. Australian
dietitians: where you have relevant learning goals, CPD hours related
to this resource can be included in your APD log.)
You can obtain 2 CPD points for reading
this newsletter and answering the accompanying questions. This
newsletter with questions has been accredited for dietitians.
CPD reference number: DT04/3/040/13
HOW TO EARN YOUR CPD POINTS
1. Complete your personal details below.
2. Read the newsletter and answer the questions.
3. Indicate your answers to the questions by making a "X"
in the appropriate block.
4. You will earn 2 CPD points if you answer more than 75% of the
questions correctly. If you score is between 60 and 75%, 1 CPD
point will be allocated. A score of less than 60% will unfortunately
not earn you any CPD points.
5. Make a photocopy for your own records in case your answers
do not reach us.
6. Cut and paste the area indicated below into a e-mail message
and e-mail it to firstname.lastname@example.org
to be received no later than 31 May 2004. Answer sheets received
after this date will not be processed.
PLEASE ANSWER ALL THE QUESTIONS
(There is only one correct answer per question.)
1. Which of the following is the most effective means of diagnosing
(a.) Clinical history and physical examination
(b.) Elimination diet and oral food challenges
(c.) Skin prick and/or serum-specific IgE tests
(d.) A combination of the above
2. Which of the
following is not true regarding the benefits of skin prick tests?
(a.) Quick to perform, with immediate results
(c.) Safe, even for persons who have experienced anaphylactic reactions
to the allergen
(d.) Highly reproducible
3. True or false:
Skin prick tests cannot be performed on children under the age of 4
4. Which of the
following is not true regarding the interpretation of skin prick tests?
(a.) The wheal size should always be compared to the positive and negative
(b.) The negative predictive value of skin tests for the highly allergenic
foods such as egg, peanut, wheat, milk, fish, and tree nuts is >95%.
(c.) The positive predictive value of skin tests is >50%.
(d.) The larger the wheal size, the higher the probability that the
skin test is truly positive.
5. True or false:
A positive skin test in isolation cannot be considered proof of clinically
relevant hypersensitivity, whereas a negative test virtually rules out
IgE-mediated food allergy to the food in question.
6. True or false:
In some instances, a skin prick test performed using the prick-to-prick
technique will be more accurate than one using commercial extracts.
7. True or false:
The materials used for pricking the skin and the technique used (the
area, depth and pressure at which the allergen is introduced) may negatively
influence the results.
8. Which of the
following is not a reason for a false-positive skin prick test?
(a.) The food can cause a nonspecific ("irritant") positive
(b.) The skin on which the test is performed was not clean and free
of active eczema or dermatographism.
(c.) The person has been sensitized to a cross-reactive allergen, without
having a clinical reaction to it.
(d.) The person being tested is very young or very old.
Cut and paste this section below into an e-mail message
Skin prick testing
CPD Reference number: DT04/3/040/13
HPCSA number: DT
Surname as registered with the HPCSA:
Please make an "X"
in the appropriate block for each question
a [ ] b [ ] c [ ] d [ ]
a [ ] b [ ] c [ ] d [ ]
a [ ] b [ ]
a [ ] b [ ] c [ ] d [ ]
a [ ] b [ ]
a [ ] b [ ]
a [ ] b [ ]
a [ ] b [ ] c [ ] d [ ]