Volume requirements will vary with the number of specific IgE assays requested.
Absolute minimum volumes for paediatric samples are 500uL blood for one allergen and 100uL for each additional allergen.
Test performed on weekdays.
Allergy to environmental and food allergens is common, and many patients will be referred for allergy testing. It is important to note that the available tests are for the detection of specific IgE (sensitisation) and are not necessarily diagnostic for clinical allergy. Allergen specific IgE can be detected in vivo (skin prick tests, SPT) and in vitro (previously known as 'RAST', now known as specific IgE tests (sIgE))
The presence of specific IgE does not always indicate the presence of allergic disease because up to 50% of positive tests, whether SPT or sIgE, will not be associated with a clinical disease. A patient may be sensitized to grass pollen but have no summer hay fever, or may have a positive test to peanut, but eat peanuts regularly without adverse effect. These individuals are sensitized, but not clinically allergic.
Conversely, patients occasionally have clinical allergy (even anaphylaxis) but no detectable specific IgE.
Testing for specific IgE is unlikely to be useful in patients where symptoms are not usually associated with IgE antibodies, e.g. chronic urticaria, irritable bowel syndrome, migraines etc.
It's therefore essential to take a detailed clinical history before ordering any tests and interpreting these tests requires an understanding of their relationship to allergic disease.
There are some situations where skin testing is preferred over serum sIgE testing, and some vice versa.
It is usual for SPT to be performed in preference to sIgE testing in the initial management of patients with food and environmental allergies, except when clinical circumstances make SPT unsafe or unreliable (e.g. patients who should not stop antihistamines, patients with dermographism).
Skin testing in the community laboratories is not available for all ages, nor for all relevant allergens, and for these patients sIgE may be preferable.
Which test is chosen will therefore depend on many factors including the allergen to be tested, availability of the tests, cost, and other patient factors such as eczema and medications, and clinical history.
If there is a strong clinical suspicion of allergy and an initial test is surprisingly negative then testing with the other type of test is appropriate.
Interpretation of SPT
These are always performed with positive and negative controls, using reputable reagents, and should be performed by trained staff. Some allergens are easily degraded and testing with fresh foods may be required, usually in specialist settings.
Interpretation of SPT results for patients with aeroallergen sensitization should be relatively straightforward in the context of the relevant clinical history (e.g. allergic rhinitis).
For patients with food allergy who have a convincing clinical history of an IgE mediated event shortly after food ingestion, e.g. urticaria, angioedema, anaphylaxis, a positive skin test can be taken as confirmatory. Where the history is less clear (e.g. a child thus far unexposed to a potential allergen but with other definite food allergies) there is data to suggest that a strongly positive skin test will predict likely allergic reaction on exposure.
Intradermal and fresh food skin tests carry higher risk of allergic reactions and are often more complicated to interpret. Such testing is generally limited to allergy specialists.
Interpretation of sIgE
The ADHB laboratory uses an ImmunoCap (Phadia) autoanalyser, as do other NZ laboratories. Results have historically been reported as a grade (0, 1+ to 4+) but recently quantitative reporting in specific allergy units KAU/L has also been made routinely available for all allergens. In the past, results <0.35KAU/L were designated as grade 0 (negative) but recent advances can now reliably detect levels as low at 0.05KAU/L. The clinical relevance of results in the 0.05 to 0.35 range (reported as Grade 0), is difficult to interpret. Patients may sometimes react to allergens with levels <0.35, even <0.05KAU/L, and sometimes may not react to allergens where high levels of sIgE are detected. This emphasizes the importance of the clinical history when interpreting these tests. If the clinical history is not suggestive of an allergic reaction to this particular allergen then results of this level are unlikely to be significant.
Skin testing has higher sensitivity, so where sIgE levels are unexpectedly negative, skin testing may be useful. Where sIgE testing for food allergen is being undertaken to support a clinical history of a food reaction, a positive sIgE may be taken as confirmatory of a diagnosis of allergy.
In the absence of known exposure to an allergen, the higher the sIgE level, the more likely there will be a reaction to that food on exposure, but the type and severity of reaction cannot be predicted.
Positive predictive values for a reaction have been identified for some allergens in particular populations, but are not readily applicable to a NZ community-based population and should be used with that understanding.
Component testing
In recent years individual allergens within foods or pollens have been identified. Testing for such components may have value in particular cases, such as to suggest the likelihood of persistence or severity of allergy, and help in distinguishing the cross reactivity that is often seen between pollen and food allergens. Their use is currently limited to allergy specialists.
Drug allergy testing
Skin testing is available at ADHB for some drugs including beta lactams, neuromuscular blockers and other anaesthetic agents. The sIgE test for penicillin has low sensitivity so must not be considered to exclude IgE- mediated penicillin allergy.
Venom testing
This is relevant where patients have a history of a systemic reaction to an insect sting. Commonly a sIgE is performed first, with subsequent skin testing (hospital based) where necessary.
Conclusions
Skin testing and sIgE are tests of the presence of specific IgE and are not diagnostic of the presence or absence of clinical allergy. Their usefulness relies heavily on a clinician's ability to take a careful allergy history and use that and the test data to discern between a confirmed clinical allergy and sensitization of no clinical relevance. If there is doubt about the interpretation of these tests, please consult an immunopathologist or clinical immunologist. Associate Professor Rohan Ameratunga, Dr Richard Steele and the immunopathology team. Contact immunology@adhb.govt.nz
Acknowledgement
We thank Drs Penny Fitzharris, Brian Broom and adult and paediatric immunologists at ADHB for their input.
References
Lack G. Clinical Practice Food Allergy. N Engl J Med 2008 Sep 18;359(12): 1252-60 Review
Sicherer SH , Sampson HA Food Allergy: Recent Advances in Pathophysiology and Treatment. Annu Rev Med 2008 Aug 19
For further information contact the laboratory (09) 307 4949 ext 22000 or:
Associate Professor Rohan Ameratunga , Immunopathologist: Locator 93-5724
Or the LabPLUS Immunology Team
Grass Mix
: Bermuda Grass, Rye Grass, Timothy, Meadow Grass, Johnson Grass, Bahia Grass
Nut 1 Mix
: Peanut, Hazel Nut, Brazil Nut, Almond, Coconut
Nut 2 Mix
: Pecan Nut, Cashew Nut, Pistachio, Walnut
Food mix
: Egg white, Cow's milk, Codfish, Wheat, Peanut, Soybean.
Environmental:
: Cat D.pteronyssinus, Grass
Please note:
If any of the food mixes (Fish, Nuts, Food) are positive, the individual components will be tested automatically.