Skin Prick and Specific-IgE Tests

There are only two scientifically validated allergy tests for the investigation of IgE-mediated food allergy; the Skin Prick Test (SPT) and Specific-IgE antibody testing (previously called RAST testing, now called Immuno-Cap Testing). Dr Du Toit makes use of both these validated tests as well as the recent ISAC testing and IgE component testing.

Dr Du Toit will always take time to obtain a rigorous clinical history prior to ordering state of the art allergy tests. The clinical history is very important in the practice of allergy, as it guides the testing to follow. Never trust an allergy test that is taken on the high street without a clinical history. In young children it is important to realise that food allergy may masquerade as a ‘food aversion’. For example, babies with egg allergy may refuse - rather than react to - egg, thus asking, “does your child have egg allergy?” to the parents may be misleading, even if the parents answer “no”. The history aims to establish if the child can eat an age-appropriate quantity of the food. For instance, a non-allergic 5 year old child should be able to tolerate a peanut butter sandwich, a whole egg, a whole slice of bread, or a full glass of milk. Only if these criteria are fulfilled can it be said that the child is tolerant to that food. Dr Du Toit will also perform a physical examination to determine if your child is growing well and to exclude signs suggestive of nutritional deficiencies such as iron deficiency and rickets. The respiratory and dermatological examination will seek to document the presence or absence of concomitant allergic conditions such as asthma and/or eczema.

 

Bogus Tests to Avoid

Please avoid the many bogus allergy tests to be found on the high street/Internet; examples include Iridology, Vega's Testing, IgG, IgG4 testing, Cytotoxic testing, Kinesiology...  Never perform allergy test at home without first obtaining a qualified allergy opinion. Herewith some additional topics that may be of interest to you:

Allergy tests should be performed to ‘candidate food allergen/s’ as identified on clinical history, this restricts the dangerous practice of testing to endless allergens that are already tolerated, or unlikely to be relevant allergens. Allergy testing is performed to ensure that the history correctly identified the cause of the index food reaction. Dr Du Toit will also perform testing to establish if the patient is co or cross-allergic to other food allergens, particularly if the patient has yet to eat these foods or if the foods were previously tasted but ‘disliked’. 

 

There are Other Factors to Consider

Dr Du Toit will also test to identify allergens which may influence the control of other allergic disorders; for example, pet dander allergy may exacerbate asthma which in itself is a risk factor for more severe food-induced allergic reactions. If a detailed history is unable to clearly identify a candidate allergen, then a screening allergen panel may be required. Screening panels should not be open-ended and should include allergens which are relevant to the patient’s age, allergic condition and geographical location. In practice, testing to peanut, tree nuts, egg white and cow’s milk will account for the majority of food allergies in young children. Wheat, soya and kiwi can be added to these panels, if not previously tolerated or eaten. In older children and adults, fin-fish, shellfish, kiwi and sesame should also be added to the panels, unless an unequivocal history of tolerance to these foods is obtained.  

There are distinct advantages and disadvantages between Skin Prick Tests with Specific-IgE Blood tests. For example, Skin Tests provide an answer within 15-20 minutes of testing whereas blood tests take days to return. Skin Prick Test values can vary between centres whilst Phadia-based IgE tests are standardised between Labs. Some labile foods such as Kiwi, Lentil, Wheat, and Sesame perform better on Skin Testing when fresh/uncooked foods are used. Specific-IgE Allergy blood tests are not influenced by concurrent antihistamine use whereas Skin Prick Tests are. It is difficult to test to many allergens on Skin Prick Testing, Specific IgE Allergy tests allow for testing to many allergens. 

Allergy tests - both the Skin Prick test and Specific-IgE blood testing - are able to predict for the likelihood of future allergic reactions if accidental exposure were to occur, however, allergy tests do not predict for the severity of these reactions.

Allergen Component testing is increasingly of use in predicting for the severity of future reactions as well as the likelihood that the allergies have been outgrown. The use of allergy test predictive values significantly reduces the need for diagnostic dietary investigations. Supervised incremental oral challenge tests remain the gold standard investigation for the diagnosis of food allergy.

 

Specific-IgE Allergy Blood Testing

IgE testing can assist with the following aspects of management; Diagnosis (of allergy vs tolerance), Prognosis (ie severe or not), and resolution.  Furthermore, these test results can also be of help in assessing for a response to immunotherapy.

Specific-IgE Blood testing (previously called RAST testing, more recently called ImmunoCAP) - is possible to a myriad of allergens. Dr Du Toit makes use of bespoke panels to foods, aeroallergens, drugs....  Allergy in-vitro testing with ImmunoCAP gives reliable results with a high reproducibility that usually corresponds well with the clinical diagnosis of the allergy specialist. ImmunoCAP (one of the commercial IgE measurement assays) is World Health Organisation calibrated and thereby offers quantitative results which make it possible to monitor the allergy development in a patient over time.

Dr Du Toit only makes use of the ImmunoCAP tests, and detailed IgE component tests. Only one blood sample is needed and it can be saved and used several times if complementary tests are requested. Specific IgE tests are neither affected by pharmaceutical treatment, nor by skin status (i.e. atopic dermatitis).

 

Interpretation

Low IgE antibody levels indicate a low probability of clinical disease, whereas high antibody levels to an allergen show good correlation with clinical disease. Test results will be interpreted by Dr Du Toit. The new detailed IgE component tests are often used by Dr Du Toit to facilitate a better diagnosis.