There are a number of different tests which an allergist might suggest a patient take in order to determine the scale and type of insect sting allergy. However, wherever possible a detailed history of the patients is probably more important than any test described below due to the somewhat imprecise results of these tests and that the body does change its allergic state quite suddenly.
ESTABLISHED AND ACCEPTED IN CURRENT MEDICAL PRACTICE
Skin Prick Test - the skin is pricked and a low dose of different venom is introduced onto the broken skin. The level of response is monitored and this can be an excellent way of determining the scale of allergy and the exact insect to which the patient is allergic. The test is cheap and simple but the major risk is anaphylaxis. This is a test favoured by many allergists and is considered to be the “gold standard”.
RAST Test - a blood test is taken and the levels of venom specific IgE in the blood is measured. This shows in theory the level of the allergy and the precise insect to which the patient is allergic. However there are some poor results and this test, whilst having the advantage of being outside the body, does not, in many professionals eyes, provide the same level of certainty as the Skin Prick Test.
Basophil Activation Test - the newest test around and has the bonus of being functional even if the patient is on anti-histamines. The blood test must be performed within four hors of the blood being drawn and measures through a number of different staining techniques the level of activation of the allergy response cells in the blood to the allergen before and after the challenge. Instead of measuring histamine the test measures CD63 which ia a marker for activated basophils (tetraspanin).
“UP AND COMING” BLOOD TESTS
Buhlmann Laboratories CAST tests - another blood test which measures the level of sulphido-leukotrines in the blood that are released on introduction of antigens into the patient. This test seems to offer higher levels of reliability and consistency than RAST tests but due to ongoing research and professional acceptance is not available yet in the wider market.
REFLAB Histamine Release Testing - another blood test whereby basophils from a blood sample are stimulated, by mixing with the antigen, to break down and release histamine. Everything is very carefully measured and therefore precise indications of the nature and extent of the allergy can be determined. This continues to be promising and is still under evaluation.
Ultimately it should be appreciated that allergy diagnosis still remains something of an art and not a science. There is still a considerable amount of interpretation involved, interpreting the results of the case history in association with the blood tests. Assuming that a patient comes into the allergy clinic with a historic systemic response to an insect sting then my own personal feelings are as follows::
- A positive skin prick test indicates that venom immunotherapy should take place and will give an indication of the type of venom needed for the treatment
- A negative skin prick test indcates that the patient is probably no longer allergic but a further test could take place three months later to confirm
- A positive RAST test will confirm that venom immunotherapy should take place and will give an indication of the type of venom needed for the treatment
- A negative RAST would suggest that the patient should consider a skin prick test
There are four generally accepted reasons for inaccurate false negative RAST tests.
1. The use of antibiotics immediately after anaphylaxis can lead to a sudded reduction in IgE
2. General reduction in IgE due to the passage of time (usually many years in the case of insects)
3. Unstable allergens in the RAST substrates (especially food allergens)
4. Use of RAST mixes can miss out the single allergens eg mixed vespid test is less sensitive than vespula or polistes tests
RAST tests are measured on a scale of either 1 to 6 (older method) or 0 to 100 (newer method). It is generally accepted that these are quantitative because they measure the amount of IgE in the blood and in theory this tells us how allergic the patient might be. In practice these are poor indicators of the severity of future allergic responses.