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For the Curious

Allergy testing - skin or blood - which one is best?

Doctors say; 'A diagnosis of an allergic disease is not complete until the trigger or triggers of the reaction are identified'. There are two common clinical ways to test for allergy triggers. Skin prick testing or blood tests often called laboratory RAST or specific-IgE testing. Patients can be confused about the difference. In this article we note what The Royal College of Physicians said about these tests.
'Skin prick or laboratory tests can help to confirm the clinical diagnosis in allergy. However, these tests are of limited value without a detailed clinical history. Skin prick tests are cheap and easy to perform once staff are trained, whereas serum (blood) IgE tests are more costly but still relatively cheap.'

Skin prick testing has been used successfully by doctors for over 130 years and the results are almost immediate. It is described as a safe, slight pin-prick delivery of an allergen or allergens just under the skin, usually on the arm. It is used to test and measure the sensitivity, of IgE driven histamine reactivity to foreign elements that may have found their way into the body. Most of these elements are proteins such as from cats, dogs or house dust mites. It is important that patients having this type of allergy test must not use anti-histamine medication for several days prior to the test. Blocking histamine reactions through medication may interfere with important readings. If a reaction happens (usually a red itchy wheal or bump) it is taken as a reliable 'YES' to the presence of IgE released histamine. It is not confirmation of an allergic disease. Many people can have positive skin prick test reactions without allergy symptoms. This is why it so important to have allergy testing carried out in a clinical setting with the background notes of the patient's medical history available. In most cases, skin prick testing is available in an allergy specialist's clinic or hospital setting.

Blood tests, often called RAST, CAP-RAST or specific-IgE, measure the exact amount of circulating levels of IgE antibody in the blood. In this test, a doctor or nurse can request either the total amount of IgE in the blood (adding all triggers together) or specify just those allergens they suspect may be triggering symptoms, such as house dust mites, cat, dog or from a wide-range of different allergens, including foods, tree pollen, grasses, moulds or fungi. With the test results, the doctor can confirm the relative levels of allergic sensitivity to various suspected triggers, and, in conjunction with a clinical history and examination, tailor treatments and advice on avoidance techniques to improve symptoms. If the results are negative and symptoms persist, further investigations may be required. This type of allergy testing can take a week or two to come back from the laboratory. Unlike skin prick testing, there is no need to stop taking anti-histamines. These tests are usually routinely available and can be requested by a family doctor or his nurse, just like any other blood test. The important thing is that whoever requests the test, takes a full allergy history to help interpret the results.

Allergy symptoms can threaten quality of life, yet allergy testing is often ignored in treating or diagnosing allergic disease. Here again, the Royal College of Physicians speaks out about the consequences.

'Until blood tests became widely available, general practices and hospitals usually had little, if any, resources for establishing the presence (or absence) of sensitisation to specific allergens. In consequence, most allergic disease has been treated with drugs, with little attention being paid to establishing causative agents and allergen avoidance strategies'.

The time honoured first-step in treating allergy is avoidance of known triggers.


References

Royal College of Physicians' Report 2003, 'Allergy, the Unmet Need', Chapter 9, Diagnostic Tests, page 73 and Preface xi