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Allergy diagnosis: blood testing for inhaled allergens

Sue Cross
RN BSc(Hons)
International Project Manager
Respiratory Education Resource Centres
University Hospital Aintree

We would never dream of commencing a patient on treatment for diabetes without performing the necessary blood glucose tests. Yet in allergy we often do just this. We may advise patients to avoid something that we think is aggravating their condition, which may cause the patient much anxiety (such as getting rid of a much-loved pet), or have large cost implications (such as buying expensive house dust mite-proof bedcovers), without confirming our suspicions at all!
Identification of the triggering allergen is essential for successful treatment. The National Institute of Health guidelines state clearly that either in-vitro testing or skin testing should be used to diagnose all individuals with allergic symptoms.

How to test
A blood test is taken for measurement of the specific IgE of the suspected allergen. A 4-7ml blood sample is required (in a standard orange-top SST or red-top vacutainer), which can be sent to a local NHS pathology laboratory, along with information on the tests required and a detailed history. This takes a relatively short time to complete, and there are certain advantages:(1)

  • It is not influenced by any current drug treatment.
  • It can be performed when there is widespread skin disease.
  • It is completely safe.
  • The specificity can be as high as 90% for inhaled allergens.

What the results show
The amount of specific IgE to each allergen can accurately be determined from the test and can be expressed as a quantitative result, such as 0.35kUa/l- 100kUa/l, or as an ImmunoCAP class 0-6 (0 being normal/low and 4-6 showing high sensitivity). The higher the level, the greater the amount of specific IgE antibodies to that allergen circulating in the blood. (Test results should always be compared back to case history for clinical relevance.)
The detection of IgE antibodies provides information that the sensitisation processes have been initiated and, along with symptoms and a positive case history, confirms the causative allergen. This then allows the health professional to give positive, informed advice on avoidance strategies.
However, a positive test without symptoms may predict the later development of allergic disease.

What to tell the patient
With the information gained from the tests, along with the history and examination of the patient, appropriate advice, guidance and treatment strategies can be agreed upon.
As well as providing crucial information to make an accurate diagnosis, the test results provide vital information and education for the patient. So often a patient will come to see us convinced that s/he is allergic to something in their environment. A negative result is as useful as a positive result and has saved the life of many a moggy! A positive result, however, along with a positive history, will provide the patient with the information needed to take appropriate action to avoid the allergy triggers, as well as take the necessary treatment, and, most importantly, get the most out of their life without being controlled by their allergies.

1. Durham S, editor. ABC of ­allergies. London: BMJ Publishing Group; 1998.