Allergy blood tests

Diagnosing allergy is a subject that is very commonly misunderstood. One of the commonest misunderstandings concerns the role of blood tests in managing allergies. When considering the use of medical tests, there are several pertinent questions that need to be answered.

• How sensitive is the test ? In other words, what percentage of patients with the disease will the test identify ?
• How specific is the test ? That means, what percentage of patients without the disease will the test exclude ?
• What is the positive predictive value ? That means, what percentage of patients with a positive test result really has the disease ?
• What is the negative predictive value ? That means, what percentage of patients with negative test result really does not have the disease ?

And of course, the most important question: How does the test result affect the management of the patient ?

Appropriate use of allergy blood tests

Specific IgE blood tests are useful when skin testing is impractical, such as when patients have active urticaria (hives) or other skin conditions, or if patients cannot stop antihistamine treatment. Only tests that have been validated, i.e. tests that have received FDA approval or clearance, should be used. Blood test results must be interpreted with care, and positive results should be verified by challenge testing unless there is strong clinical evidence to support the results. The level of specific IgE might be useful in monitoring the progression of food allergies, since a declining trend of food-specific IgE is a good prognostic factor for eventually outgrowing that allergy.

The most commonly used FDA-approved allergy blood tests include RAST, MAST, CAP, UniCAP and chemiluminescence assay.

Can blood tests be used to test for "food intolerance" ?

Many laboratories performing non-validated blood tests claim that these tests can be used to identify food intolerance. Such tests typically test for dozens or even hundreds of foods using crude methods such as ELISA. Food intolerance is not a diagnosis, rather an umbrella term for any condition that produces symptoms due to food ingestion. Food allergy, for example, is a form of food intolerance. Other examples include enzyme deficiencies that can lead to indigestion or malabsorption of certain nutrients, pharmacological effects of food chemicals such as caffeine etc.

Since none of these conditions, with the exception of food allergy, involve food-specific antibodies, antibody blood tests have no role in making diagnosis. Any laboratory that claims that a blood test can be used to diagnose food intolerance is committing healthcare fraud.

Reference information:  www.allergy.hk

The information aims to provide educational purpose only. Anyone reading it should consult physician before considering treatment and should not rely on the information above.