If you have allergies — or a child with allergies — you have probably spent some time in an allergist’s office getting skin-prick testing and/or blood testing. But what do all the numbers mean? Which tests are necessary (and which are not)? And are there any new or better tests on the horizon?
We spoke recently with Dr. Zainab Abdurrahman, a Toronto-based allergist, about the myths and realities of food allergy testing.
Question: Let’s run through a typical testing scenario. When would an allergist usually order skin-prick tests? When would a blood test (RAST) be ordered? At what point might a food challenge be used?
Dr. Abdurrahman: A skin prick test is ordered as part of the investigation of a child or an adult who has a history of a reaction after ingesting a food. Sometimes if the skin prick test is negative but the history is very suggestive, then an allergist may follow up with a blood test. Another situation is if a person was previously positive in the past to a particular food but now has a negative skin test, then a blood test may be ordered to confirm this finding.
A food challenge can be used when both the skin prick test and the blood test are not suggestive of food allergy. Hence, we often use the food challenge as the final test of the possibility of food allergy. When administered in a certain fashion then it is considered the gold standard test for food allergy. This is when a patient does two separate food challenges and is not aware on which day they actually ingest the food of concern.
Question: How can we interpret the numbers on a skin or blood test? What do higher or lower RAST levels mean? What about the size of the wheal (i.e., the swelling of the skin at the site of testing)? How do allergists interpret the results from year to year?
Dr. Abdurrahman: The interpretation of any of these types of tests requires training and expertise in the field of allergy and clinical immunology. A certified allergist can perform the testing and interpret the results for you. Once you have a positive skin test to a food that you have had a reaction to, then you have a food allergy. Having accidental exposure to this identified food can result in a severe reaction and possibly anaphylaxis.
Question: No matter whether it is a small number or a large number?
Dr. Abdurrahman: The exact size of a positive test does not change the fact that a person is at risk of anaphylaxis. We follow the skin tests with repeat testing at certain intervals (months to years) to see if they become negative. At this time, we will follow up with corollary testing such as the blood test and possibly a food challenge if indicated. The blood Specific IgE levels (sometimes referred to as RASTs) differs not only by food but sometimes also by age.
Lower numbers do not indicate a milder allergy. The cut offs for specific IgE levels differ by food. Hence, the numbers may seem numerically lower for one food allergen compared to the numbers of another food allergen, but both levels are still considered positive. And once positive, a patient is at risk for anaphylaxis.
Question: What about IgG testing, as opposed to the typical IgE testing? Are these other tests scientifically valid?
Dr. Abdurrahman: The antibody related to food allergy is IgE, Immunoglobulin E. IgG, Immunoglobulin G, is another type of immunoglobulin that often reflects your body having seen a particular antigen.
Testing levels of IgG to foods is not a validated means of assessing reactions to foods. In fact, our society, Canadian Society of Allergy and Clinical Immunology, CSACI, has a position statement that summarizes this issue very well: “The CSACI does not support the decision of licensed physicians and our pharmacist colleagues to offer such testing, given the overwhelming consensus against the validity of such tests. The CSACI strongly discourages the practice of food-specific IgG testing for the purposes of identifying or predicting adverse reactions to food.”
Question: I have heard about a new blood test that provides more information than the RAST. It’s called the Diagnostic Allergy Test [DAT]. Do you have any thoughts or information on this to share?
Dr. Abdurrahman: This new blood test is currently being used on a research level and is not currently available for physicians within Canada. We are hoping with more work being done in the area of food allergy, tests such as these as well as other tests will be able to be added to the toolkit of an allergist to help optimize the diagnosis and management of patients with food allergy.
Question: How can parents prepare their children before an in-office food challenge?
Dr. Abdurrahman: To prepare for an in-office food challenge, let your child know that they will be eating the particular food in the clinic with the doctor on that day. If they are worried about a reaction, let them know they are being monitored and if any thing should happen the physician and their team are prepared. Depending how long you will be at the office, I suggest you bring some form of entertainment, a bottle of water, and another snack (something they usually eat and have never had a reaction to). I also ask parents to bring along something that complements the food the child is eating. For example if it is a butter of some sort, then crackers or bread or apple slices can help facilitate eating this food.
Question: Thank you so much, Dr. Abdurrahman. This information is very helpful to us all!
Dr. Abdurrahman: You are most welcome.