The gold standard of allergy testing is an oral food challenge, conducted in an allergist’s office. During a food challenge, small amounts of the allergen are eaten over time while symptoms are closely monitored. In theory, a true “food allergy” may only be diagnosed if someone actually reacts to ingesting a food.
That said, skin and blood tests are used routinely to determine reactivity and sensitivity to foods. This is a great explanation by Dr. Paul Erlich of the various types of testing and what the results mean:
No single test is considered diagnostic of a food allergy except an oral food challenge (OFC), which obviously you don’t want to do indiscriminately because of the risk it poses to the patient. Of the stepping-stone tests, RAST, which measures allergen-specific IgE in the blood, doesn’t diagnose allergy, only sensitization. Skin prick tests show reactivity, but not the extent of the allergy – i.e. the skin may react and wheal sizes are significant, but it may not translate into a measure of other reactions, such as hives, cardiac, or respiratory symptoms. Component testing shows which epitopes within the proteins you are allergic to, some of which, in the case of peanuts, are more dangerous than others, generally speaking.
FARE is a great resource and has very detailed explanations of skin prick testing and blood testing. If you suspect you or your child has a food allergy, I highly recommend seeing a Board Certified allergist. Pediatricians and general practitioners simply don’t have adequate experience or the appropriate background to recommend what testing you may need.
Skin Prick Testing (SPT)
Many allergists first conduct a skin prick test (SPT). They will likely test for the suspected allergen as well as Top 8 allergens or closely related allergens. During the test skin on the forearm or back is ‘pricked’ with a small needle and exposed to the allergen. After 30 minutes, the doctor will measure the reaction – a hive usually referred to as a ‘wheal’. The size of the wheal can show the level of reactivity, but cannot predict the type or severity of an allergic reaction. Additionally, SPTs have a very high false positive rate. Meaning your skin may react to the test, but you could safely eat the food. Bubba’s first SPT was around 5 or 6 months old. It went fairly well. She didn’t know what was happening (meaning she wasn’t scared or anxious) and only cried for the minute it took to do the pricks. As she has gotten older, SPTs are harder. She knows what’s coming and tries to fight. I always bring a small new toy, a lollipop, and an iPad for distraction while we wait for results. I also explain before and after why we’re doing this. I want her to always trust me, so I am always honest when something is gonna hurt. We still do all SPTing on her back so she can’t scratch.
Any positive allergens on a SPT are usually followed up with a blood test. There are two types of blood tests – traditional and ImmunoCAP.
Traditional Blood Tests
Traditionally, allergy blood tests were called RAST (for radioallergosorbent) tests because they utilized radioactivity. Current tests no longer utilize radioactivity but are sometimes still referred to this way. Traditional blood tests measure IgE antibodies to specific foods in the blood. These are the nasty guys that trigger an allergic reaction. These tests measure sensitivity to foods. There are less false positives than with a SPT, but more false negatives. In case that seems confusing, just remember that SPTs and blood tests work well in conjunction with each other. Blood tests also are not indicative of the type or severity of an allergic reaction. For some allergens, IgE numbers are fairly accurate in predicting the likelihood of reacting to a food if exposed. You will need to discuss your results with your allergist. Traditionally, IgE levels are reported as Class 0 to Class 5 or 6. Class 0 indicates no allergy. Class 5 or 6 indicates a high allergy. But again, class levels do not predict the severity of a reaction, only the likelihood of having one.
CLASS 0 <0.35 KU/L
CLASS 1 0.35 – 0.7 KU/L
CLASS 2 0.71 – 3.5 KU/L
CLASS 3 3.51 – 17.5 KU/L
CLASS 4 17.51 – 50 KU/L
CLASS 5 50.01 – 100 KU/L
CLASS 6 >100 KU/L
For some (common) allergens the significance of the class varies by food and age. For example:
- Egg IgE greater than 7KU/L (greater than 2 KU/L in infants under 2 yrs) indicates a 98% chance one will react when eating eggs (2% do not react).
- Milk IgE greater than 15 KU/L (greater than 5 KU/L in infants under 2 yrs) indicates a 95% chance one will react when ingesting milk (5% do not react).
- Peanut IgE greater than 14 KU/L indicates a 100% chance of reacting when eating peanuts. (IgE of 1 KU/L indicates a 50% chance).
- Fish IgE greater than 20 KU/L indicates a 100% chance one will react when eating fish.
- Tree nut IgE greater than 15 KU/L indicates a 95% chance one will react when eating tree nuts (5% do not react).
- Soybean IgE greater than 30 KU/L indicates a 73% chance one will react when eating soy-although some forms of soy may be tolerated while others may cause a reaction.
- Wheat IgE greater than 26 KU/L indicates a 74% chance one will react when eating wheat. Wheat is often positive at low levels in people who are allergic to grass pollen and may not cause reactions in that situation.
ImmunoCAP is a specific type of IgE testing that can provide more information about what specific proteins in a food the individual is reactive to. This can be especially helpful for milk, egg, and peanut allergies. For more information on peanut ImmunoCAP or “component” testing, see my previous post, and for more information on the implications of egg component testing see this post.
If you take nothing away from this post, here’s what you need to understand. Test results are great for helping you know if there is a good chance you or your child has an allergy. The specific numbers can be helpful as a ‘likelihood of reacting’ indicator. They can also be useful to track over time so you know if the allergy might be outgrown. But test results mean NOTHING, absolutely nothing, in predicting the severity or type of allergic reaction that may occur. Reactions can and do change over time. Bubba’s numbers are ‘relatively’ low. Many of her allergens are only Class 2. But she has experienced anaphylaxis over five times.
Finally, a word of caution
Under the supervision and at the direction of an allergist, testing should be done discriminately. Testing should not be conducted for foods that are clearly tolerated. In this excellent scientific article, Dr. Scott Sicherer explains, “Testing large panels of allergens without consideration of the history, geographic relevance, and disease characteristics may result in many clinically irrelevant positive results, which, if overinterpreted, may lead to costly and socially, emotionally, and/or nutritionally detrimental actions of unnecessary allergen avoidance. Similarly, caution is advised when testing is negative despite a convincing history. ”
Sensitization and cross-reactivity among food proteins can result in false positives. The example Dr. Sicherer provides is especially applicable to Bubba, “>50% of patients with peanut allergy test positive to other legumes, but <5% have clinical symptoms of allergy from ingestion of legumes.” Bubba tests highly allergic to peanuts but has never been exposed. She is probably allergic to them. She also tests positive to virtually all other legumes. Because she tolerates black beans, we have never tested her for them. Because she had anaphylactic reactions to peas, we know she’s allergic. For soy, she reacted and tested positive, but eventually outgrew it. For all other legumes we are slowly food challenging them all off her list. Success with chick pea so far! Please consult your own allergist to make an appropriate testing plan for your child.