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Diagnosing Food Allergies

It may be tempting to diagnose and treat a food allergy on your own.  But suspected food allergies should always be evaluated, diagnosed, and treated by a qualified medical professional, such as a board-certified allergist.   

Only an experienced physician can provide an accurate diagnosis and prescribe medications, including epinephrine—the only treatment that can stop a severe allergic reaction. Self-diagnosis also can lead to unnecessary dietary restrictions and inadequate nutrition, especially in children.  Misdiagnosis is another concern.  For instance, some people think they are allergic to a food, but they actually have a food intolerance (an inability to digest a certain food). 

Your primary care physician can refer you to a qualified allergist.  You also can search the American Academy of Allergy, Asthma, and Immunology’s physician referral directory.  

Physicians use several important tools to diagnose food allergies:

• Medical History
• Prick Skin Tests
• Blood Tests
• Food Challenges
• Elimination Diets

Medical History
The first step in diagnosing a food allergy is a thorough medical history.  Your allergist will ask a series of questions to determine if food allergy is a cause of your symptoms and toidentify suspect foods.  After taking the case history, the allergist will perform a thorough physical exam.

To help you prepare for your first visit to the allergist, here is a sample set of questions.   Before your visit, you may find it helpful to jot down your answers so you won’t need to rely on your memory for this important information.

• What are the typical symptoms of your reaction and what is the order in which the symptoms occur?
• What exactly was eaten? (Note: It may be helpful to bring labels from a manufactured food product or an ingredient list.  For chronic symptoms, such as rashes, keeping a diet record for days or weeks with notes about your symptoms may be helpful.)
• Did your reaction cause you any breathing, skin and/or digestive symptoms?
• What was the length of time between your consumption of the suspect food and the first sign of your reaction?
• How much of  the food did you eat to trigger the reaction?
• Does a similar reaction occur each time you eat that food?
• How was your reaction treated?  How long did it take for you to resume your normal activities?
• Did anyone else who ate the same food get sick?
• Were you taking any prescription medications or over-the-counter drugs at the time of your reaction?
• Have there been any recent changes in your living situation—for example, new pets, remodeling, moving to a new home?
• Does anyone in your family have allergies? If so, who has allergies and to what are they allergic?

The Testing Process
Next, your allergist may conduct tests to help identify a food allergy, based upon  your medical history.  Your doctor should explain which tests will be performed, how the tests will be administered, how long it will take to get the results, and what information these tests will provide.  As we’ll discuss later, allergy tests alone do not always provide clear-cut answers.  To arrive at an accurate diagnosis, the allergist will combine your test results with the information given in your medical history. 

To understand how diagnostic tests work, it helps to understand how food allergies affect your immune system.  For many types of food allergies,  your body produces abnormally large amounts of an antibody called immunoglobulin E (IgE).  IgE clings to the surface of mast cells, which are found in the parts of the body associated with allergies:  your nose and throat, lungs, skin, and gastrointestinal tract.  Researchers describe IgE antibodies as “food-specific.”  This means that they are genetically programmed to recognize and attack a specific food protein, or allergen.   For example, if you have an egg allergy, the IgE antibodies you produce will be on the alert for the protein found in eggs.   Now, let’s say you accidentally eat a dish that contains eggs.  As your body breaks down the food, IgE targets the egg protein as the “enemy” and signals the mast cells to attack.  The mast cells respond to this command by releasing histamine and other chemicals, which trigger your allergy symptoms.     

Diagnostic tests focus on some aspect of this faulty immune response.  Keep in mind, however, that different kinds of tests provide different information.  Depending on your medical history and initial test results, you may have to take more than one test before receiving your diagnosis.     

Prick Skin Tests
Prick skin tests identify IgE to an allergen by exposing your skin to a tiny amount of a suspect food and watching for a mild, localized response. The allergist places a drop of a solution containing the food allergen on your back or forearm.  (When testing fruits and vegetables, fresh food may be used instead of a solution.) Using a small plastic probe or needle, the doctor produces pricks or scratches that allow the solution to enter the surface of the skin. This may sound unpleasant, but don’t be anxious:  Prick skin tests actually cause little discomfort, since the allergist is only scratching the surface of the skin--similar to a fingernail scratch.   

Depending on your case history, you may be tested for one food or several.  Results appear about 10-15 minutes after the prick skin test is administered.  Positive results are indicated by a wheal—a raised white bump surrounded by a small circle of itchy red skin. The wheal is a sign that your mast cells are reacting to the food protein by releasing histamine.  If no wheal appears, it is unlikely that you are allergic to the test food.  If a wheal does appear, the results are considered positive.  In general, a large wheal is more likely to indicate a true food allergy, but size is not always an accurate predictor.  

Prick skin tests seldom produce “false negatives” (erroneous results indicating that you are not allergic to a food, even though you really are).  Negative results almost always mean that you are not allergic to a food.  More frequently, however, prick skin tests produce “false positives”:  You have a positive test, even though you aren’t really allergic to the food being tested.  This is why an experienced allergist will not base your diagnosis on the results of a single test, but will also consider your case history and order additional tests, if necessary.

Blood Tests
You may hear the blood test for allergy called a “RAST” test.  This is short for Radio Allergo Absorbent test, an older diagnostic method that is rarely used today.  However, allergists still apply the general term RAST to blood tests that measure the presence of IgE antibodies to specific foods.

In this test, a sample of your blood is sent to a laboratory, which measures the level of IgE antibodies in the blood sample. The results take about a week to arrive and are based on a numerical scoring system. Not all RASTs are the same, and different laboratories sometimes use different “brands” of the test.  In addition, the tests may use different scoring systems.  Like skin tests, RASTs sometimes produce false positives. In general, however, the higher your RAST score, the more likely you are to be allergic to the food being tested.

A test known as the CAP-RAST, ImmunoCAP, or CAP-FEIA (shorthand for “CAP fluorescent enzyme immunoassay”) is a brand that has been studied more than others for its use in food allergy.  The CAP RAST reports IgE levels on a scale from less than 0.35 (undetectable) to over 100 in a measurement called “kUA/L.”  Over time, researchers have pinpointed scores that indicate, with good accuracy (above 95%), that a patient is allergic to specific foods.  For example, if a young child has a CAP RAST level of more than 7 kUA/L to egg, 14 kUA/L to peanut, or 15 kUA/L to milk, it is very likely that child is  allergic to that food.   

Over a period of years, an allergist can monitor CAP RAST scores to help determine whether or not patients are outgrowing their allergies.  For instance, most children eventually outgrow their egg or milk allergy.  When the test shows that the IgE level is low enough, the allergist can carefully supervise the reintroduction of important nutrients into a young patient’s diet.   

A Note about False Positives
About 50-60% of all skin and blood tests are false positives.  These results occur for two reasons:

• When you eat a food, your digestive system gradually breaks down the food proteins, chopping them into small pieces.  But diagnostic tests can’t mimic this allergen-reducing process.  Since food proteins are bigger when they interact directly with your skin or blood, it is easier for the IgE antibodies to “see” and attack them.  As a result, your tests may show that you are more sensitive to a suspect food than you really are. Thanks to the digestive process, the allergenic proteins are small enough to fly under the IgE radar—and the food is safe for you to eat.
• Members of a food “family” often share similar proteins.  For example, if you are allergic to peanuts, your tests may show a positive response to other members of the legume family, such as green beans, even if eating green beans has never been a problem for you. This is known as cross-reactivity.  The test is positive because it recognizes a similar protein in peanuts and green beans.  But the test is not detecting the real culprit—an entirely different protein found only in peanuts.  

You may be wondering if these tests are really effective, since the rate of false positives is so high.  But, in the hands of an experienced physician, allergy tests are extremely helpful—especially when the results are interpreted in the context of your medical history.  For example, if your case history suggests that you have had several reactions after eating soy products, and prick skin tests show a positive reaction to soy proteins, it is very likely that you do, in fact, have a soy allergy.

Test Selection
You also are probably wondering which tests your doctor may select and why.  Your doctor may suggest performing either or both of the skin and blood tests, depending upon many factors.  Prick skin tests are inexpensive, produce immediate results, and are sometimes less likely than blood tests to produce false negatives.  On the other hand, although RASTs are more expensive, they are very useful if you suffer from skin conditions that leave little space for the tests or are taking antihistamines, which block the skin tests but not the blood tests.   Antihistamines can interfere with skin tests, so if stopping your medication, even for a short time, will make your symptoms worse, blood tests may be the best choice. Sometimes the allergist will perform the blood test if the food allergy is already known or quite likely to exist, and the only reason for testing is to monitor if the allergy may be fading away.  

Food Challenges (Medically Supervised Feeding Tests)
Sometimes, even after performing skin and blood tests, an allergist is unable to arrive at a definitive diagnosis.  In this case, you may be asked to undergo an oral food challenge (OFC).  Although they are highly accurate, OFCs are performed only in specific circumstances, since they have the potential to trigger a severe reaction.  However, when administered by an experienced allergist who is prepared to respond immediately if a reaction occurs, these tests are very safe.  There are two types of food challenges.

During a medically supervised food challenge or feeding test, the suspect food is given in measured doses, starting with small amounts.  Following each dose, you are observed for a period of time for any signs of a reaction.  In the absence of symptoms, increasingly larger doses are given.  If you show any signs of reaction, the food challenge will be stopped and medications given if needed.  The general goal is to feed a full serving size of the tested food.  If this is tolerated, then adding the food back into the diet is unlikely to be a problem.  If the test confirms that you have a food allergy, your allergist will discuss food avoidance techniques and prescribe appropriate medications.         

Feeding tests can be conducted in several ways.

Double-Blind, Placebo-Controlled Food Challenge (DBPCFC)  
This test is considered the “gold standard” for diagnosing a food allergy.  In fact, it is so accurate that it is often used to verify the results of other tests. During this test, the potential allergen may be hidden in a capsule or mixed into another food that you normally eat with no difficulty. (For example, if you are being tested for a milk allergy, milk powder may be mixed into a piece of hamburger).  In addition to ingesting the capsule or eating the food that contains the allergen, you will be given a placebo—an identical capsule or food that is perfectly safe, because it does not contain the allergen.  Since you are given the allergen and the placebo at different times, neither you nor your doctor knows which you are receiving (hence the term “double-blind”).  This ensures that the test results are objective:  Neither the patient’s anxiety nor the allergist’s preconceptions can influence its outcome.  After the test shows that you were able to tolerate the food in a hidden form, you eat it in its usual form to confirm that you can tolerate it.

Open Challenge
This type of food challenge is more commonly done when there is not a strong suspicion that nervousness may affect the results.  You will be asked to eat a suspect food that is not disguised in any way, so you will know what you are eating.  As mentioned above, this is done gradually, under supervision. 

Trial Elimination Diets 
Some food allergies result in frequent or daily symptoms.  Examples include skin problems like eczema (atopic dermatitis) and allergies affecting the gut.  For these problems in particular, your doctor may recommend a trial elimination diet.  You eliminate suspect foods from your diet for 2-4 weeks, while your doctor monitors your symptoms.  If one or more of these foods really is causing your allergy, your symptoms should disappear by the end of this period. Your doctor will then gradually reintroduce each food to your diet.  If a food causes your symptoms to return, then you truly are allergic to that food.  If the results of the elimination diet are inconclusive, then your allergist may recommend food challenges to confirm the results.  These trials should only be undertaken under medical supervision.

A word about unproven diagnostic tests: The tests discussed in this article are all proven diagnostic methods. However, some methods of allergy testing are considered controversial, since no definitive studies have shown that they are effective in diagnosing food allergies.  Some may even increase the risk of a reaction. Learn more.