Food Allergies Diagnostic Step By Step

Diagnosing a food allergy is about as challenging as finding an allergist who can see you the next day. Your allergist is likely to take a detailed history, perform a physical examination, and proceed with a battery of diagnostic tests to figure out what’s going on and which foods are the prime suspects.

The diagnostic process is more complicated if your food allergy is not IgEmediated, since the most common allergy tests — skin tests and RASTs — both rely on the presence of IgE antibodies. These tests are useless in diagnosing non-IgE mediated food allergies.

Medical History

Your medical history is of tremendous value in diagnosing food allergy, so don’t forget to bring the self-screening test you completed in “Food Allergies Diagnostic,” to your first appointment. In addition to what you’ve already recorded, your allergist is likely to ask you a series of probing questions to determine whether true food allergy is likely, and whether the allergy is likely to be IgE mediated.

You can prepare for the doctor to review your medical history by jotting down your answers to the following questions:

  • What foods do you suspect you are allergic to?
  • At what age did the suspected food allergy begin?
  • What were your specific symptoms?
  • What was the timing of the suspected reaction(s)?
  • Did your reaction occur soon after eating the suspect food?
  • Does a reaction always occur when you eat that food?
  • Had you previously tolerated that food? How often have you eaten it?
  • How much did you eat before you had a reaction?
  • Did anyone else who ate the same food get sick?
  • How was the food prepared? How likely is it that the food had hidden ingredients?
  • Were any medications used to treat the reaction? If medications were used, which meds were used and were they effective?
  • What were you doing prior to or during the time you had the reaction? Just eating? Exercising? Taking a bath?
  • Have similar reactions occurred on prior occasions?

Using your answers to these questions, your allergist formulates your medical history — one of the most important and useful diagnostic tools. Your allergist may also ask you to keep a food diary to find out even more about your diet and symptoms in an attempt to identify any consistent pattern to your reactions.

Although your history is the most important diagnostic tool, its usefulness varies depending on the results. Typically, the history leads to one of the following two common scenarios:

  • Bingo! The history is precise and virtually diagnoses your specific food allergy all on its own. If you break out in hives the first time you eat an egg, for example, it’s pretty obvious that you’re allergic to eggs. In this case, further testing usually confirms the suspicion.
  • I dunno. With more subtle symptoms, especially eczema and some of the non-IgE mediated gastrointestinal food allergies, reactions may be quite delayed. You may have had three or four meals and several snacks before you started feeling ill, making any clear link between cause and effect very difficult.

Several studies show that further testing confirms a history of suspected food allergy only about 30 to 40 percent of the time. Your allergist must interpret your history, including food diaries, with great caution.

Physical Exam

You could place a pretty safe bet that by the time you get in to see the allergist, your symptoms will have magically disappeared. Your allergist asks to see the hives, and your skin is perfectly clear. Your tummy, which was as bloated as a beach ball two days ago, is as flat as an ironing board today. Even your sinuses are clear.

Even so, your allergist is going to perform a physical exam to look for signs of a possible food allergy, such as eczema and other rashes and symptoms of other allergic reactions, such as hay fever. What does hay fever have to do with food allergies? Well, food allergy is more likely to occur in those who have other types of allergy, and this can be an important clue.

The physical exam, in and of itself, is not always conclusive. Your exam may reveal no abnormalities, even if you have severe reactions to multiple foods. After all, the only good thing about food allergy is that you usually look and feel fine as long as you’re abstaining from the problem foods.

One important bit of information a normal exam provides is that it reassures you and your allergist that you’re probably okay with your current diet. Once your allergist commences testing, you may begin to wonder if you mistakenly stepped into the acupuncturist’s office.

Allergists commonly perform skin-prick tests in which they poke or scratch the skin with an extract of the suspect food and observe any reactions on the surface of the skin. Your allergist may use any of the following three methods:

  • The allergist places a tiny drop of food extract on the skin and pricks the skin through the drop with a small needle or plastic probe.
  • Using a pricking device soaked in the food extract, the allergist pricks the skin. The allergist inserts a small amount of the food extract into the skin using a small needle. This method, called intradermal skin test, is rarely, if ever, indicated for food allergy diagnosis.

A positive skin test results in a mosquito bite-like reaction at the site of the test within minutes, indicating the presence of histamine, which causes the skin to swell. After 10–15 minutes, your allergist takes a reading and compares all tests to a control prick (to test your reaction to salt water, which shouldn’t cause a reaction).

A larger reaction — a larger bump on your skin — typically shows an increased likelihood that you’re allergic to the tested food. How many times can you expect to get poked? If you’re presenting symptoms of a nonfood allergy, such as hay fever or asthma, your allergist performs a fairly standard set of tests consisting of 20 to 40 skin pricks.

With food allergy the number of tests is typically based on patient history, so you can’t expect a set number. An allergist usually performs skin prick tests for only suspect foods. If you’ve had only one reaction to milk, for example, your allergist is likely to test only for milk to confirm suspicions.

If the history identifies no particular problem food — you have eczema, for instance, but no specific food reactions — then, your allergist is likely to test for only the five or six most common food allergens, including milk, egg, soy, wheat, and peanut. Be cautious of any allergist who recommends dozens of tests for food allergies.

A few allergists out there routinely test every patient who walks into their office for 120 different foods, even if they have no reason to suspect a food allergy. Studies show that in children with eczema, if the skin prick tests or RASTs results are negative for the most common food allergens, the children are highly unlikely to be allergic to less common allergens.

Interpreting a skin test seems like a snap. Either it’s positive or negative. What’s so tricky about that? Actually, it’s tougher than it sounds. The easiest result to interpret is when the test is completely negative. When you test negative for a particular food, the likelihood that you have an IgEmediated reaction to that food is next to nothing.

The main exceptions occur in babies in the first six to nine months of life, during which time occasional false negatives occur. Skin tests are positive only if you have IgE antibodies to the food being tested. Skin tests do not detect non-IgE mediated allergies. Interpreting positive food skin test results is more problematic.

Positive tests indicate that IgE is present but do not, without confirmation from other sources, prove that a reaction will occur when you eat the food. In other words, the test can show a positive result or a false positive (a skin reaction even though you don’t react to the food when you eat it). False positives occur in the following scenarios:

  • You have a small amount of IgE antibody to a food but are not be truly allergic to that food. You can eat the food and experience absolutely no reaction to it.
  • Some proteins in foods are cross-reactive with similar proteins in other foods or even environmental allergens like pollens. This cross reactivity can lead, for example, to a falsely positive skin test for soy in a person with peanut allergy, or a positive test to wheat in a person with grass pollen allergy.

Take the results of skin tests with a dose of salt.

Overall, up to 60 percent of all positive food skin tests turn out to be incorrect (falsely positive) upon further evaluation. Some studies show that the larger the skin test (the bigger the bump on your skin at the site of the test), the more likely a true allergy is at work, although this has not proven true in other studies. Skin test results are only one component of the diagnostic picture, allergists should evaluate them carefully and in the context of the big, clinical picture.

Cross-reactivity can occur when your immune system confuses one protein for another one. This typically happens with members of the same food family, but can occur between two allergens you may not imagine are related. A person who’s allergic to tree pollen, for example, may not be able to eat apples or cherries. Someone who’s allergic to ragweed may be sensitive to cantaloupe or banana.

Allergists remain vigilant of cross reactivity for two reasons:

  • Cross-reactivity can often provide clues to other foods you may be allergic to or may become allergic to in the future.
  • Cross-reactivity can thoroughly confuse test results.

When evaluating skin tests, your doctor needs to be aware of the possibility of cross reactivity, because the test may return a false positive result for a food you can safely eat.

Allergy skin testing is generally a very safe procedure. However, because it exposes you to a food that you may be highly allergic to, caution is always in order. An occasional patient may in fact be considered too allergic for administering a safe skin test. Incidence of systemic (whole body) reactions, however, are very low — estimates run at about 30 reactions per 100,000 tests.

Only allergists trained in the treatment of severe allergic reactions should perform skin tests, just in case you experience a severe reaction. Your allergist must have emergency equipment and drugs on hand for the treatment of anaphylaxis whenever performing a skin test.

RASTs Test

Another test that requires a needle is the RAST, a test that measures the amount of allergen-specific IgE in your blood. This test doesn’t require an allergist; your GP can perform the test, but an allergist may be more qualified to interpret the results and is usually the doctor who performs the test.

RAST consists of drawing a small amount of blood and then having the blood tested — by sending it out to a lab. Doctors can perform RASTs for almost any food or airborne allergen. The most important point about RASTs is that they’re not all the same.

Some types of RASTs are more accurate than others, and the results of one type of RAST are not interchangeable with the results of another type. For diagnosing food allergies, the type of RAST that has the best track record is the Pharmacia CAP fluorescent enzyme immunoassay.

Wrap your mouth around that one! To simplify the nomenclature, doctors refer to this type of RAST as CAP-FEIA or CAP-RAST. As with skin testing, negative RAST results are quite accurate in ruling out an IgE-mediated food allergy, but positive RAST results do not necessarily mean you have a true food allergy.

False positive results occur with RASTs for the very same reasons they occur with skin testing. However, because the RAST is more of a true measure of the amount of IgE in your system, differentiating a true positive test from a false positive test is generally easier than it is in the case of skin tests.

When your doctor gets the results, she looks at your RAST score and interprets the results based on the following criteria:

  • The higher the RAST score, the more likely that the results represent a true food allergy.
  • More importantly, for some of the most common food allergens, the doctor may compare your RAST levels to predetermined cut-offs, above which a true food allergy is almost certain. For example, using the CAPRAST, which gives results on a scale from 0 (zero) to 100, an IgE level of more than 7 to egg, over 15 to milk, 14 to peanut, and 20 to codfish is highly predictive (greater than a 95 percent chance) that you’re allergic to that food.

Your doctor can often use RAST results to track your levels of specific IgE antibodies over time. RAST levels that decrease over time are an excellent indication that you’re outgrowing your allergy to a particular food. My colleagues and I typically decide when to try to re-introduce a food into a patient’s diet based on the RAST result.

The Pros And Cons Of RASTs And Skin Tests

As you probably realized by now, neither skin tests nor RASTs are the perfect tests. Results can range from highly successful at best, to inconclusive, to misleading at worse. When discussing with your doctor which test would be most useful in your case, weigh the pros and cons of each. Skin tests have a couple advantages over RASTs:

  • A skin test is cheap.
  • Skin test results are available almost immediately.
  • Skin tests generally produce fewer false negatives. However, in food allergy testing, false negatives are uncommon with either test method, so this is not a huge issue.

RASTs have several advantages over skin testing:

  • Certain medications, such as antihistamines, can interfere with skin testing, so you have to stop the medication beforehand. If you have trouble stopping your antihistamines, a RAST is an attractive alternative.
  • Widespread skin conditions, especially hives or severe eczema, may preclude accurate skin testing.
  • RASTs are less risky for patients who are susceptible to severe anaphylaxis.
  • RASTs may be better at discerning a true positive from a false positive test.
  • RASTs provide more information on the progress of an allergy over time.

Neither skin-test nor RAST results are very good at predicting the type or severity of an allergic reaction. Although higher RAST levels generally indicate more severe reactions, numerous exceptions prevent RAST results from functioning as accurate predictors of a future reaction.

This is due in part to the dose effect described in earlier article, but even with the same dose (amount of a problem food) three people with the same skin-test or RAST result may have hugely different reactions with exposure to the food. One person may eat peanuts regularly without symptoms, the second may experience minor hives, and the third may experience severe anaphylaxis.

You’ve been interviewed, examined, and poked, and your allergist can provide you with no definitive diagnosis. It happens, especially when you’re experiencing delayed reactions. Hope, however, remains. Your doctor has some additional tricks up his sleeve, including both eating the food (a food challenge) and not eating the food (an elimination diet).

Food Challenges

When your allergies prove too elusive for skin tests and RASTs, your doctor may try to dare the allergies out of hiding by challenging them to react to suspect foods. This test, commonly called an oral food challenge, consists of feeding you increasing amounts of the suspect food under your doctor’s supervision, while observing you for symptoms.

Food challenges are considered the only foolproof test for most food allergies. In addition to identifying elusive allergies, food challenges serve three useful purposes:

  • Verify the accuracy of a positive skin test or RAST.
  • Determine if a patient has outgrown an allergy.
  • Diagnose cases of non-IgE-mediated reactions. An oral challenge may be the only definitive way to diagnose a non-IgE mediated food allergy.

Don’t try this at home. Food challenges carry a risk of serious reactions. Only trained personnel with emergency treatment immediately available should perform these tests. Your doctor can choose to perform a food challenge using either of the following three methods:

  • Instruct you to eat the suspect food in increasing doses while observing you for signs of a reaction. (With this method, unlike the following two methods, you know what you’re eating.) This is the most common way to do a food challenge.
  • Have you to eat something that contains the suspect food without your knowing that you’re eating the suspect food. Your doctor mixes samples of the offending food with another food or adds it as an ingredient to another food, so you can’t recognize it by sight, smell, or taste.
  • Have you swallow a capsule containing the allergen. In some cases, the doctor uses placebo tablets, as well, to keep you in the dark about whether you’re eating the suspect food or some inert substance.

The ideal way to perform a food challenge test is to do a “double-blind, placebo-controlled challenge.” With this method, neither the allergist nor the patient is aware of which capsule or food contains the suspected allergen. In order for the test to be effective, you must also take capsules or eat food that does not contain the allergen (placebos). This ensures that any observed reaction is due to the allergen and not some other factor, such as stress or anxiety.

When you eat something and it makes you sick, the logical thing to do is stop eating it. This is essentially what you do with an elimination diet. When you have a food allergy, your doctor often places you on an elimination diet permanently, or until you’ve outgrown the allergy (if you do outgrow the allergy), but doctors often use elimination diets on a temporary basis to diagnose allergies.

You may have already performed this test on your own by avoiding a particular food and then re-introducing it to your diet and finding that your symptoms returned. If you’ve already done this, your doctor should have included this piece of information in your history.

If you haven’t performed the test yet, your doctor may recommend it to confirm skin test or RAST results or simply as a logical next step — “we can’t figure this out so let’s avoid certain foods and see what happens.” An elimination diet typically spans the course of several weeks but consists of only two steps (plus a step that’s sometimes recommended):

  1. Eliminate specific foods and ingredients from your diet, typically over a period of two to three weeks. During this time, you carefully read food labels and find out about food preparation methods when you’re dining out. Over the course of the elimination period, your doctor monitors symptoms, all of which should disappear in three weeks if you’re following the right diet.
  1. With your doctor’s okay, you begin to gradually reintroduce the foods that were eliminated, one at a time, while carefully recording any symptoms that arise when you partake of each food. If your symptoms return, your doctor can usually confirm the diagnosis.
  1. In some cases, your doctor may direct you to once again eliminate the remaining suspect foods from your diet to reinforce the diagnosis.

Maybe you’re thinking that elimination diet is just a fancy term for food challenge, and it sort of is. You eliminate the food and then challenge it. The elimination diet is not foolproof, and it can be risky. Psychological and physical factors can affect the diet’s results.

For example, if you think you’re sensitive to a food, a response could occur that may not be a true allergic one. And if you’ve experienced severe reactions to certain foods, your doctor should consider reintroducing the food only in the controlled setting of a food challenge.

Some types of allergy involve other parts of the immune system and are undetectable with the most common allergy tests — skin tests and RASTs. These are mostly gastrointestinal reactions, although occasionally eczema and other rashes may occur due to non- IgE mediated food allergies.

In such cases, allergists typically have to fall back on your history, which may range from tremendously helpful to completely misleading, especially if you experience delayed reactions or you and your doctor can’t pin down a specific food. If the history reveals little useful information, your doctor may recommend one of the following next steps:

  • The first next step is usually an elimination diet followed by either an oral food challenge or a gradual re-introduction of the food(s) at home.
  • The second diagnostic approach is to perform a biopsy of the esophagus, stomach, or intestine (obtained through a scope by a gastroenterologist). To obtain the most definitive results, your allergist (with the assistance of your gastroenterologist) may repeat the scope and biopsies after an elimination diet or after you’ve re-introduced the food into your diet.

Another type of skin testing called patch testing has shown some promise in the diagnosis of non-IgE mediated food allergy. When used for food reactions, small amounts of a pure food are placed in tiny cups, which your doctor tapes to your back. The foods are chosen based on diet, knowledge of common allergens, and previous reactions.

Your doctor removes the patches after 48 hours and reads them at 72 hours. During the writing of this book, no standardized reagents, application methods, or guidelines for interpretation are available, and patch testing is still finding its place in the diagnosis of food allergy.

Avoiding The Untested And Unproven

Admittedly, skin tests and RASTs, are less than perfect, but some allergy tests, often purported to be superior, are untested at best, proven to be worthless at their worst, and are usually pretty costly (and not covered by insurance). I commonly see patients who have spent thousands of dollars of these tests and who have been placed on broad avoidance diets based on totally inaccurate test results.

Remember those quackologists I talked about earlier in this article. They’re typically the misinformed, misguided souls who mislead their patients with these phony tests. Here’s a list of the most common dubious tests to watch out for:

  • IgG and IgG4 tests
  • Sublingual or intradermal provocation tests
  • Lymphocyte activation tests
  • Kinesiology
  • Cytotoxic tests
  • Electrodermal testing

Certain foods can make you miserable even though you’re not allergic to them. If your doctor examines your body, your history, and your test results and rules out food allergy, he may begin to suspect a food intolerance — an adverse food-induced reaction that does not involve the immune system.

Lactose intolerance is one example of a food intolerance. If you have a lactose intolerance, you lack an enzyme (lactase) that’s essential for digesting milk sugar. In this case, the milk sugar is the culprit. In the case of a milk allergy, a milk protein is the perpetrator.

When a person with lactose intolerance consumes milk products, symptoms such as gas, bloating, and abdominal pain may occur. Your doctor can perform a specific test for lactose intolerance called a breath hydrogen test, but for most other food intolerances no specific diagnostic test is available.

To diagnose an intolerance to other foods, such as wheat, your doctor is likely to re-examine the data he’s already collected:

  • History: A history that shows a predominance of gastrointestinal symptoms and a tolerance of small doses of the food commonly points to food intolerance. With food intolerance, symptoms typically occur when you eat larger quantities of the suspect food. While this can occur with food allergy, allergic reactions tend to occur at much smaller doses than food intolerances.
  • Test results: If your skin tests and RASTs show no signs of food allergy, a food intolerance may be triggering your symptoms.

Treatment for a food intolerance is very similar to food allergy treatment. Your doctor is likely to instruct you to avoid the offending foods or at least limit your consumption. The same food substitutes can often help you vary your diet without missing the foods you love. In the case of lactose intolerance, your doctor may prescribe lactase supplements to help you digest the milk sugar.