How Food Allergies Are Diagnosed

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To diagnose food allergies, your doctor first will talk to you about your reactions to certain foods and conduct a physical exam and detailed medical history. Skin prick tests or blood tests can be used to check for reactions to specific food allergens. An oral food challenge may be done if other testing is inconclusive. Ultimately, your doctor will use all this information to diagnose you with food allergies.

Self-Checks/At-Home Testing

Your doctor will likely ask you to take several steps at home that will help determine if you have a food allergy. These steps include keeping a food and symptom diary and you might be asked to participate in an elimination diet, both of which can provide helpful diagnostic information.

Be wary of home testing kits for food allergies. These are not accurate (they test for the wrong type of antibodies—IgG instead of IgE) and often are falsely positive. This might lead to unnecessarily avoiding healthful foods.

Keeping a Food Diary

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If your doctor is unsure whether food is the cause of your symptoms, he or she may recommend that you keep a food diary. A food diary can also be used to monitor dietary patterns to better pinpoint the cause of your symptoms.

The diary should be a complete record of not only the foods you've eaten over a given period of time (usually a week), but also an accurate record of the timing and characteristics of any symptoms you may have experienced.

Your doctor may go so far as to ask you to record any activities you may have done prior to your symptoms to assess whether they may have contributed. In some cases, stress and physical exertion can play as much a role in your symptoms as the foods you eat.

A food diary is often used in combination with other diagnostic tools or as a first step in opening an investigation.

Elimination Diets

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Your doctor may place you on an elimination diet to help pinpoint a food allergy. You keep track of your symptoms as you eliminate suspected foods from your diet.

While doctors will conduct an elimination diet in different ways, the basics are similar: to exclude the foods you suspect are causing your symptoms and then to reintroduce them after several days or weeks to see if symptoms reappear.

Elimination diets should only be done in consultation with your allergist, who should recommend what to avoid and for how long. Never reintroduce a food you suspect as having triggered an anaphylactic reaction. Staying on an elimination diet might lead to nutritional imbalance.

Labs and Tests

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Your medical history can help your allergist pinpoint potential allergens and choose which testing methods might be most appropriate. These can include blood tests and skin tests. An oral food challenge may be used to confirm a diagnosis.

Prick Test

A prick test (also called a scratch test or a skin test), is often used to test a number of potential allergens at one time. Despite the name, this isn't a painful test, and it can provide a lot of information fairly quickly.

To perform a series of prick tests, your allergist will use either the thin skin of your forearm or your back. A drop of a solution that includes the food allergen is placed on the skin. The allergist scratches the skin to allow for a very minimal amount of the solution to enter just below the surface.

If the test is positive, you'll develop a hive, or wheal, in the area of the prick or scratch. A wheal is a raised white bump surrounded by a circle of itchy skin. All prick testing is done within your doctor's office, under close supervision, in case you have a serious allergic reaction.

Prick tests can provide lots of useful information, but sometimes they simply raise questions. An inconclusive prick test will usually be followed by a more sensitive test.

Blood Tests

Blood tests are used to test for food allergies that are IgE-mediated, which are usually those with rapid onset after exposure (acute reactions), as opposed to food allergies that have delayed reactions. The name of the test is often related to the method used, and may be called immunoCAP , enzyme-linked immunosorbent assay (ELISA) , or radioallergosorbent (RAST) test.

To perform the test, your doctor will draw a sample of your blood. With these tests, you never have to be exposed to the allergen directly—instead, your blood is exposed to it in the lab. A positive test result indicates that the body has produced antibodies to an allergen and is primed for a reaction.

These blood tests are accurate and can help in situations where a skin test isn't recommended—for example, to determine if a child has outgrown a serious allergy or when a patient has severe eczema or another skin condition. However, they do have some downsides. They're more expensive than skin prick tests, and they take days or weeks (as opposed to minutes) to produce results.

Oral Food Challenge

In a food challenge, you ingest suspected allergens and are observed over a number of hours to determine whether you have an allergic reaction. An oral food challenge is risky and always should be carried out under close medical supervision, but it will show the presence of an allergy conclusively. You should never attempt an oral food challenge without consulting with your doctor since you could have a serious allergic reaction.

According to the American College of Allergy, Asthma and Immunology, an oral food challenge is the only test that confirms a food allergy.

Imaging

Endoscopy...
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Imaging is not typically used in diagnosing food allergies. Endoscopy and biopsy might be recommended if you are experiencing digestive tract bleeding, chronic constipation or diarrhea, or severe persistent stomach pain. It is more typically used to check for celiac disease, which is an autoimmune reaction to gluten.

Using an endoscope, a flexible tube with a light and camera, your doctor can view images of your small intestine on a digital monitor and take a sample of tissue (biopsy) for evaluation in the lab. For this test, the endoscope is lowered into the stomach, usually through your esophagus.

Differential Diagnoses

It is very common for people with symptoms of a food allergy to actually have a different condition. The suspect food may indeed be a trigger of unpleasant symptoms, but the reaction is not allergic in nature. For example, gastric reflux can be provoked by some foods, and some people have irritant reactions to hot or spicy foods (including a runny nose).

In the case of a severe acute reaction, a food allergy might be suspected but the doctor will want to explore whether the true allergen was instead a medication, insect sting, etc. Food poisoning can often mimic a food allergy and should always be considered.

Lactose intolerance is a common problem that can mimic food allergies. According to research, up to 36 percent of Americans have some level of lactose malabsorption, as measured in a hydrogen breath test. While many will have mild to severe lactose intolerance symptoms (bloating, abdominal pain, diarrhea after consuming milk products), others will have no symptoms at all.

In children, atopic dermatitis may be associated with food allergies, but an eczema flare might be due to other factors such as irritants, humidity, or skin infection.

A Word from Verywell

It can be tricky for your doctor to determine what is causing your symptoms of food allergy. Symptoms often appear hours after you've eaten, so trying to pin cause to effect is not always easy.

Diagnosing a food allergy is ultimately a process of trial and error, and there is no pretending that the process is easy. Elimination diets can be hard to implement, especially if you eat out frequently, and food diaries don't always capture everything you eat.

Even so, your best bet is to follow your doctor's advice diligently, even if it means giving up fast food for lunch or taking a few minutes after lunch to write down everything you've just eaten. If you are patient and remain diligent, you are more likely to find the cause of your problem.

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