Some people have such an intense and immediate reaction to a food that it seems clear what is causing the problem. Even in these cases, it’s important to make sure which ingredient or contaminant someone is reacting to. Other people have more subtle or delayed reactions, and it’s more complicated to figure out which food or foods are responsible. There isn’t one conclusive test to diagnose a food allergy. Instead, the allergist gradually narrows down potential food allergens to identify the most likely cause of the symptoms in each person.

How Are Food Allergies Diagnosed?

Allergists have many techniques to identify potential food allergens. Some tests provide quick results, while others, such as elimination diets, may take weeks to complete.

Tests and What They Show

Some tests can provide proof of allergic reactions in the body, while others are performed to rule out other conditions. There’s no test that can accurately determine how severe a food allergy might be.

Medical History

The allergist will take a thorough history, asking about symptoms over time and family medical history. A clear medical history can help a doctor identify risk factors, which may either confirm the likelihood of food allergies or rule out other conditions.

Physical Exam

The allergist will perform a physical exam to look for ongoing allergy symptoms, such as rash, abdominal pain, dark skin under the eyes, or discolored and swollen membranes inside the nose. They will likely listen to your heart and breathing to look for changes that could be caused by allergies.

Skin Testing

Allergists commonly use the skin prick test to check for food allergies. Before the test, they will ask you to stop taking any antihistamine medications for a few days, as these drugs can make the test results inaccurate. During the test, they will draw a grid on your back or forearm, each square representing a different food. One square will contain pure histamine, which almost everyone reacts to, helping to make sure the test is working correctly.

Histamine is a chemical the body releases during allergic reactions. It makes the immune system treat harmless substances (allergens) as threats, leading to symptoms like itching, swelling, and sneezing as the body tries to defend itself. Histamine is used to show the allergist what a positive test reaction will look like on each person’s skin. A drop of a food allergen is placed in each square. Next, the allergist will lightly prick your skin with a small medical device like a lancet or probe to allow the allergen to enter the skin. The pricking can be uncomfortable. The allergist waits about 15 to 20 minutes, then examines the skin to see which allergens caused a response. A positive response shows up as a small, itchy swelling much like a mosquito bite.

A negative response (no swelling) to a skin test is a fairly reliable indicator that you’re not allergic to a particular food. A positive response to the skin testing may not mean you’re allergic. The skin prick test is a useful tool to narrow down the foods that are most likely causing allergy symptoms, but more testing may be needed to confirm the allergy.

Blood Test

If you cannot tolerate skin testing, are taking medications like antihistamines that could interfere with the test results, or if your skin test results are unclear, you may be given a blood test. Immunoglobulin E (IgE) is an antibody that triggers allergic reactions when the body detects allergens like pollen or certain foods. The specific IgE test, also called the RAST or ImmunoCAP test, measures the level of IgE in the blood that reacts to the allergen.

Blood testing is generally less reliable than skin testing because it can produce many false positives, showing an allergy when one doesn’t exist. Allergists often use a combination of both skin and blood tests when a food allergy is unclear based on medical history.

Elimination Diet

If certain foods test positive for the skin or blood test and it’s not clear if these foods are causing symptoms, the next step may be an elimination diet. In an elimination diet, you avoid the suspect foods completely for usually two weeks or one month. During this process, keep a record of any symptoms. If the eliminated foods were causing your symptoms, they should fade as the foods leave your body. After avoiding the food for a while, if your initial symptoms were mild or not life-threatening, the allergist may ask you to start eating the food again, either slowly or in larger amounts, and report how your body reacts. If symptoms return, the allergist will consider it likely that you are sensitive to that food.

Food Challenge

If the results from skin or blood tests and an elimination diet are unclear, the allergist may suggest a food challenge. A food challenge is done at the allergist’s office while you’re being watched closely. Food challenges are considered extremely accurate. There are three types of food challenges: open, single-blind, and double-blind.

In an open food challenge, you know that you’re eating the suspect food. You’ll eat a small portion of the suspected allergen and then wait for a few minutes. A nurse will check on any reactions periodically. If you have little or no reaction, you’ll eat a larger portion of the food, followed by waiting and regular check-ins. You may be given a final, larger portion of the food item and wait yet again for a reaction to occur. You can expect to spend two or three hours undergoing a food challenge.

In a single-blind food challenge, you don’t know whether you’re receiving the suspected food allergen or a placebo (safe food), but the allergist knows what you’re getting. In a double-blind food challenge, neither you nor the allergist know whether you’re receiving the allergen or a placebo. Single- and double-blind food challenges may require multiple office visits on the same or different days.

Endoscopy or Colonoscopy

It’s rare, but an allergist may suggest that you see a gastroenterologist to have an endoscopy or colonoscopy to examine your intestines. This might be done in a case of ongoing gastrointestinal symptoms when the allergist is uncertain whether problems are caused by a food intolerance, or another condition altogether.

An endoscopy is a test in which a small camera is inserted into your stomach via the mouth and esophagus (upper endoscopy) or into the rectum to examine the lower colon or the entire colon (colonoscopy). These tests can look for celiac disease (an autoimmune condition triggered by gluten) acid reflux, inflammatory bowel disease, and other conditions that may mimic food allergies or intolerances. The intestines will likely appear normal in the case of a food intolerance.

Unfortunately, there are no specific tests to diagnose food intolerances. The only way to identify them is through trial and error by avoiding certain foods and seeing if your symptoms improve.

Ruling Out Other Conditions

Many other conditions can cause symptoms similar to those of food allergies. The process of ruling out similar conditions is referred to as differential diagnosis. Conditions that can mimic symptoms of food allergies include:

• Lactose intolerance
• Celiac disease (a condition when the body cannot tolerate gluten)
• Reactions to food additives such as dye, sulfites, or monosodium glutamate (MSG)
• Food poisoning
• Crohn’s disease
• Histamine toxicity from eating foods high in histamines, like spoiled fish
• Gallstones
• Anxiety
• Chronic hives
• Chronic obstructive pulmonary disease (COPD)

Your doctor may be able to rule out many of these conditions quickly based on your age, medical and family history, or simple blood tests. Other disorders may require time, repeated tests, medication trials, or visits to other specialists before they can be confirmed or ruled out. Having other health conditions along with food allergies can make it harder to figure out what’s causing the symptoms and might lead to multiple diagnoses.

Commercial Food Allergy Tests

There are many commercial products marketed as being able to diagnose food allergies or sensitivities. Customers are told to send in a sample of blood or hair for testing and are promised valid diagnostic results. According to the American Academy of Allergy, Asthma and Immunology and the U.S. National Institute of Allergy and Infectious Diseases, these commercial tests do not provide valid results and should be avoided.

Talk to Your Doctor

Diagnosing food allergies requires different tests and methods like skin prick tests, blood tests, and elimination diets. Because no single test can give a definite answer, allergists often combine several approaches to find out which foods are causing symptoms. It’s also important to rule out other conditions that might look like food allergies. By working closely with a doctor, you can better understand and manage food allergies and stay safe.

Find Your Team

On MyFoodAllergyTeam, the social network for people with food allergies and their loved ones, more than 41,000 members come together to ask questions, offer support and advice, and connect with others who understand life with food allergies.

Have you undergone any tests, such as skin prick tests or elimination diets, to diagnose a food allergy or intolerance? What was your experience like? Share in the comments below, or start a conversation by posting on your Activities page.