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 is important to make sure which ingredient or contaminant someone is reacting to. Other people have more subtle or delayed reactions, and it is more complicated to figure out which food or foods are responsible. There is not 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.
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.
Some tests can provide proof of allergic reactions in the body, while others are performed to rule out other conditions. The allergist will request that no antihistamine medications be taken for several days before allergy testing takes place. Antihistamine drugs can render allergy tests ineffective.
There is no test for food allergy that can determine the severity of a food allergy.
The allergist will take a thorough history, asking about symptoms over time and family medical history. A clear picture may emerge from the medical history that will help a doctor assess risk factors that may strengthen the suspicion of food allergies or rule out other conditions.
The allergist will perform a physical exam to look for chronic allergy symptoms, such as rash, abdominal pain, dark skin under the eyes, or pale and swollen membranes inside the nose. The allergist will likely listen to the heart and breathing to look for changes that could be caused by allergies.
There are two ways to test for allergies on the skin. The skin prick test is the most common. In the skin prick test, a grid will be drawn on your back or forearm. Each square corresponds to a specific food. One square will be pure histamine, which provokes a response in nearly every person. Histamine is used to show the allergist what a positive reaction will look like on each person’s skin. A drop of a food allergen is placed in each square. The skin is then lightly pricked with a medical lancet or probe to allow the allergen to enter the skin. The pricking can be uncomfortable. The allergist waits a few minutes, then examines the skin to see which allergens provoked a response. A positive response will cause 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 are not allergic to a food. A positive response to the skin testing, whether skin prick or intradermal, may not mean you are allergic. The skin prick test is a useful tool to narrow down the food or food that are most likely causing allergy symptoms, but more testing may be needed to confirm the allergy.
If a person cannot tolerate skin testing, is taking certain medications that would interfere with results of skin testing, or has skin testing results that are inconclusive, a blood test may be administered instead. The specific IgE test — also called the RAST or ImmunoCAP test — measures the level of immunoglobulin E (IgE) in the blood that reacts to that allergen. Blood testing is a much less reliable test than skin testing since it produces many false positives — results often indicate that an allergy exists when it does not.
If certain foods tested positive for the skin or blood test, the next step may be an elimination diet. In an elimination diet, you avoid the suspect foods completely for a period, usually two weeks or one month. During this process, keep a record of any symptoms. If the eliminated foods were causing your symptoms, the symptoms should fade as the foods leave your body. At the end of the period of avoidance, the allergist may ask you to add the eliminated food back into your diet, either gradually or in large quantities, and report the results. If symptoms return, the allergist will consider it likely that you are indeed allergic to that food.
If the allergist considers results uncertain after skin or blood tests and an elimination diet, they may recommend a food challenge. A food challenge is conducted at the allergist’s office under close medical observation. Food challenges are considered extremely accurate. There are three types of food challenge: open, single blind, and double blind.
In an open food challenge, you know that you are eating the suspect food. You will eat a small portion of the suspected allergen and then wait for a few minutes. A nurse will check on any reactions periodically. If small or no reactions occur, you will eat a larger portion of the food item, followed by waiting and 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 do not know whether you are receiving the suspected food allergen or a placebo, but the allergist knows which you are getting. In a double-blind food challenge, neither you nor the allergist know whether you are getting the allergen or a placebo. Single- and double-blind food challenges may require multiple office visits on the same or different days.
Rarely, an allergist may order an endoscopy or colonoscopy to examine the intestines for food allergy symptoms. This might be done in a case of chronic gastrointestinal symptoms when the allergist is uncertain whether problems are caused by a food allergy, a food intolerance, or another condition altogether.
An endoscopy is a test in which a small camera is inserted into the stomach via the mouth and esophagus (upper endoscopy) or into the rectum for examination of the lower colon or the entire colon (colonoscopy).
Dozens of other conditions can produce symptoms like 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:
Your doctor may be able to rule many of these conditions quickly based on your age, medical and family history, or simple blood tests. Other disorders may require time and repeated tests before they can be confirmed or ruled out. The presence of other conditions in addition to food allergies may complicate the differential diagnosis and eventually result in multiple diagnoses.
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 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.