Patient education: Food allergy symptoms and diagnosis (Beyond the Basics)

FOOD ALLERGY OVERVIEW — 

Reactions to food are common and cover a wide range of symptoms that can be broadly divided into two categories: food allergy, which involves your immune system, and nonimmune food reactions. It is important to know the difference between food allergies and other types of reactions because the risks and management of each are different.

●Food allergy develops when the body's immune system has an abnormal reaction to one or more proteins in a food. Food allergies can lead to serious or even life-threatening allergic reactions including anaphylaxis.

●Other nonimmune food reactions are far more common than food allergies. These reactions are less likely to be serious or life threatening and include examples such as lactose intolerance, heartburn (gastroesophageal reflux), bacterial food poisoning, and sensitivity to caffeine.

This article discusses the signs and symptoms of food allergy and tests that may be recommended to diagnose food allergies. The management of food allergy generally involves strictly avoiding that food; this is discussed separately. (See "Patient education: Food allergen avoidance (Beyond the Basics)".)

CLASSIC (IgE-MEDIATED) FOOD ALLERGY — 

In people with "classic" food allergy, the immune system reacts to proteins in certain foods as foreign or harmful and produces antibodies in response. These antibodies are called immunoglobulin E (IgE) antibodies. When the person is exposed to that food protein again through eating the food, these IgE antibodies bind to the food protein, triggering a release of chemicals. These chemicals cause the symptoms of an allergic reaction, which typically occur quickly, within minutes to two hours after eating. A person with a food allergy can also have a "local reaction" to a food. If the allergic food touches their skin, they may develop hives or a rash at the site of contact. Skin exposures and local reactions are not expected to lead to serious or life-threatening allergic reactions.

You may hear classic food allergy referred to as "IgE-mediated" food allergy.

Sudden-onset food allergy symptoms — The symptoms of a food allergy typically happen within minutes to two hours after eating and can vary from mild to severe or even life threatening. Symptoms experienced during a previous reaction do not predict how severe a future reaction will be. As an example, a person could have a mild reaction with hives after eating peanuts on one occasion and then have a serious anaphylactic reaction after eating peanuts another time (see 'Anaphylaxis' below). However, reactions do not necessarily get worse after each exposure.

The sudden-onset symptoms of food allergy can include any combination of the following:

●Skin – Itching, flushing, hives (itchy bumps, also called "urticaria"), or swelling (angioedema)

●Eyes – Itching, tearing, redness, or swelling of the skin around the eyes

●Nose and mouth – Sneezing, runny nose, nasal congestion, swelling of the lips or tongue, or increased mucus production

●Lungs and throat – Difficulty getting air in or out, chest tightness, repeated coughing, wheezing or other sounds of labored breathing, throat swelling or itching, hoarseness or change in voice, or a sensation of choking

●Heart and circulation – Dizziness, weakness, fainting, changes in heart rate (fast, slow, or irregular), or low blood pressure

●Digestive system – Nausea, vomiting, abdominal cramps, or diarrhea

●Nervous system – Anxiety, confusion, or a sense of impending doom

Specific presentations

Anaphylaxis — Anaphylaxis is the most serious type of allergic reaction and can cause life-threatening signs and symptoms, including difficulty breathing, swelling of the upper throat and/or tongue, low blood pressure, or cardiac arrest (the heart stops beating). (See "Patient education: Anaphylaxis symptoms and diagnosis (Beyond the Basics)".)

Anaphylaxis related to food allergy generally begins within 5 to 60 minutes of exposure to the food, although, in rare cases, symptoms don't start until several hours after eating. A person who develops symptoms of anaphylaxis must be treated immediately with epinephrine. For this reason, if you or your child has been diagnosed with a food allergy that has the potential to cause anaphylaxis, your clinician will prescribe emergency epinephrine to keep with you at all times. It comes in two forms:

●An autoinjector (sample brand names: Auvi-Q, EpiPen), which has a shot you can give yourself or your child (figure 1)

●A nasal spray (brand name: neffy) (figure 2)

Your doctor will also prescribe extra doses if needed (for example, to keep at school or with a caregiver).

The treatment of anaphylaxis is discussed separately. (See "Patient education: Anaphylaxis treatment and prevention of recurrences (Beyond the Basics)" and "Patient education: Using an epinephrine autoinjector (Beyond the Basics)".)

Allergic rhinitis and conjunctivitis — Food allergies can trigger allergic symptoms in the nose, eyes, or throat. The most common nose, eye, and throat symptoms include a runny nose; congestion; sneezing; nasal itching; itchy or watery red eyes; and an itchy mouth, tongue, or throat. These can happen along with or before other whole-body symptoms such as hives, difficulty breathing, vomiting, etc but rarely occur as the only sign of a classic food allergy. One exception is that an itchy throat or mouth without any other symptoms may represent a unique type of food allergy called pollen-food allergy syndrome. (See 'Oral allergy syndrome' below.)

Oral allergy syndrome — Oral allergy syndrome, or pollen-food allergy syndrome, is seen in up to 50 percent of people with allergic rhinitis caused by pollen (also known as seasonal allergies). In this condition, people who are allergic to pollen can have an allergic reaction after eating certain raw (uncooked) fruits or vegetables. The reaction is immediate and can cause itching, irritation, and mild swelling of the lips, tongue, roof of the mouth, and throat. A list of pollens and foods that cross react is available in the figure (figure 3).

Symptoms of oral allergy syndrome may be more noticeable during pollen season. Symptoms usually resolve within minutes after the person stops eating the food. Most people have only localized symptoms (that is, affecting only the mouth).

Less than 10 percent of people develop body-wide symptoms from fruits and vegetables (eg, vomiting, coughing), and 1 to 2 percent of people develop anaphylaxis (see 'Anaphylaxis' above). People with a history of body-wide symptoms should carry emergency epinephrine.

The reaction does not usually occur if the fruits or vegetables are cooked. Tree nuts and peanuts may be an exception to this as they are associated with a higher risk of severe and body-wide reactions. If you have a history of an oral allergy to nuts, your allergist may recommend avoiding them in all forms (raw, roasted, or cooked) and/or limiting the amount you eat at once.

Food-dependent exercise-induced anaphylaxis — Some people develop anaphylaxis after eating a certain food and then exercising afterwards (within about four hours of eating the food). This is called "food-dependent exercise-induced anaphylaxis." The reaction can occasionally occur after exercising first and then eating the food. Importantly, with this rare form of food allergy, the food does not cause anaphylaxis if the person does not exercise, and exercise does not cause anaphylaxis if the person does not eat the food.

The most common foods associated with this condition include wheat, celery, and seafood, although some people react after eating any food and then exercising. Not eating for several hours before exercise can usually prevent this type of reaction.

MIXED IgE- AND NON-IgE-MEDIATED FOOD ALLERGIES — 

There are several conditions that may be food related, such as eosinophilic gastrointestinal disorders (eosinophilic esophagitis) and atopic dermatitis (eczema). Eczema is discussed in detail separately. (See "Patient education: Eczema (atopic dermatitis) (Beyond the Basics)".)

NON-IgE FOOD ALLERGIES — 

It is possible to have a food allergy that does not involve immunoglobulin E (IgE) antibodies. With this type of food allergy, symptoms are usually slower to develop and last longer than those of classic (IgE-mediated) food allergies. Because IgE is not involved, non-IgE food allergies do not have a risk of anaphylaxis.

The three main types of non-IgE food allergies are:

●Food protein-induced enterocolitis syndrome (FPIES) – This is a serious type of allergy that mostly affects infants; it causes severe gastrointestinal symptoms like continuous vomiting and diarrhea and can also cause low blood pressure. FPIES is most often triggered by cow's milk or soy protein, which are found in many infant formulas. It often resolves by the age of three to five years.

●Food protein-induced allergic proctocolitis (FPIAP) – This condition affects the lower part of the colon. It can cause rectal bleeding and diarrhea in infants.

●Celiac disease and dermatitis herpetiformis – Celiac disease is a condition in which the immune system responds abnormally to a protein called gluten, which then leads to damage to the lining of the small intestine. Gluten is found in wheat, rye, barley, and many prepared foods. Dermatitis herpetiformis is a skin condition that is also related to gluten sensitivity. (See "Patient education: Celiac disease in adults (Beyond the Basics)".)

FOOD ALLERGY DIAGNOSIS — 

If you suspect that you or your child may have a food allergy, it's important to see a clinician for evaluation. They will learn about your history (including past exposures to the food and what symptoms you have experienced) and likely do tests to determine whether you have a true food allergy and need to avoid a particular food. Taken together, all of this information can help a clinician diagnose a food allergy.

Medical history — During a medical history, the clinician will ask questions about your past reactions to food, such as:

●What symptoms did you have?

●What particular food do you think caused the reaction? Had you eaten this food before? If yes, how often were you eating it, when did you last eat it, and had you eaten the food without having a reaction?

●How much of the food did you eat?

●What other foods did you eat at that time? Do you know all the ingredients of the food you ate? Include all foods: appetizer, main dish, sauces, dressings, breads, beverages, and side dishes.

●How was the food prepared? As an example, could the food have been fried in oil used to prepare other foods?

●Were any of the following eaten: peanuts, tree nuts, sesame, shellfish, fish, milk, eggs, wheat, or soy?

●How much time passed between eating the food and the first symptoms?

●Did you exercise or do other physical activity after eating?

●Did you take any medications, herbs, vitamins, nonprescription medications, or drink any alcohol before or after eating?

●How was the reaction treated? Did it resolve without treatment, or did you take any medications? How long were the medications continued, and were there any later symptoms?

These questions will help the clinician to determine if a food allergy might be likely as well as the type of food allergy that might be involved. Confirming classic food allergy often involves allergy testing. The clinician may decide to order blood tests (see 'Blood tests' below). In other cases, they will refer you to a specialist (such as an allergist or gastroenterologist) for further evaluation.

Allergy testing — Testing for classic food allergy often includes skin testing and/or blood tests to determine if IgE to the food is present. Depending upon the situation, testing may also be done to determine if a person is allergic to pollens, insects, latex, and other allergens. However, allergy testing is only recommended if the person is suspected to have a food allergy. As an example, if a person had a reaction after eating peanuts but has never reacted to wheat or eggs and eats them regularly, it is not necessary to test for allergy to wheat or eggs. Neither the skin test nor the blood test alone is sufficient to diagnose food allergy; the clinician must also consider the person's medical history and other supporting information.

Skin testing — Skin testing is an office-based procedure that involves pricking/scratching the skin with a tiny device that is coated with food extract or fresh food. It is done by a trained clinician (usually an allergy specialist) and can be done on both adults and children. The pricks are usually done on the forearm or upper back after the skin is cleaned with alcohol. This should not be very painful, and any mild discomfort should resolve quickly. Antihistamine medications can interfere with skin testing and need to be stopped several days prior to testing.

Fifteen minutes after the testing is applied, an itchy bump (hive) may form where the skin was pricked, indicating if IgE is present. The size of the bump will be measured and recorded. This result will be considered along with the history and any additional testing to determine if a food allergy is present.

Blood tests — Blood tests are another way to determine if a person has IgE antibodies to certain foods in their system. Different than skin testing, which provides results immediately after testing, blood tests typically require several days to be completed. Unlike with skin testing, antihistamine medications do not interfere with blood testing. Blood tests are widely available and do not require an allergy specialist to perform the test. However, positive results are possible in people who are not allergic to the food. Thus, consultation with an allergy specialist may be recommended to interpret the results of the test.

Elimination diets — An elimination diet is a specially designed diet that removes one or more foods or groups of food from a person's diet as part of the process of determining if a person has food allergies. After a predetermined amount of time, the food is then added back to see if signs or symptoms of an allergy develop. It is critical that an allergist or dietitian is involved in designing an elimination diet to first ensure that the suspected food is completely avoided and second to prevent the risk of malnutrition, especially in infants and children. As with other forms of allergy testing, an elimination diet by itself does not often lead to the diagnosis of food allergy and should be interpreted in combination with the patient's history.

During an elimination diet, it is important to read food labels carefully. In the United States, the Food Allergy Safety, Treatment, Education, and Research (FASTER) Act mandates that nutritional labels on food packages plainly identify nine specified food allergen sources (milk, eggs, fish, crustacean shellfish, tree nuts, peanuts, sesame, wheat, and soy), although other foods may still appear under multiple names. (See "Patient education: Food allergen avoidance (Beyond the Basics)".)

In addition, it's important to understand that "substitute" foods, which reduce or eliminate fat or other components of a food, may still contain the allergenic proteins. As an example, some egg substitutes (which are lower in cholesterol) still contain egg white proteins.

Food diary — Your clinician may suggest keeping a complete record of everything you eat over a period of time, including all foods, drinks, condiments, and candies. A form to record this information is available here (form 1). As is true with an elimination diet, it is important to remember that a food diary by itself does not often lead to the diagnosis of food allergy.

Food challenges — If it is not clear if a person has a food allergy based upon their medical history and the results of allergy testing, a clinician might suggest a medically supervised "food challenge." Food challenges may also be recommended to determine if a known the food allergy is improving or has resolved. In addition, some foods such and egg and milk become less allergenic when they are extensively heated (eg, baked in bread or muffins), and a food challenge may be performed to find out if the allergic person is able to eat the food in these cooked forms.

A food challenge is done in a hospital or allergy clinic; it involves giving the person a tiny amount of the potentially allergenic food to eat. After eating the first sample of food, the person is observed for 10 to 30 minutes. If there is no reaction, a slightly larger amount of the food is given and again followed by observation. This pattern is continued until an age-appropriate serving size is eaten. If the person develops signs or symptoms of an allergic reaction, the food challenge is immediately stopped, and treatment is given if necessary.

Food challenges should only be performed in a setting where the personnel and equipment needed to treat anaphylaxis are available.

●Preparing for the food challenge – The clinician will provide specific instructions, including what can be eaten on the day of the food challenge and when certain medications may need to be stopped prior to the challenge. If you have epinephrine, you should bring it with you to the food challenge in case you develop a delayed allergic reaction on the way home.

●If there is no reaction during the food challenge – If you did the food challenge to find out if you have an allergy, and you do not have any signs of an allergic reaction during the food challenge, you are most likely not allergic to that food. However, you could still have allergies to other foods, so be sure you understand whether and when foods should continue to be avoided.

If you already have a known allergy to a food (such as egg or milk) but pass a challenge without symptoms for that food in extensively heated form, you will still need to be careful to avoid the food in raw or less cooked form. For example, a person with a milk allergy might be able to eat bread or processed foods that contain milk but still need to avoid drinking milk and eating dairy products like cheese or yogurt. A clinician will discuss the results of your food challenge and give recommendations on what to do moving forward.

WHEN TO SEEK HELP — 

It is sometimes difficult to know if a reaction is caused by a food allergy or a nonimmune food intolerance. Anyone who has one or more of the following symptoms suddenly after eating should seek medical care:

●Nausea or vomiting

●Cramping, abdominal pain, or diarrhea, especially if there is blood or mucus in the stool

●Itching or raised red hives on the skin

●Swelling, especially of the lips, mouth, face, or throat

●Wheezing, coughing, or difficulty breathing

●Lightheadedness or passing out

Having a food allergy can be challenging. But it is possible to have a full life and enjoy cooking and eating, as long as you are prepared to recognize and treat symptoms of an allergic reaction.

WHERE TO GET MORE INFORMATION — 

Your health care provider is the best source of information for questions and concerns related to your medical problem.

This article will be updated as needed on our website (www.uptodate.com/patients). Related topics for patients, as well as selected articles written for health care professionals, are also available. Some of the most relevant are listed below.

Patient level information — UpToDate offers two types of patient education materials.

The Basics — The Basics patient education pieces answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials.

Patient education: Food allergy (The Basics)
Patient education: Starting solid foods with babies (The Basics)
Patient education: Lactose intolerance (The Basics)
Patient education: Angioedema (The Basics)
Patient education: Eosinophilic esophagitis (The Basics)
Patient education: Allergy skin testing (The Basics)
Patient education: Peanut, tree nut, and seed allergy (The Basics)

Beyond the Basics — Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are best for patients who want in-depth information and are comfortable with some medical jargon.

Patient education: Food allergen avoidance (Beyond the Basics)
Patient education: Anaphylaxis symptoms and diagnosis (Beyond the Basics)
Patient education: Anaphylaxis treatment and prevention of recurrences (Beyond the Basics)
Patient education: Eczema (atopic dermatitis) (Beyond the Basics)
Patient education: Celiac disease in adults (Beyond the Basics)

Professional level information — Professional level articles are designed to keep doctors and other health professionals up-to-date on the latest medical findings. These articles are thorough, long, and complex, and they contain multiple references to the research on which they are based. Professional level articles are best for people who are comfortable with a lot of medical terminology and who want to read the same materials their doctors are reading.

Allergic and asthmatic reactions to food additives
Clinical manifestations of food allergy: An overview
Clinical manifestations and diagnosis of oral allergy syndrome (pollen-food allergy syndrome)
Management and prognosis of oral allergy syndrome (pollen-food allergy syndrome)
Diagnostic evaluation of IgE-mediated food allergy
Management of food allergy: Avoidance
Food allergy in schools and camps
Food-induced anaphylaxis
Food allergy management: Allergen-nonspecific therapies
Oral food challenges for diagnosis and management of food allergies
Primary prevention of allergic disease: Maternal diet in pregnancy and lactation
Respiratory manifestations of food allergy
Peanut, tree nut, and seed allergy: Clinical features
Seafood allergies: Fish and shellfish
Milk allergy: Clinical features and diagnosis
Egg allergy: Clinical features and diagnosis
The impact of breastfeeding on the development of allergic disease
Food allergy in children: Prevalence, natural history, and monitoring for resolution
Anaphylaxis in infants
Clinical manifestations and diagnosis of eosinophilic esophagitis (EoE)
Food protein-induced allergic proctocolitis of infancy

The following organizations also provide reliable health information.

●Medline Plus (medlineplus.gov/foodallergy.html; available in Spanish)

●American Academy of Allergy, Asthma & Immunology (AAAAI) (www.aaaai.org/conditions-and-treatments)

●Food Allergy Research & Education (FARE) (www.foodallergy.org/)

●American College of Allergy, Asthma & Immunology (ACAAI) (acaai.org/)

●National Institute of Allergy and Infectious Diseases (NIAID) (www.niaid.nih.gov/)

●Asthma and Allergy Foundation of America (AAFA) (www.aafa.org/)

●US Food and Drug Administration (FDA) (www.fda.gov/)

●US Department of Health and Human Services (healthfinder.gov/FindServices/)

●US Centers for Disease Control and Prevention (CDC) (www.cdc.gov/healthyschools/foodallergies/toolkit.htm)