How to Read a Food Allergy Skin Test: A Step-By-Step Guide

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A close friend of mine found out about her allergy to bananas (weird, I know) the hard way- she ended up in the ER with an epi-pen. Her experience compelled me to learn more about food allergy skin tests, and I’ll share here with you the critical pieces of information I’ve gathered. 

How do we read a food allergy skin test correctly? To correctly read a skin test, monitor your skin after the test for a reaction. If you have an allergy, a red, itchy bump, which may look like a mosquito bite will develop. The bump’s size should be measured for changes, and the results recorded.

As you may have guessed, there’s a lot more to testing for a food allergy than watching an itchy bump. Read on for more information on food allergy skin tests, and the questions that surround diagnosing and treating a food allergy.

How Does an Allergy Skin Test Work?

If you or your physician suspect you’ve got a food allergy, it’s imperative to get a skin test immediately. Food allergies can be very dangerous, and knowing upfront if you have one could save your life.

There are different kinds of food allergy skin tests, and lots of variables to consider when getting a skin test. Individuals often have different reactions to different types of skin tests. Your doctor or if you have a Board-Certified Allergists will determine which test is right for you.

A skin test for food allergies works the same as for other kinds of allergies- by exposing your body to different potential allergens and monitoring the results. Here’s what to expect when interpreting a food allergy skin test:

  • Your doctor or allergist will place a drop of solution containing the food allergen on your back or forearm. When testing for allergies to fruits or vegetables, fresh foods may be used instead of the derived solution.
  • Using a tiny plastic probe or needle, the physician gently pricks or scratches the skin. This allows a tiny amount of the solution to enter just below the surface of the skin, exposing the skin to suspected allergy-causing substances (allergens).
  • The area is monitored for 15-30 minutes after the skin pricks. An itchy red bump that looks like a mosquito bite (called a wheal) will develop if you’re allergic to one of the tested substances.
  • A nurse or doctor measures the size of the bump and records the results. Then he or she will clean your skin with alcohol. Measurements may take time, as the wheel may change in size and appearance after your skin is exposed to the allergen.

It’s crucial at this point to emphasize it’s recommended to take a food allergy skin test under the care of a professional. Home skin tests are available, but when testing for allergies, your body is being exposed to allergens that may cause severe reactions. 

Some patients are concerned that a skin test may be painful. Skin tests aren’t usually painful, the scratching feels a little like a fingernail scratch, and there’s no bleeding. They’re also inexpensive and often produce immediate results.

If an allergy test determines you have a food allergy, your doctor will develop a plan with you to help treat that allergy. Some specific allergy treatments can even eliminate a food allergy.

Preparing for the Allergy Skin Test

Both you and your doctor will cover your medical history together to determine if a skin test is the best for your needs. This usually involves asking detailed questions about your signs and symptoms and how you’ve been treating them.  The American College of Allery, Asthma and Immunology has an FAQ guide on allergy testing and how to prepare for one.

Based on this information, your doctor may be able to determine if genetically, allergies run in your family. They will also determine if an allergic reaction is what’s most likely causing your symptoms.

Your doctor may also do a physical exam, paying specific attention to your skin, throat, eyes, ears, nose, and chest to gather more information and clues about what’s causing your symptoms.

How to Get Your Skin Allergy Results

Before leaving your doctor or allergist’s office, you’ll know the results of a skin prick test. Sometimes, effects occur immediately. Other times it may take a few minutes, but the visual results of a skin test usually become apparent rapidly.

The same is true for an intradermal test. A blood test and a patch test, however, may take several days or more to receive results. (We’ll take a more in-depth look at different kinds of allergy tests a little later in this article.)

If your skin test returns positive, you may be allergic to the substance to which you were exposed. If bigger patches (wheals) of irritation emerge, it’s usually an indicator of heightened sensitivity when exposed to an allergic substance.

Like any medical exam, skin tests aren’t always 100% accurate. Sometimes, they can show you’re allergic to something when you’re not (creating a false positive), or they may not show a reaction to something you are allergic to (creating a false negative).

Reactions to the same test performed at different times can also vary, or you may even show sensitivity to an allergen during the test, but not show symptoms in daily life.

How Might my Allergies be Treated?

If your allergy tests return positive, your doctor will work with you to develop a treatment plan. Be sure to ask questions about anything you’re not clear about regarding your diagnosis or treatment. Your doctor should be more than happy to answer them.

Allergy treatment plans are diverse and depend on the individual’s situation. Possible treatments include:

  • Medications
  • Changes to work or home environmental conditions
  • Immunotherapy
  • Dietary changes

Different Types of Allergy Skin Tests

When visiting your doctor, she or she will determine which kind of skin test is best for you and your specific needs. There are four main types of skin tests:

Blood testing

Blood testing is considered a safer option for people who may be at higher risk for exhibiting a life-threatening reaction such as anaphylaxis when undergoing allergy tests.

Blood testing may also be recommended for people with asthma or heart disease, or who have eczema and/or extensive rashes, as these symptoms can make skin testing difficult.

If a patient is using certain medications that they can’t or would rather not stop taking, blood tests are a better option as well, as the skin test requires you to cease taking your medication for three days before the test for an accurate test result.

In a blood test, blood is drawn from a vein, usually in the arm, by a phlebotomist. A specialized laboratory then tests the blood’s components for specific results for several conditions, including allergies.

Skin Prick Testing (AKA the Scratch Test or Puncture Test)

Skin prick tests can measure immediate allergic reactions up to 40 different substances at once, and are usually done to determine allergies to more common irritants including, but not limited to:

  • Mold
  • Pollen
  • Pet-related allergies
  • Foods
  • Dust mites
  • Grass

Skin tests are usually painless and without bleeding. Testing for children is generally done on the upper back, and on the forearm in adults. The needles used in this type of testing (called lancets) are tiny, and barely break the skin’s surface.

A nurse will clean the area of the test site with alcohol, then will draw tiny marks on the skin. He or she will put a drop of allergen extract specific to the test next to each mark.

The lancet is then used to barely prick the skin’s surface, allowing the allergen extracts to absorb just below the first dermal layer. The nurse must use a new lancet for each allergen.

It’s necessary to scratch two additional substances into the skin to determine if your skin is reacting normally to the allergen. These are:

  • Histamines- Histamines cause a skin response in the majority of people, but if you’re in the small minority who don’t react to it, a skin test may not reveal an allergy, even if you have one. 
  • Glycerin or saline- Unlike in histamines, in most people, glycerin or saline doesn’t usually cause a reaction. If there is a reaction, your skin may be overly sensitive, and test results could indicate an allergic reaction where there is none.

Skin injection test

With a skin injection test, a thin needle is used to inject the allergen extract just under the skin on your arm. The amount used is small. Results should appear about 15 minutes after the test if you have an allergic reaction. This test commonly is recommended for allergies to penicillin or insect venoms.

Patch test

Allergic skin irritation, commonly known as contact dermatitis, is usually tested with patch testing. Patch tests are useful for detecting allergic reactions, which may take longer to develop than others. Patch tests are not commonly used to identify food allergies.

Instead of needles, the allergen solution is applied to the patch, which is then placed on your skin. During the test, your skin may have exposure to 20-30 extracts of substances, which commonly cause skin irritation due to allergic reactions.

Substances which are known to induce contact dermatitis commonly include:

  • resins
  • latex
  • hair color
  • fragrances and perfumes
  • medications
  • preservatives
  • metals

The patch should stay securely on the back or arm for up to 48 hours. The patient should avoid bathing and activities which cause heavy sweating, as moisture breaks down the adhesiveness of the patch.

After 2 days (or 48 hours), the doc will take off the patches. If the skin is irritated at the patch site, an allergy may be present, although it can take a couple of days for a patch test to reveal it’s results.

Are Skin Tests Safe?

On rare occasions, skin tests can produce a severe allergic reaction. These can often come swiftly, so it’s essential to go to an office with appropriate treatment such as medication and emergency medical equipment when getting a skin test. 

A staff trained in how to treat severe adverse allergic reactions effectively should be available.

Are Skin Tests Best for Detecting Food Allergies?

Skin tests are regularly recommended to determine food allergies. Skin testing for food allergies is very similar to screening for other kinds of allergies.

A nurse places a small drop of liquid food extract on the skin, testing for each food that may be causing an allergic symptom. Then, like in other allergy tests, the skin is pricked or scraped lightly.

Within 15 to 20 minutes, a wheal may appear, which looks much like that seen in other allergic reactions. If a wheal appears, you are most likely allergic to that food. A wheal is basically a red swollen mark on the skin.

A skin prick test can not only detect food allergies but also reveal food sensitivities. Unless there has already been a reaction to the food, however, the test may not always show an accurate response.

This is not well known but allergy blood tests may be used to test for food allergies too. Blood is normally taken from a vein located in the arm. In these tests, it usually takes a week on average to receive the results. 

A professional, board-certified allergist should perform skin tests or blood tests when testing for food allergies to increase the reliability of the accuracy of the results of the test.

Oral Food Challenge to Diagnose Food Allergies

It’s not uncommon for someone to display an allergic reaction to a particular food through a skin test or a blood test, but still be symptom-free when they eat that food.

Your allergist may suggest an oral food challenge to confirm or refute test results. In an oral food challenge, you eat or drink small portions of the suspected food allergen, then increase the amounts over time.

An oral food challenge done under a physician’s strict supervision will accurately help to reveal an allergic reaction. An oral food challenge should also be done with regular check-ins to a doctor’s office or in a hospital setting to ensure patient safety.

When should I avoid a skin test?

Even though skin tests are usually safe for adults, kids of all ages, and infants, there are circumstances where skin tests are not recommended. 

Your doctor may not recommend a skin test if:

  • You have ever had a severe or immediate allergic reaction. Some people can have such severe sensitivities to specific allergens, that even the smallest of amounts (like those used in skin tests) could trigger a dangerous anaphylactic reaction.
  • You are taking medications that may interfere with the test results. Some antidepressants, antihistamines, and heartburn medicines are some examples.
  • Your doctor decides that it’s more critical for you to continue taking your medications than to stop them in preparation for a skin test temporarily.
  • You have certain skin conditions. In cases of severe psoriasis or eczema which affect large areas of skin where a skin test would be conducted (like the back or arms), it may be challenging to do a reliable test.
  • You have other skin conditions that can cause unreliable test results, such as dermatographism.

Medications can Interfere With Your Results

Your doctor will need to review a list of all of your prescription and over-the-counter medications before you go in for your skin test. Some medicines can interfere or even negate some results because they suppress allergic reactions. Also, certain medications can possibly increase your risk of developing a life-threatening response during the test.

Medications metabolize in your system at different rates, because some take longer than others, your doctor may ask you to stop taking your medicines for three to ten days before your test.

Medications That May Alter Skin Test Results:

  • Tricyclic antidepressants. Ask your doctor about desipramine or nortriptyline. These brands have been shown to interfere with allergy skin tests.
  • Some heartburn medications
  • Some popular prescription antihistamines
  • Over-the-counter antihistamines
  • Xolair (A brand of asthma medication)

It’s imperative to notify your doctor about any medications you are taking, including over the counter medications and supplements.

What Causes an Allergic Reaction?

When understanding how to interpret the results of a skin test for food allergies correctly, it’s helpful to know what happens in the body to cause an allergic reaction in the first place.

If your immune system detects you are allergic to something, it may overreact to that allergen, causing reactive symptoms. Other than certain foods, examples of common allergens include, but are not limited to:

  • Pollen
  • Weeds
  • Grass

As a result of your immune system reacting, it produces antibodies called “Immunoglobulin E” (IgE). These antibodies travel through the bloodstream to cells that release chemicals, causing an allergic reaction.

Allergic symptoms most commonly manifest in the body through the skin, the nose, in the lungs, or the throat. Your doctor monitors these areas closely during an allergy test.

Each type of Immunoglobulin E has it’s own specific detectors for each type of allergen. This explains why folks are only allergic to one thing (for example, they only have the IgE antibodies specific to bananas); while others have allergic reactions to multiple allergens because they have many more types of IgE antibodies.

What if I’m not sure it’s a Food Allergy?

Erring on the side of caution is always best. If you’re not sure, see your doctor.

That said, there is a difference between having an allergy and having an intolerance or sensitivity to certain foods. The difference between a food allergy and a food sensitivity is the body’s response.

If you happen to have a food sensitivity or intolerance, the reaction is triggered by the digestive system instead of the immune system. While food intolerance can undoubtedly be very uncomfortable (think stomach virus for some), it isn’t usually life-threatening.

Symptoms of Food Intolerance Include:

  • Constipation
  • Cramping
  • Bloating
  • Diarrhea
  • Nausea
  • Gas

A full-blown allergy attack can be deadly, particularly if you’re caught unprepared. A food allergy can make your immune system flare up the reaction rather than the digestive system.

Symptoms of Food Allergy Include:

  • Anaphylaxis
  • Dizziness
  • Hives
  • Itching
  • Swelling

Allergy skin tests are true and trusted for detecting allergies to airborne substances (dust mites, pollen, etc.)

But because food allergies can be complicated, you may need additional tests or procedures.

Allergy tests will be able to confirm whether or not something you eat is causing your symptoms. Your physician will also use your medical history to diagnose your symptoms and determine a treatment.

How Do Allergists Determine Which Foods Make Me Sick?

For some people, it’s immediately apparent what food causes a food allergy in their system. For instance, many people with a shellfish, or a peanut allergy will quickly break out in a rash, or exhibit more dangerous allergic symptoms.

For others, the cause isn’t so obvious, and they may need a doctor’s help to find what foods are causing their allergies. One factor that can make it difficult to tell if you’re allergic to a particular type of food is that the symptoms may not even show up until hours after they have eaten that food.

When determining if you have a food allergy, your doctor will review your medical files and will have a standard list of questions for you.

Your doctor may ask about:

  • Whether or not your reaction occurs with other foods
  • If the reaction occurs every time you eat the food.
  • How much of the food you had eaten before the reaction.
  • Symptoms you’re having after eating the food.
  • How long it takes for the symptoms to occur after eating the food.
  • How often the reaction has occurred.
  • Details about any type of medical treatment you have had.

Your physician will also probably ask questions about your family’s medical history, your diet, and your home or living conditions. These variables may be working individually, or in tandem to aggravate your allergies. Your doctor will carefully record the information you provide for them. These questions will assist your allergist in narrowing down what is causing your allergies, or what may be making your symptoms worse.

Narrowing down the cause of a food allergy isn’t always as straightforward as eating food and having a reaction. This is why it is so important to see a specialist when diagnosing food-related allergies.

For example, for some people, an allergy to certain airborne pollens such as ragweed pollen can cause itching and swelling in the mouth and throat if they’ve eaten particular foods, such as melons. 

Can Special Diets Help Pinpoint the Problem?

Allergists often recommend their patients to use particular dietary practices to determine if they have a food allergy. Some key practices when using a dietary strategy include:

  • Restricting certain foods, or even adding certain foods to your diet. This may help narrow down which foods may be causing your symptoms.
  • Your allergist may ask you to keep a food diary to list all of the food you eat and the medications you take in order to narrow the search for foods causing allergies. Any symptoms should be noted in this diary as well.
  • It’s essential to be thorough when keeping a food diary. Detailed descriptions of your daily food intake habits can help your doctor tremendously to pinpoint the foods that are causing your symptoms.
  • If you and your doctor can narrow down only one or two foods that seem to trigger your allergic symptoms, eliminating them would help to determine if they are actually the source of your allergies.
  • It is important to be thorough about the elimination phase. Be sure to check labels and ask questions about how your food is prepared at restaurants. Don’t be shy about asking in-depth questions about what is in your food!
  • The suspected allergen foods should be avoided entirely for one or two weeks, then resumed again at the end of the prolonged testing time. The reintegration of suspected allergen foods can be risky and should be done under the supervision of your doctor.
  • If there is a marked decrease in allergic symptoms during the restrictive period, but the allergic symptoms return upon eating the food again, most likely, that is the food causing your allergy. At this point, you and your doctor will determine a course of action.
  • Making dietary choices about which foods to avoid for what length of time, and when you should begin eating the food again (if at all) can be confusing. An allergen can be dangerous in specific amounts, but harmless in others.

Some allergies, such as shellfish or nut allergies, can be so severe for some people that even dermal contact with other foods prepared in places where the allergen had previously been prepared can cause anaphylaxis.

It could be potentially dangerous to try to eat even a small portion of any foods your allergist has found to be a risk factor for anaphylaxis, so always consult your allergist before attempting. He or she will want to monitor your diet testing closely.