How Allergies Are Diagnosed

Your symptoms, and their timing, can guide your healthcare provider in making an allergy diagnosis. While that information may be enough to give your practitioner confidence that what you're experiencing is an allergic reaction, and treat you accordingly, various tests are usually required to formally confirm specific diagnoses. The testing procedures used, such as blood tests and skin tests, will depend on the type of allergic disease in question.

Self-Checks

Keeping a diary of your symptoms (what they are, when you experience them), activities, what you eat, and so on can reveal patterns and associations that can inform a diagnosis. Not only can you share this with your healthcare provider, but you can use it to inform changes you can make to avoid allergic reactions.

You may be enticed to try home allergy test kits. Know, though, that though do-it-yourself tests for a variety of conditions are readily available today, that doesn't mean they are all approved by U.S. Food and Drug Administration (FDA). Furthermore, many such allergy tests actually test for the wrong type of antibody—immunoglobulin G (IgG) rather than immunoglobulin E (IgE).

In fact, major medical organizations in the U.S., Canada, and Europe recommend against using these kits. If you choose to use one, doing so should never replace seeing a board-certified allergist to get an accurate diagnosis of what may be causing your symptoms.

Physical Examination

Diagnosis of allergies begins with your primary care provider or an allergist taking your medical and family history, listening to your report of symptoms, and doing a physical examination.

The provider will listen to your lungs and check your nose and throat for drainage, especially if you are reporting respiratory symptoms. You can expect your provider to also check your skin for signs of rashes that are often seen with allergies.

Your provider will then determine what types of testing or imaging may be needed to reach a diagnosis or rule out other conditions.

Labs and Tests

Diagnostic tests and criteria differ based on the type of allergy. The only two valid types of allergy testing are skin testing and specific IgE (sIgE) blood testing.

Other tests, such as spirometry or a food challenge, may be done if a particular potential diagnosis requires them.

Skin Testing

Skin testing for allergies has been standard for over 100 years. There are three options that a healthcare provider can consider using, and which they opt for will depend on the type of allergy suspected. In each case, a red, raised bump (like a mosquito bite) indicates a positive reaction. The procedures used are:

• Prick/puncture procedure: A scratch is made on the skin and a drop of a specific allergen is placed, then inspected for a reaction.
• Intradermal skin testing: This can be more sensitive, but also has more false positives. Instead of scratching, the allergen is injected just under the top layer of the skin.
• Patch testing: Patches with a small amount of the suspected allergen are placed on the skin and kept there for about two days to see if a reaction ensues.

Blood Testing

Your healthcare provider may have blood drawn to be tested for IgE antibodies to specific allergens. Radioallergosorbent test (RAST) may be used, but it has largely been replaced by newer options: enzyme-linked immunosorbent assay (ELISA) testing and ImmunoCAP testing.

Imaging

Imaging is not commonly used for diagnosing allergies. However, an X-ray of your lungs or sinuses may be done to rule out other conditions. Studies such as a sinus computed tomography (CT) scan might be used if chronic sinusitis is suspected.

By Condition

What tests your healthcare provider uses to diagnose, if any, depends on what they suspect is to blame for your symptoms. Here's a sense of what is likely to be used in the diagnostic process for common allergic conditions.

Atopic Dermatitis

There is no blood test that can be used to diagnose atopic dermatitis. Instead, a healthcare provider must examine you and review your history of symptoms. In most cases, people with atopic dermatitis should have skin testing performed to airborne allergens (pollens, molds, pet dander, and dust mite) as well as to common food allergens.

There are three criteria that must be present in order to diagnose atopic dermatitis:

• Allergies: The person must have allergies (at least one positive skin test) and/or a family history of allergic diseases in close relatives. There may be rare cases in which a person has atopic dermatitis without evidence of allergies.
• Itching: The patient must have itching and scratching in order for the rash to occur. If the skin or areas of the rash do not itch or have not been scratched, then the person does not have atopic dermatitis.
• Eczema: Eczema refers to the appearance of the rash in patients with atopic dermatitis; it also occurs in those with other skin diseases. The rash appears red with small blisters or bumps. These may ooze or flake with further scratching. Over the long term, the skin appears thickened and leathery.

Allergic Contact Dermatitis

In addition to a good history and physical exam, your healthcare provider may do patch testing to see how your skin reacts to different allergens.

Nasal Allergies

In many cases, allergic rhinitis is diagnosed when a person has convincing symptoms and physical exam findings consistent with this disease. However, in order for allergic rhinitis to be properly diagnosed, allergy testing is needed.

Positive allergy tests are required to diagnose allergic rhinitis; negative allergy testing suggests non-allergic rhinitis. While allergy testing can be accomplished with skin testing or blood tests, skin testing is still the preferred method.

Food Allergies

The diagnosis of food allergies is made when typical symptoms occur after eating a specific food and a patient receives a positive result after undergoing an allergy test to the food in question. Testing for the allergic antibody is best accomplished with skin testing, although can be done with a blood test as well.

The blood test can be helpful in predicting if a person has outgrown a food allergy. This is especially true since, in many cases, the skin test can still be positive in children who have actually outgrown a food allergy.

If the diagnosis of food allergy is in question despite testing, an allergist may decide to perform an oral food challenge. This involves having a person eat increasing amounts of the suspect food, over many hours, under close medical supervision.

Allergic Asthma

The diagnosis of asthma is suggested by the presence of asthma symptoms; however, spirometry is needed to make a firm diagnosis. Spirometry can easily be performed in people 5 years of age and older.

For example, someone with asthma may display a decrease in the amount of air they can exhale due to airway obstruction and show a certain increase in lung function after using a bronchodilator, such as Albuterol. This test can help your doctor to assess the severity of your asthma.

If the diagnosis of asthma is still in question despite performing spirometry, asthma specialists can perform additional tests to determine if a person has asthma. These include bronchoprovocation (causing a drop in lung function with the inhalation of certain chemicals, such as methacholine) and measurements of markers of inflammation in exhaled air, sputum, and blood.

Formally determining that one's asthma is allergic asthma—i.e., triggered by allergens—involves skin or blood tests. You may have triggers that are seasonal (like pollen) or they may be allergens that are present year-round (like dust mites).

Differential Diagnoses

Your healthcare provider will consider whether rhinitis has a non-allergic cause, which can be environmental or due to a virus like the common cold. Acute or chronic sinusitis can also produce some of the same symptoms.

Food allergy symptoms could be caused by conditions including celiac disease. But the symptoms may also have a non-allergic cause including chronic infection, enzyme maladies, and even psychosomatic reactions.

Atopic dermatitis can look like many other skin conditions, including psoriasis, seborrheic dermatitis, scabies, ringworm, and more. Allergic contact dermatitis shares symptoms with irritant contact dermatitis.

A Word From Verywell

It can be quite a relief to find what exactly is triggering your allergies or to rule out allergies as the cause. If allergies are diagnosed, you can begin to find ways to avoid the allergens and effective treatment for your symptoms.