How do doctors diagnose asthma?

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According to the Centers for Disease Control and Prevention (CDC), 7.8% of people in the United States had asthma in 2020, including 8.4% of adults and 5.8% of children.

No single test exists for doctors to diagnose asthma conclusively. However, several objective tests together can support a diagnosis.

Medical history

A doctor will review a person’s medical history before diagnosing asthma. They may ask questions about the following:

• a person’s symptoms
• any triggers for symptoms — for example, dust or pollen
• whether there is a family history of asthma or other medical conditions
• whether a person smokes or has exposure to secondhand smoke
• where a person works, as there is a possibility of exposure to asthma-causing substances

A history of allergies or eczema puts a person at higher risk of developing asthma.

Physical exam

During a physical exam, a person’s doctor looks out for signs of asthma and related conditions. It typically involves examining:

• the upper airways, nose, and throat
• blood oxygen levels by placing a pulse oximeter device on the person’s finger
• the skin for eczema or another allergic reaction
• the chest and lungs to assess a person’s breathing

Doctors will look out for common symptoms and signs of asthma during a physical exam and when taking a medical history. With asthma, people may experience:

• coughing
• breathing difficulties
• sensitivity to cold air, exercises, or exposure to certain allergens
• wheezing

Spirometry

Spirometry uses a spirometer machine to measure how much air the lungs can blow out and how quickly this happens.

The person takes a deep breath before exhaling as fast and hard as they can into the mouthpiece of the spirometer. This test produces a score. A lower score indicates the possibility of swollen lungs or narrow airways.

A doctor may advise doing this test several times to monitor asthma.

Learn more about spirometry.

Bronchodilator reversibility test

If a spirometry test shows that a person’s airways have become narrow, the doctor may order a bronchodilator reversibility test. This involves using a bronchodilator, a medication that helps open airways and makes breathing easier.

After taking the medication, a person waits 15–20 minutes before repeating the spirometry test to check if the score changes. If the score improves, it is likely because the airways have widened.

Learn more about bronchodilators and their role in asthma treatment.

Bronchial challenge test

A doctor may recommend a bronchial challenge test to check how a person’s airways respond to asthma triggers. Another name for this test is the direct challenge test. This test can only take place at a hospital or medical center.

It involves deliberately irritating the airways with histamine or methacholine to see how they respond. A spirometry test follows to measure lung function.

Learn more about asthma triggers.

FeNo test

During a fractional exhaled nitric oxide (FeNo) test, a person breathes into a machine that measures the amount of nitric oxide in their breath. Nitric oxide is a marker of lung inflammation. The lungs typically produce nitric oxide in response to someone breathing in something they are allergic to.

A person inhales deeply with an open mouth before breathing slowly and steadily into the mouthpiece on the machine. If the results show a high level of nitric oxide, this could mean the airways are inflamed.

Learn more about what to expect during lung function tests.

Peak flow test

As with spirometry, this test involves blowing as hard and fast as possible into a handheld device called a peak flow meter. A person can do this daily and record it in a peak flow diary to check for changes. Typically, a doctor may check readings over 2–4 weeks.

Rather than diagnosing asthma, it helps those who have recently received a diagnosis manage their condition.

Learn how to use a peak flow meter.

Doctors are not always able to perform tests for asthma in children under 5 years old. Therefore, they rely on caregivers to provide information about the child, including:

• the symptoms they are experiencing — for example, wheezing
• if they have allergies or eczema
• whether there is a family history of asthma or allergy

The doctor may prescribe asthma medications for a trial period of 4–6 weeks to see if they provide symptom relief or until the child is old enough to undergo tests.

Once a person has received an asthma diagnosis, doctors may recommend further tests to help guide treatment or provide more details. These can include:

• allergy testing, which involves a skin-prick exam
• blood testing to check for the amount of eosinophils, a white blood cell, and the antibody immunoglobulin E, which could signal severe asthma
• imaging tests, such as lung X-rays, to check for scarring or blockages
• gas transfer tests to measure the amount of oxygen that travels from the lungs to the blood. This test can help to monitor a person with severe asthma

If a person receives an asthma diagnosis, they will discuss their individualized treatment plan with their healthcare professional. This includes which asthma medications they will use.

A person must understand how and when to use these medications to manage their symptoms. Frequent follow-up visits may be necessary to achieve this. Doctors may use questionnaires to determine how asthma affects a person’s daily life.

Learn about asthma treatments.

Doctors can diagnose asthma in adults using a range of tests, including spirometry, peak flow testing, and FeNo testing. Further exams, such as blood tests, may be useful to determine the body’s response to asthma.

Performing certain lung function tests may not always be possible in children under 5 years old, so doctors rely on physical examinations and medical history for a diagnosis. They may prescribe asthma medication to see if it clears up the symptoms.

A person with a diagnosis of asthma will receive an individualized treatment plan from a doctor to manage their symptoms effectively.