Doctors may use breathing tests like spirometry to confirm an asthma diagnosis. In younger children, an examination of symptoms alone may be enough for a diagnosis.

Asthma is a chronic lung condition affecting nearly 25 million people in the United States, or about 8% of the population.

People with asthma experience repeated flare-ups of wheezing, coughing, and shortness of breath. Every year, this leads to millions of missed school and work days as well as millions of doctor’s office and emergency department visits.

Once you’ve received a diagnosis of asthma, a doctor can help treat and control your symptoms. But although asthma is very common, the diagnosis can be surprisingly challenging, especially in young children.

Keep reading to learn more about how doctors arrive at an asthma diagnosis.

Symptoms of asthma

Asthma symptoms include:

  • wheezing
  • chest tightness
  • shortness of breath
  • coughing, especially nighttime coughing spells

Asthma symptoms often come and go. Triggers include cold viruses, exercise, temperature changes, allergens, and irritants like smoke or fragrances.

You may also notice symptoms of related atopic conditions, such as eczema or allergies.

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In a diagnostic spirometry test, a healthcare professional will ask you to take a deep breath and blow hard into a large tube. You’ll do this before and after taking a bronchodilator such as albuterol. A machine will then measure your breathing patterns and lung function, determining whether you have the reversible airway obstruction seen in asthma.

Spirometry is the best diagnostic test for asthma. Later, it can also help doctors measure your response to asthma treatment.

But spirometry isn’t perfect. It requires cooperation, coordination, and forceful breathing. Accurate spirometry typically isn’t possible in children under 5 years old. Some healthcare professionals don’t use spirometry to test for asthma in children until they’re age 8 years or older.

Other potential breathing tests for asthma include:

  • Methacholine challenge: In a methacholine challenge test, you’ll inhale another medication before repeating spirometry.
  • Fractional exhaled nitric oxide (FeNO): In FeNO testing, you’ll briefly blow into a tube attached to a small machine. If you have airway inflammation or irritation due to asthma, you’ll exhale slightly higher levels of nitric oxide.
  • Peak flow meter: Peak flow meter testing involves blowing into a small handheld device to move a toggle. This test is most useful for monitoring older children and adults who already have an asthma diagnosis.

Allergy tests can’t diagnose asthma. But they can identify allergies that could be causing or worsening your asthma symptoms.

Researchers estimate that allergies are an asthma trigger in up to 80% of children and more than 50% of adults. If allergies aggravate your asthma, treating them can help get your asthma under better control. Typical allergy tests include skin prick testing and blood testing for allergen-specific antibodies.

Sometimes, exercise triggers asthma symptoms. If initial testing doesn’t reveal symptoms of asthma, a healthcare professional might ask you to perform spirometry after exercising, such as on a treadmill or stationary bike, in the clinic.

Blood tests alone can’t diagnose asthma. But a doctor might order blood allergy testing, an eosinophil count, or additional blood tests to check for different medical conditions that can cause shortness of breath.

During an acute asthma exacerbation, doctors might need other blood tests to monitor your overall condition.

Eosinophils and asthma

Eosinophils are part of your immune system. They’re white blood cells that help you fight off infections. In eosinophilic asthma, eosinophils cause inflammation and swelling in your airways.

Eosinophilic asthma is rare. A doctor can confirm it with a blood test to check your eosinophil levels.

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Although asthma symptoms frequently begin in early childhood, asthma is hard to diagnose in young children. This is because wheezing with cold viruses is very common in preschoolers. However, most of these children won’t go on to develop asthma.

More definitive asthma tests, like spirometry or FeNO, aren’t typically useful until the age of 5 years.

If you’re concerned that your young child might have asthma, doctors can often make a clinical diagnosis after talking with you and examining your child.

For older children and adults, doctors can order diagnostic spirometry and additional supporting tests to confirm a suspected asthma diagnosis.

Here are answers to some common questions about asthma diagnosis.

At what age does asthma start?

Asthma usually begins in early childhood, but it’s difficult to diagnose in very young children. In the United States, asthma prevalence is highest among teenagers ages 12–17 years.

But asthma can begin at any age, including during adulthood. Global research shows a second asthma incidence peak around age 65 years.

Does asthma show up on an X-ray?

Some chest X-ray findings, such as lung hyperinflation or bronchial wall thickening, may suggest asthma. However, a chest X-ray alone cannot diagnose asthma, as such findings could indicate other conditions as well.

Still, a doctor may order a chest X-ray, especially during an acute asthma attack. They do this to make sure there’s no other cause for your symptoms, such as a lung infection.

How can I self-diagnose asthma?

If you suspect that you or your child might have asthma, it’s very important to bring the symptoms to the attention of a healthcare professional.

Repeated episodes of wheezing, shortness of breath, or prolonged coughing might indicate asthma.

If you can, make note of when your symptoms started and what seems to trigger them, and review your family’s medical history thoroughly. Then, contact a doctor to discuss evaluation for possible asthma.

Diagnostic breathing tests, like spirometry and FeNO, are common ways for doctors to confirm an asthma diagnosis in older children and adults. Allergy testing may also be helpful.

But asthma often begins in young childhood, and there’s no definite diagnostic test for this age group. Instead, doctors will suspect and treat asthma based on a child’s symptoms and examination, and they’ll continue to monitor the child closely as they grow.

Although asthma is a chronic condition, effective treatment can preserve lung function and control symptoms. Talk with a doctor if you suspect that you or your child have asthma.