Lung tests

Lung tests (pulmonary function tests) such as spirometry are often used to help confirm an asthma diagnosis. These tests check how well your lungs are working. During spirometry, you take a deep breath and forcefully breathe out (exhale) into a tube connected to a machine called a spirometer. This records both the amount (volume) of air you breathe out and how quickly you can exhale. If certain key measurements are below normal for a person your age, it may be a sign that your airways are narrowed by asthma.

After taking lung test measurements, your doctor may ask you to inhale an asthma drug to open air passages. Then, you'll do the lung tests again. If your measurements improve significantly after taking the medication, you may have asthma.

Challenge test
With this test, your doctor tries to trigger asthma symptoms by having you inhale a substance that causes the airways to narrow in people with asthma, such as methacholine (meth-uh-KO-leen).

If you appear to have asthma triggered by exercise (exercise-induced asthma), you may be asked to do physical activity to see whether it triggers symptoms.

After taking action to trigger your symptoms, you'll retake the spirometry test. If your spirometry measurements are normal, you probably don't have asthma. But if your measurements have fallen significantly, it's possible you do.

Lung tests in children
Doctors seldom do lung tests in children under age 5. Instead, diagnosis is generally based on a child's signs and symptoms, medical history, and physical examination. It can be especially difficult to diagnose asthma in young children because there are many conditions that cause asthma-like symptoms in this age group.

If your child's doctor suspects asthma, the doctor may prescribe a bronchodilator — a drug that opens the airways. If your child's signs and symptoms improve after using the bronchodilator, your child may have asthma.

Exhaled nitric oxide test
With this test, you breathe into a tube connected to a machine that measures the amount of nitric oxide gas in your breath. Nitric oxide gas is produced by the body normally, but high levels in your breath can mean your airways are inflamed — a sign of asthma.

Additional tests: Ruling out conditions other than asthma

Depending on your age, your medical history and initial testing, your doctor may suspect that you have a condition other than asthma. Conditions that can cause asthma-like symptoms include:

  • Something blocking in the airways (airway obstruction). Breathing a piece of food or a small object into the airways is especially common in infants and young children.
  • Another lung disease such as emphysema or chronic obstructive pulmonary disease (COPD).
  • A tumor or tumors in the airways.
  • Bronchiolitis (BRONG-ke-oh-LIE-tis). This type of airway inflammation often stems from a viral lung infection such as respiratory syncytial (sin-SISH-ul) virus, particularly in children under 2.
  • Lung infection (pneumonia) caused by a bacteria or virus.
  • A blood clot in the lung (pulmonary embolism).
  • Heart failure (congestive heart failure).
  • Vocal cord dysfunction, which is also called paradoxical vocal cord movement or laryngeal dysfunction.

To rule out possible causes of your breathing problems, you may need tests such as:

  • Chest and sinus X-rays
  • Blood tests — for example, a complete blood count (CBC) test
  • Computerized tomography (CT) scans of the lungs
  • Gastroesophageal reflux assessment
  • Examination of the phlegm in your lungs (sputum induction and examination) for signs of a viral or bacterial infection.

Your doctor may also want to see whether you have any other conditions that often accompany asthma and can make symptoms worse. These include:

  • Heartburn (gastroesophageal reflux disease, or GERD)
  • Hay fever
  • Sinusitis

Your doctor may also perform allergy tests. These can be either skin tests or blood tests, or both. Allergy tests aren't specifically used to diagnose asthma. But, they can help identify an allergic condition, such as hay fever, that may be causing your symptoms or worsening existing asthma.

For some people, asthma diagnosis is straightforward. But for others, diagnosing the cause of breathing problems is a challenge. It can be difficult to differentiate asthma from other conditions — particularly in young children. When existing asthma is worsened by another condition, such as hay fever, it can further complicate diagnosis. Your doctor will need to piece together all of the information possible to determine what's causing your symptoms. Even if a diagnosis isn't certain, your doctor may prescribe medications or other treatment to see whether it helps.

Because numerous conditions can cause asthma-like symptoms, it may take some time — and patience — to get the correct diagnosis and determine the best course of treatment.

Jun. 23, 2011 See more In-depth