Asthma: Steps in diagnosis

Diagnosing asthma involves a thorough medical history, physical exam and lung function tests.

If you have bouts of wheezing, coughing and shortness of breath, your doctor may suspect asthma. But symptoms alone aren't enough to diagnose asthma. Your doctor will also ask about your symptoms and other health conditions. After gathering that information, your doctor may recommend a lung function (breathing) test or other tests. If your doctor suspects asthma, he or she may give you medication that reduces asthma symptoms. If the medication works, this suggests that you may have asthma. In some cases, other tests, such as lung function tests, are needed to confirm the diagnosis — and to make sure symptoms are not caused by something else.

History and physical exam: Detecting signs of asthma

Your doctor will want to know symptoms you have, when they occur and what seems to trigger them. Common signs and symptoms of asthma include:

Recurrent wheezing
Coughing
Trouble breathing
Chest tightness
Symptoms that occur or worsen at night
Symptoms that are triggered by cold air, exercise or exposure to allergens

Your doctor will also want to know if you have any allergies — which can be linked to asthma, and whether you have a family history of asthma or allergies.

A physical examination of your upper respiratory tract, chest and skin generally follows the history. Your doctor may look inside your nose for signs of increased nasal secretions, a swollen nasal lining and nasal polyps.

Your doctor may also use a stethoscope to listen to the sounds your lungs make as you breathe. Wheezing — high-pitched whistling sounds when you breathe out — is one of the main signs of asthma and indicates obstructed airways.

Finally, your doctor may examine your skin for signs of allergic conditions such as eczema and hives, which are often associated with asthma.

Lung function tests

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Spirometer

Although your symptoms, medical history and physical examination may suggest that you have asthma, lung (pulmonary) function tests may be needed to confirm an asthma diagnosis. Lung function tests may include one or more of the following tests.

Spirometry
This noninvasive test measures how well you breathe. During spirometry, you take deep breaths and forcefully exhale into a hose connected to a machine called a spirometer. Spirometry testing reveals two measurements that are important in diagnosing asthma:

Forced vital capacity (FVC), which is the maximum amount of air you can inhale and exhale.
Forced expiratory volume (FEV-1), which is the maximum amount of air you can exhale in one second.

Your doctor will compare these two measurements. If certain key measurements are below normal for a person your age, it may be a sign that your airways are obstructed. Your doctor may ask you to inhale a bronchodilator drug used in asthma treatment to open obstructed air passages and then try the test again. If your measurements improve significantly, it's likely that you have asthma. Your doctor may still suspect that you have asthma even if your initial spirometry measurements are normal. If so, you may need additional tests.

Challenge test
During this test, your doctor deliberately tries to trigger airway obstruction and asthma symptoms by having you inhale an airway-constricting substance or take several breaths of cold air. If you appear to have exercise-induced asthma, you may be asked to do vigorous physical activity to trigger symptoms.

After triggering your symptoms, you retake the spirometry test. If your spirometry measurements are still normal, it's likely that you don't have asthma. But if your measurements have fallen significantly, it may mean you have 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:

Other lung disease such as chronic obstructive pulmonary disease (COPD)
Airway tumors
Airway obstruction
Bronchitis
Lung infection (pneumonia)
Blood clot in the lung (pulmonary embolism)
Congestive heart failure
Vocal cord dysfunction
Viral lower respiratory tract infection

If your doctor suspects that you have another condition, you may have other tests or assessments, such as:

Chest and sinus X-rays
Complete blood count
Computerized tomography (CT) scans of the lungs
Gastroesophageal reflux assessment
Sputum induction and examination

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

Heartburn/Gastroesophageal reflux disease (GERD)
Hay fever
Sinusitis

Your doctor may also perform allergy tests. Although allergy tests aren't used to diagnose asthma, they can help identify substances that may be causing or worsening your asthma.

Diagnosing asthma in children

When assessing children under age 5, doctors seldom conduct lung function tests because young children usually have trouble following the instructions. Instead, when a child's signs and symptoms, medical history and physical examination suggest 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, an asthma diagnosis is likely.

A new diagnostic approach: Exhaled nitric oxide

Because diagnostic tests such as spirometry aren't always accurate, doctors are seeking better ways to diagnose asthma. One newer approach is to measure a chemical marker of asthma — nitric oxide — in exhaled air. In general, higher levels of nitric oxide correspond with higher degrees of asthma severity. Although initial results appear promising, this test is costly, requires specialized equipment and is still being evaluated.