Lung Function Tests To Diagnose Asthma: Spirometry, Peak Airflow, Feno And Provocation Tests

Asthma is a chronic inflammatory disease of the airways. Although it usually affects children, adult-onset asthma is also a big burden on both modern society and the patients it affects.

How is asthma diagnosed?

As for any other disorder, there is a predefined procedure that physicians need to follow in order to diagnose asthma properly. It starts with an anamnesis, which basically means gathering information from the patient about their symptoms, assessing their medical history, and investigating if there is a family history of asthma. This is followed by a physical examination, which consists of auscultation of the lungs and inspection for skin changes.

This initial assessment is usually enough to suspect asthma, but the diagnosis cannot be confirmed without performing additional tests. These include spirometry, bronchodilatation tests, bronchoprovocation tests, peak expiratory flow test, and a FeNO test. Of course, not all patients need all these tests, and doctors decide which tests should be employed in each particular case.

Spirometry

The procedure is conducted by applying a plastic mask to your mouth and nose or a tube in your mouth (depending on the type of the machine). Then, the patient is usually instructed to take three normal breaths, followed by maximum inhalation and immediate maximum exhalation. This process can be repeated a few times in order to confirm the correctness of the procedure. Some people do not succeed in following this sequence of instructions on the first try, and this can be especially hard with children. The machine measures the air flow in and out of the tube and calculates all the necessary volumes and capacities.

The most important parameters for an asthma diagnosis are:

• Forced vital capacity (FVC) – the total amount of air blown out by forced expiration after maximum inspiration.
• Forced expiratory volume in the first second (FEV1) – the amount of air blown out during the first second of forced expiration after maximum inspiration.
• FEV1/FVC ratio – which, multiplied by 100, represents the percentage of the patient’s total lung capacity blown out in the first second of expiration.

Both FVC and FEV1 values should normally be 80 percent or more of expected. FEV1/FVC ratio should be more than 85 percent in a healthy person. If these values are lower than normal, there is a strong possibility of an existing pulmonary disorder. Still, the following test is necessary to confirm the diagnosis of asthma.

Bronchodilatation test

After getting lowered values in the initial spirometry, the patient usually undergoes the bronchodilatation test. The patient is given an inhaled bronchodilator, usually albuterol, in order to check whether the lung capacities and volumes will improve. Asthma is characterized by reversible narrowing of the airways. Therefore, if the results of the second spirometry are improved by at least 12 percent, asthma is the most probable diagnosis. If the improvement is less than 12 percent, the more likely cause is chronic obstructive pulmonary disease (COPD), which causes an irreversible airway obstruction.

Bronchoprovocation tests

A bronchoprovocation test is not always necessary, but it is very specific for asthma, because it tests for hypersensitivity of the airways. Methacholine or histamine can be used to stimulate the bronchi, which is followed by spirometry. If there is a significant decrease in FVC and FEV1, that is a sign of hyperresponsiveness of the airways, probably caused by chronic inflammation, which is at the core of asthma pathology.

Peak expiratory flow

Peak expiratory flow is the maximum speed of expiration. This is an easy test which patients can perform themselves by blowing in a small, tube-like, meter. This is not a standard test for diagnosing asthma, although it can help patients with asthma to control their current level of airway obstruction. The test can be valuable in the absence of other, more sophisticated techniques, such as spirometry.

FeNO test

A FeNO test measures the exhaled amount of nitric oxide. People with airway obstruction have difficulties exhaling, and therefore are not able to promptly eliminate nitric oxide from the organism. Residual nitric oxide levels in patients with asthma are higher than in healthy persons. This test is very convenient because it can be performed easily in children. It is used for tracking the efficiency of asthma treatment and sometimes as an additional diagnostic tool in the diagnosis of disorders with airway obstruction.