How to interpret spirometry results: a practical guide

Spirometry is one of the most common pulmonary function tests. It measures the volume of air a patient can inhale and exhale, providing valuable insights into how well the lungs are working. It is a key asset in diagnosing and monitoring chronic lung conditions like asthma or COPD, as well as lung health and functionality in general.

But how are spirometry results to be interpreted? In this article, we will examine the key parameters of spirometry, providing a simple explanation of their meaning.

How spirometry works

In order to understand the parameters measured by spirometry, let’s take a look at how it works. During a spirometry test, patients are asked to take a deep breath and then exhale as quickly as possible into a mouthpiece shaped like a tube, connected to a device called a spirometer. Simple as that.

The main parameters measured by the spirometers are:

1. Forced Vital Capacity (FVC): the amount of air a patient can forcefully exhale after taking a deep breath
2. Forced Expiratory Volume in 1 Second (FEV1): the amount of air exhaled in the first second of a forced breath
3. FEV1/FVC Ratio: the ratio of FEV1 to FVC

These values are compared to the so-called “predicted values”, an average calculated based on age, sex, height, and ethnicity to determine whether lung function is healthy and expressed as a percentage of said values.

Examining FEV1

FEV1 is a crucial value for determining the presence and severity of airway obstruction, i.e. obstacles like mucus or narrowing in the airways that make it hard for a patient to exhale.

If the result is higher than 80%, lung function in relation to this parameter is considered normal. Lower results may be indicators of obstructive patterns, whose severity is calculated as follows:

• Mild Obstruction: FEV1 71-80% of predicted
• Moderate Obstruction: FEV1 50-70% of predicted
• Severe Obstruction: FEV1 30-49% of predicted
• Very Severe Obstruction: FEV1 < 30% of predicted

Understanding FVC

FVC measures the volume of air that a patient can breathe out by blowing as fast as possible. It is a key parameter in the identification of obstructive and restrictive patterns, particularly when compared with FEV1 and FEV1/FVC ratio. As for the other parameters, the results will be expressed as a percentage of the predicted values.

If the FEV1/FVC ratio is normal or near normal, but FVC is lower than 80% of predicted, this may suggest the presence of restrictive lung diseases like pulmonary fibrosis or sarcoidosis. Restrictive diseases mean reduced lung volume, often due to stiff lungs or chest wall abnormalities.

If both FEV1 and FVC are low, but the FEV1/FVC ratio remains normal or elevated, that may be an indicator of restrictive patterns instead.

Read our dedicated article to understand the difference between restrictive and obstructive diseases.

Examining FEV1/FVC Ratio

The FEV1/FVC ratio is a key value in distinguishing between obstructive and restrictive lung disease. Again, the results will be shown as a percentage of the predicted value and can be interpreted as follows:

• Normal FEV1/FVC Ratio: around 70-80% for adults
• Low FEV1/FVC Ratio: lower than 70%. This suggests obstructive lung disease, where airflow is reduced due to narrowed airways

Common restrictive conditions include COPD, or Chronic Obstructive Pulmonary Disease, asthma, and chronic bronchitis.

Using spirometry to assess treatment efficacy in obstructive diseases

Another common use of spirometry is evaluating the efficacy of treatments and whether the airflow obstruction is reversible with the use of medication.

In order to do that, the patient will repeat the test after taking a bronchodilator. If the medication is effective, test results will show a significant improvement in FEV1.

Summing it up

In conclusion, the interpretation process can be roughly summed as follows:

1. FEV1/FVC ratio helps identify obstructive disease
2. FEV1 helps determine the severity of obstruction
3. FVC helps assess for restrictive disease
4. Comparing values to predicted norms allows us to gauge severity
5. Result differences before and after taking medication help assess the efficacy of treatments

Don’t forget: spirometry is a powerful tool for diagnosing and managing respiratory diseases, but results should always be interpreted in the context of the patient’s clinical history and symptoms. If you’re unsure, consult with a healthcare professional for a thorough evaluation.