Spirometry Interpretation: A Simple Guide

Spirometry is a common method used to assess lung function. It can help differentiate and diagnose obstructive and restrictive lung disease, monitor disease progression, and evaluate the effectiveness of a current treatment plan.

Introduction to interpreting spirometry results

Interpreting spirometry results depends on patient effort. It’s essential to provide patients with graphic instructions before the test and verbal cues throughout the spirometry test to achieve maximal effort.^* Verbal encouragement shouldn’t be underestimated; it makes a big difference, particularly toward the end of the maneuver.

Common problems associated with poor patient effort include:

• Sub-maximal inhalation
• Slight hesitation when blowing out at the start of the initial exhalation
• Coughing in the first second
• Failure to completely exhale
• Exhalation speed is reduced due to mouthpiece blockage by tongue, teeth, or biting.^*

To achieve an accurate spirometry reading, guidelines from the American Thoracic Society (ATS) and European Respiratory Society (ERS)^* suggest the test should have at least three acceptable maneuvers. A proper spirometry maneuver includes:

• The back-extrapolated volume (BEV) must be <5% of the FVC or 0.100 L, whichever is greater
• A maneuver performed with maximal inhalation and exhalation
• No coughing, inhalations during the trace, or evidence of leaks

One of the following 3 recommended indicators are required to reach End-of-Forced-Expiration (EOFE):

• There is less than a 0.025-L change in volume for at least 1 second (a “plateau”). This is the most reliable indicator of complete expiration
• The patient has reached a forced expiratory time (FET) of 15 seconds
• The patient cannot exhale long enough to achieve a plateau (e.g. children with high elastic recoil or patients with restrictive lung diseases). In this case, it is important that the patient achieves the same FVC

There is no requirement to reach a minimal forced expiratory time.

An accurate measurement should also have consistent (repeatable) results for both FVC and FEV1. For a test to achieve repeatability, the difference between the largest and second largest values for FVC and FEV1 should be within 0.15 liters (150 ml), and up to a maximum of eight additional breathing maneuvers can be attempted to meet the criteria for validation. 

The table below represents the 2019 ATS/ERS grading system considering acceptability and repeatability.^*

Common spirometry tests and parameters

Normal parameters for a spirometry test vary from person to person and depend on the following:

• Age
• Height
• Race
• Gender

A score is considered “normal” if it is 80% or more of the predicted value based on the above criteria.

Forced vital capacity (FVC) is the maximum volume of air a patient can exhale when blowing out as hard and fast as possible. The FVC measurement will be low when a patient can’t inhale deeply or exhale completely. An abnormal FVC measurement could be due to restrictive or obstructive disease, so other spirometry measurements must factor in to determine which type of lung disease is present.

Forced expiratory volume (FEV1) is the volume of air a patient can force out of the lungs in one second and helps to evaluate the severity of the disease. An FEV1 reading below the normal parameter shows a possible breathing obstruction.

A predicted value of FEV1:

• 80% or greater is normal
• 70%-79% is mildly abnormal
• 60%-69% is moderately abnormal
• 50-59% is moderate to severely abnormal
• 35-49% is severely abnormal
• Less than 35% is very severely abnormal^*

FEV1/FVC ratio (FEV1%) helps differentiate whether restrictive or obstructive lung disease exists. The FEV1/FVC ratio represents the percentage of lung capacity exhaled in one second.

Interpreting obstruction versus restriction 

Measuring FEV1 or FVC alone cannot assess whether a patient has an obstructive or restrictive lung defect. The FEV1/FVC ratio is necessary to determine which problem is present. A patient with obstructive disease will not be able to achieve a normal FEV1 compared to FVC. In contrast, a patient with a restrictive disease can’t reach an adequate FVC, but the FEV1 will be normal to slightly elevated.

Obstruction

When the FEV1/FVC ratio is less than the lower limit of the normal value for the patient, there could be an obstructive defect. Patients with obstructive disorders cannot exhale adequate volumes of air because of decreased airway diameter. The decrease in diameter resulting in obstruction is often caused by inflammation, mucous plugging, or smooth muscle contraction.

Obstructive parameters include:

• Reduced FEV1
• Normal (or reduced) VC
• Normal (or reduced) FVC
• Reduced FEV1/FVC ratio
• Concave flow-volume loop

Common obstructive lung conditions include asthma, chronic obstructive pulmonary disease (COPD), and Cystic Fibrosis.

Restriction

A reduced FVC with a normal to elevated FEV1/FVC ratio can indicate a restrictive defect. Patients with restrictive lung disease are restricted from fully inflating their lungs with air.

Restrictive parameters include:

• Reduced FVC
• Normal-to-high FEV1/FEV ratio
• Normal looking shape on spirometry
• Possible high PEF

Lung conditions associated with restrictive disorders include idiopathic pulmonary fibrosis, neuromuscular disorders, pulmonary edema, and sarcoidosis.

Conclusion

Spirometry is a vital measurement for assessing, diagnosing, and managing lung disease. It’s crucial to obtain accurate measurements for precise diagnosis to manage lung disease better and improve patient outcomes.

Disclaimer:

ndd Medical Technologies is a medical device manufacturer and does not offer medical advice.

This content is intended for informational purposes only. Always consult a doctor or qualified healthcare provider regarding any questions you may have regarding your spirometry results.