Limitations of Spirometry in Measuring Lung Volumes

Introduction to Spirometry Limitations

The European Respiratory Society guidelines state that spirometry can only measure volumes of air that move in and out through the mouth, but residual volume (RV) is the air that remains trapped in the lungs after maximal exhalation and therefore cannot be exhaled or detected by a spirometer 1, 2
Spirometry measures dynamic lung volumes, including vital capacity (VC), which is the maximal volume exhaled from full inspiration to complete expiration, as defined by the European Respiratory Society 1
The European Respiratory Society guidelines also define inspiratory capacity (IC) as the volume from end-tidal expiration to maximal inspiration, which can be measured by spirometry 2
Spirometry can measure forced expiratory volumes and flows, which are important for diagnosing obstructive lung diseases, according to the European Respiratory Society 3

The European Respiratory Society recommends using body plethysmography to measure thoracic gas volume by detecting pressure-volume relationships during breathing against a closed shutter, which can assess RV and other static lung volumes 5, 6
Gas dilution techniques, such as helium dilution or nitrogen washout, can calculate lung volumes based on dilution of an inert gas, as suggested by the European Respiratory Society, although the strength of evidence for this method is not specified 5, 6, 7
Radiographic methods, such as CT scan with spirometric gating, can directly visualize and calculate lung volumes, providing a high level of accuracy, according to the European Respiratory Society 5, 6, 7

The European Respiratory Society states that you cannot calculate total lung capacity (TLC) from spirometry alone, as TLC = VC + RV, and RV cannot be measured by spirometry, which is a limitation of the technique 5, 6
The European Respiratory Society guidelines indicate that you cannot definitively diagnose restrictive lung disease with spirometry alone, as this requires demonstrating reduced TLC, which is a critical limitation of spirometry 5, 6, 7
Air trapping and hyperinflation cannot be quantified without measuring RV, which is essential for diagnosing and managing obstructive lung diseases, according to the European Respiratory Society 6, 7
Isolated elevation in RV, which occurs in airway diseases like asthma, bronchiectasis, and emphysema, will be completely missed if only spirometry is performed, highlighting the importance of using alternative methods, as recommended by the European Respiratory Society 6, 7

A reduced VC on spirometry does not prove restrictive lung disease—it is associated with low TLC only about half the time, according to the European Respiratory Society, which highlights the need for careful interpretation of spirometry results 6
The European Respiratory Society guidelines suggest that when spirometry suggests restriction (reduced VC with normal or elevated FEV₁/VC ratio), you must proceed to body plethysmography or other lung volume measurement to confirm the diagnosis, which is a critical step in diagnosing lung diseases 5, 6