Synopsis
Interpret lung volumes, carbon-monoxide gas transfer and field or laboratory exercise tests as a coherent physiological assessment, while checking technical quality, safety and the clinical consequence of each result.
- Confirm the clinical question, test quality, reference equations and lower limit of normal before interpreting a labelled abnormality; percentage predicted alone can misclassify age-related variation.
- A low forced vital capacity does not prove restriction. Restriction is physiologically confirmed by total lung capacity below the appropriate lower limit of normal.
- Raised residual volume or residual-volume-to-total-lung-capacity ratio supports air trapping, while raised total lung capacity supports hyperinflation; both must be interpreted with spirometry and method.
Key red flags
Markedly reduced TLCO despite near-normal volumes and spirometry should raise pulmonary vascular disease, early interstitial disease, emphysema or anaemia. Exertional desaturation, syncope, right-heart signs or rapid decline warrants expedited specialist assessment.
Investigation priorities
Define baseline airflow, identify obstruction and determine whether a low vital capacity needs full lung-volume measurement.
Management branches
Spirometry shows reduced FVC with a preserved or raised FEV1/FVC ratio.
- Review spirometry acceptability, expiratory time, effort and flow-volume loop, and check whether obesity, pain or neuromuscular weakness affected performance.
- Measure static lung volumes using an appropriate quality-assured method; classify low TLC as restriction and examine RV/TLC for hidden gas trapping when TLC is normal or high.