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.
- Gas transfer reported as TLCO or DLCO reflects carbon-monoxide uptake across the alveolar-capillary system and its blood-volume component; haemoglobin, smoking and inspired volume materially affect it.
- KCO is TLCO divided by measured alveolar volume, not a simple correction that makes a low TLCO normal. A high KCO with low lung volume has a different meaning from normal total gas transfer.
- Low transfer can occur in emphysema, interstitial lung disease, pulmonary vascular disease and anaemia; increased transfer can occur with alveolar haemorrhage, polycythaemia and sometimes asthma.
- A six-minute walk or incremental shuttle test measures integrated functional performance and exertional desaturation, whereas cardiopulmonary exercise testing helps partition ventilatory, cardiovascular, gas-exchange and conditioning limitations.
- Compare serial tests using the same laboratory and method where possible, looking for change beyond expected biological and measurement variability rather than reacting to tiny numerical shifts.
- ARTP standards, equipment-specific quality control, local contraindication policy and specialist interpretation govern UK laboratory practice and sign-off.
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.
Chest pain, presyncope, pallor, confusion, severe breathlessness, significant arrhythmia, concerning blood-pressure behaviour or profound desaturation requires test termination and immediate clinical review under the service emergency protocol.
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.
- Combine volumes with TLCO, imaging and clinical context to distinguish parenchymal, pleural, chest-wall, obesity-related and neuromuscular mechanisms.
- Escalate rapid decline, severe physiological impairment or unexplained discordance to respiratory physiology and specialist review.