The utility of spirometry and single breath gas transfer measurements to identify low total lung capacity.

BACKGROUND

Measurement of total lung capacity (TLC) requires large and expensive equipment. We aimed to investigate whether spirometric restriction and low alveolar volume measured by single breath gas transfer ( ) can be used to identify those with a low TLC.

METHODS

We retrospectively analysed data from adults referred to Cambridge University Hospitals between January 2016 and December 2023. We investigated the utility of spirometric restriction (forced vital capacity (FVC) < lower limit of normal (LLN) with forced expiratory volume in 1 s /FVC ≥LLN) and reduced (<LLN), to discriminate low TLC measured by plethysmography (TLC <LLN). We assessed agreement between definitions using the Cohen's kappa coefficient and the discriminative ability of spirometric restriction and reduced to identify TLC <LLN.

RESULTS

Data from 7923 patients were included. The majority (94%) of patients were of European ancestry, 51% were female. Mean age was 58 years. 11% of patients had a TLC <LLN, of which 27% also had spirometric restriction, and 95% had a <LLN. Agreement between spirometric restriction and TLC <LLN was fair (κ=0.37). Spirometric restriction had low sensitivity (27%) but high specificity (99%) for discriminating TLC <LLN. Agreement between <LLN and TLC <LLN was substantial (κ=0.63). <LLN had good sensitivity (95%) and specificity (89%) for discriminating TLC <LLN, except for in patients with the most severe airflow obstruction.

CONCLUSIONS

A FVC in the healthy range is an effective tool for ruling out restriction. Low can be used to accurately identify those with a low TLC, negating the need for formal measurement of static lung volumes when identifying restrictive lung disease.