Short term variability of single breath carbon monoxide transfer factor

Background-When monitoring patients with chronic lung disease it is important to distinguish genuine changes in gas transfer over time from natural variability. This study aims to define the coefficient of repeatability for routine measurements of single breath transfer factor (Tco) and transfer coefficient (Kco). Methods-Sixty eight subjects (32 with emphysema, 36 healthy volunteers) had Tco measured twice at a mean (SD) interval of 7.5 (1.3) days. On each occasion a standard protocol (conforming to BTS guidelines) was followed, comprising duplicate measurements satisfying standard technical criteria. The mean of these duplicates was recorded. For the pooled data changes in Tco and Kco between study days were expressed as coefficient of repeatability. Results-The coefficient of repeatability was +/-1.60 mmol/min/kPa for Tco and +/-0.24 mmol/min/kPa/1 for Kco. Correcting Tco and Kco for prevailing barometric pressure or carboxyhaemoglobin level made no significant difference to the results. Conclusions-The quoted limits for variability in gas transfer over time are valid for a wide range of clinically relevant values. Changes in Tco and Kco greater than these limits are unlikely to arise from natural variation.