Short term variability of single breath carbon monoxide transfer factor

Background-When monitoring patients with chronic lung disease it is importa nt to distinguish genuine changes in gas transfer over time from natural va riability. This study aims to define the coefficient of repeatability for r outine measurements of single breath transfer factor (Tco) and transfer coe fficient (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 occa sion a standard protocol (conforming to BTS guidelines) was followed, compr ising duplicate measurements satisfying standard technical criteria. The me an 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 a nd +/-0.24 mmol/min/kPa/1 for Kco. Correcting Tco and Kco for prevailing ba rometric pressure or carboxyhaemoglobin level made no significant differenc e 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 K co greater than these limits are unlikely to arise from natural variation.