Less is more… comparison of two repeatability protocols for diffusing capacity (DLCO).

<bold>Introduction/aims</bold> The 2017 ATS/ERS DLCO standard recommends that at least 2 acceptable measurements should be repeatable within 0,67 mmol/min/kPa (U). However, with many of our patients having ILD or PH with low DLCOs, we assumed 0,67 U (protocol B) allows for too much intra-session variability and narrowed it to 5% (protocol A). Aims of this study were to compare DLCO results between protocols A and B, the number of maneuvers needed to match the protocols criteria, and to explore potential predictors for the number of maneuvers. <bold>Methods:</bold> Data over a 2-week period in 2022 of 193 consecutive patients who performed DLCO test (following protocol A), was retrospectively collected (MEC2023-0189). DLCO, KCO and VA and the number of maneuvers were recalculated applying protocol B. We used 7% as an acceptable relative difference in DLCO between protocol A and B. <bold>Results:</bold> Mean(SD) DLCO, KCO and VA (protocol A) of all patients were resp. 5,69 U (2,18), 1,28 U/L (0,33) and 4,43 L (1,33). The mean(SD) Δ DLCO A vs. B was -0,01 U(0,09); p= 0,22. Bland-Altman analysis showed the mean relative Δ DLCO between A and B was - 0,06%, with limits of agreement at - 3,20% and 3,08%. Compared to protocol A, applying protocol B significant less patients (n=33) needed > 2 (3-5) maneuvers to match repeatability (p< 0,001, exact McNemar). In total 510 vs. 455 maneuvers were needed applying protocol A vs. B. The number of maneuvers (A) could not be predicted from age, VC, FEV1, DLCO or FEV/VC (Poisson regression). <bold>Conclusion:</bold> Applying the broader ATS/ERS repeatability criteria at our laboratory will not lead to significant other DLCO results but could reduce the number of maneuvers, thus effort for the patient, time and diffusing gas.