Inter- and intra-unit reliability of the COSMED K5: Implications for multicentric and longitudinal testing

Purpose To evaluate the intra-unit (REL INTRA ) and inter-unit reliability (REL INTER ) of two structurally identical units of the metabolic analyser K5 (COSMED, Rome, Italy) that allows to utilize either breath-by-breath (BBB) or dynamic mixing chamber (DMC) technology. Methods Identical flow- and gas-signals were transmitted to both K5s that always operated simultaneously either in BBB- or DMC-mode. To assess REL INTRA and REL INTER , a metabolic simulator was applied to simulate four graded levels of respiration. REL INTRA and REL INTER were expressed as typical error (TE%) and Intraclass Correlation Coefficient (ICC). To assess also inter-unit differences via natural respiratory signals, 12 male athletes performed one incremental bike step test each in BBB- and DMC-mode. Inter-unit differences within biological testing were expressed as percentages. Results In BBB, TE% of REL INTRA ranged 0.30–0.67 vs. REL INTER 0.16–1.39 and ICC ranged 0.57–1.00 vs. 0.09–1.00. In DMC, TE% of REL INTRA ranged 0.38–0.90 vs. REL INTER 0.03–0.86 and ICC ranged 0.22–1.00 vs. 0.52–1.00. Mean inter-unit differences ranged -2.30–2.20% (Cohen’s ds (ds) 0.13–1.52) for BBB- and -0.55–0.61% (ds 0.00–0.65) for DMC-mode, respectively. Inter-unit differences forV ˙ O 2and RER were significant (p < 0.05) at each step. Conclusion Two structurally identical K5-units demonstrated accurate REL INTRA with TE < 2.0% and similar REL INTER during metabolic simulation. During biological testing, inter-unit differences forV ˙ O 2and RER in BBB-mode were higher than 2% with partially large ES in BBB. Hence, the K5 should be allocated personally wherever possible. Otherwise, e.g. in multicenter studies, a decrease in total reliability needs to be considered especially when the BBB-mode is applied.