To evaluate the intra-unit (RELINTRA) and inter-unit reliability (RELINTER) 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 RELINTRA and RELINTER, a metabolic simulator was applied to simulate four graded levels of respiration. RELINTRA and RELINTER 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 RELINTRA ranged 0.30-0.67 vs. RELINTER 0.16-1.39 and ICC ranged 0.57-1.00 vs. 0.09-1.00. In DMC, TE% of RELINTRA ranged 0.38-0.90 vs. RELINTER 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 for and RER were significant (p < 0.05) at each step. Conclusion Two structurally identical K5-units demonstrated accurate RELINTRA with TE < 2.0% and similar RELINTER during metabolic simulation. During biological testing, inter-unit differences for and 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.
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