Background: Several indirect calorimetry (IC) instruments are commercially available, but comparative validity and reliability data are lacking. Existing data are limited by inconsistencies in protocols, subject characteristics, or single‐instrument validation comparisons. The aim of this study was to compare accuracy and reliability of metabolic carts using methanol combustion as the cross‐laboratory criterion.

Methods: Eight 20‐minute methanol burn trials were completed on 12 metabolic carts. Respiratory exchange ratio (RER) and percent O 2 and CO 2 recovery were calculated.

Results: For accuracy , 1 Omnical, Cosmed Quark CPET (Cosmed), and both Parvos (Parvo Medics trueOne 2400) measured all 3 variables within 2% of the true value; both DeltaTracs and the Vmax Encore System (Vmax) showed similar accuracy in measuring 1 or 2, but not all, variables. For reliability , 8 instruments were shown to be reliable, with the 2 Omnicals ranking best (coefficient of variation [CV] < 1.26%). Both Cosmeds, Parvos, DeltaTracs, 1 Jaeger Oxycon Pro (Oxycon), Max‐II Metabolic Systems (Max‐II), and Vmax were reliable for at least 1 variable (CV ≤ 3%). For multiple regression , humidity and amount of combusted methanol were significant predictors of RER ( R 2 = 0.33, P < .001). Temperature and amount of burned methanol were significant predictors of O 2 recovery ( R 2 = 0.18, P < .001); only humidity was a predictor for CO 2 recovery ( R 2 = 0.15, P < .001).

Conclusions: Omnical, Parvo, Cosmed, and DeltaTrac had greater accuracy and reliability. The small number of instruments tested and expected differences in gas calibration variability limits the generalizability of conclusions. Finally, humidity and temperature could be modified in the laboratory to optimize IC conditions.