Indirect calorimetry in humans: a postcalorimetric evaluation procedure for correction of metabolic monitor variability

Background: Indirect calorimetry (IC) with metabolic monitors is widely used for noninvasive assessment of energy expenditure and macronutrient oxidation in health and disease. Objective: To overcome deficiencies in validity and reliability of metabolic monitors, we established a procedure that allowed correction for monitor-specific deviations. Design: Randomized comparative IC (canopy mode) with the Deltatrac MBM-100 (Datex) and Vmax Encore 29n (SensorMedix) was performed in postabsorptive (overnight fast >8 h) healthy subjects (n = 40). In vitro validation was performed by simulation of oxygen consumption (VO2) and carbon dioxide output (VCO2) rates by using mass-flow regulators and pure gases. A simulation-based postcalorimetric calibration of cart readouts [individual calibration control evaluation (ICcE)] was established in adults (n = 24). Results: The comparison of carefully calibrated monitors showed marked differences in VCO2 and VO2 (P < 0.01) and derived metabolic variables [resting energy expenditure (REE), respiratory quotient (RQ), glucose/carbohydrate oxidation (Gox), and fat oxidation (Fox); P < 0.001]. Correlations appeared to be acceptable for breath gas rates and REE (R-2 similar to 0.9) but were unacceptable for RQ (R-2 = 0.3), Gox, and Fox (R-2 = 0.2). In vitro simulation experiments showed monitor-dependent interferences for VCO2 and VO2 as follows: 1) within series, nonlinear and variable deviations of monitor readouts at different exchange rates; 2) between series, differences and unsteady variability; and 3) differences in individual monitor characteristics (eg, rate dependence, stability, imprecision). The introduction of the postcalorimetric recalibration by ICcE resulted in an adjustment of gas exchange rates and the derived metabolic variables with reasonable correlations (R-2 > 0.9). Conclusions: Differential, metabolic, monitor-specific deviations are the primary determinants for lack of accuracy, comparability, and transferability of results. This problem can be overcome by the present postcalorimetric ICcE procedure. Am J Clin Nutr 2013;97:763-73.