Reproducibility of CO-Derived Intravascular Volumes and Hemoglobin Mass in a Diverse Cohort of Healthy Individuals using the Detalo OpCO Device

The objective of this study was to determine the test-retest reproducibility of carbon monoxide (CO) rebreathing-derived measures of intravascular volumes (e.g., total blood volume, plasma volume, and red cell volume) and hemoglobin (Hb) mass (Hb mass ) using the automated OpCO device (Detalo, Zürich, Switzerland). We hypothesized that all hematological variables would remain consistent between days. Fifteen healthy participants (7 females; mean age ± SD [range]: 53 ± 15 [22-75] years; height: 172 ± 10 [159-190] cm; body mass: 77.6 ± 11.0 [57-91] kg) performed a CO rebreathing test on two days separated by an average of 11 days (range: 2-34 days). Following a 1-minute preconditioning phase of 100% oxygen inhalation, in the semi-supine position, with legs elevated to level of the heart, participants received a single bolus of 99.997% chemically pure CO (dosing: 0.8 ml·kg -1 for males; 0.7 ml·kg -1 for females) and respired this CO-enriched gas mixture for a duration of 4 minutes. Before and immediately after the CO rebreathing procedure, 2 mL of blood was sampled from an antecubital vein from which [Hb] and fractional concentration of carboxyhemoglobin were measured by blood gas analyser (ABL90, Radiometer, Copenhagen, Denmark). Hematocrit (Hct) was also measured using capillary tubes and centrifugation. These variables were used to calculate Hb mass and, blood, plasma, and red cell volumes using standard equations. Paired t -tests and Pearson’s correlation coefficient (R) evaluated between-day reproducibility for [Hb], Hct, Hb mass , blood volume, plasma volume, and red cell volume. For all variables there were no significant Day 1 vs. Day 2 differences: i) [Hb]: 14.8 ± 1.1 vs. 14.8 ± 1.0 g·dl -1 (p = 0.788, R = 0.940); ii) Hct: 43.5 ± 3 vs. 43.4 ± 3.3% (p = 0.441, R = 0.980); iii) Hb mass : 872 ± 238 vs. 876 ± 260 g (p = 0.827, R = 0.970); iv) total blood volume: 5.85 ± 1.3 vs. 5.86 ± 1.5 L (p = 0.873, R = 0.967); v) plasma volume: 3.28 ± 0.62 vs. 3.29 ± 0.73 L (p = 0.955, R = 0.958); and vi) red cell volume: 2.57 ± 0.71 vs. 2.57 ± 0.81 L (p = 0.975, R = 0.975). CO rebreathing-derived measures of intravascular volumes and Hb mass mass using the automated OpCO device exhibited excellent test-retest reproducibility between-days even if separated by 1-3 weeks. Daniel A. Keir was supported by a NSERC Discovery Grant and the Canadian Foundation for Innovation. Robin Faricier was supported by an Ontario Graduate Scholarship. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.