Reproducibility of the CO rebreathing technique with a lower CO dose and a shorter rebreathing duration at sea level and at 2320 m of altitude.

Total hemoglobin mass (Hb _mass ) is routinely assessed in studies by the carbon monoxide (CO) rebreathing. Its clinical application is often hindered due to the consequent rise in carboxyhemoglobin (%HbCO) and the concern of CO toxicity. We tested the reproducibility of the CO rebreathing with a CO dose of 0.5 mL/kg body mass (CO _0.5 ) compared to 1.5 mL/kg (CO _1.5 ) and when shortening the CO rebreathing protocol. Therefore, CO rebreathing was performed 1×/day in eight healthy individuals on four consecutive days. On each day, either CO _0.5 (CO _0.5 -1 and CO _0.5 -2) or CO _1.5 (CO _1.5 -1 and CO _1.5 -2) was administered. Venous blood samples to determine %HbCO and quantify Hb _mass were obtained prior to, and at 6 (T ₆ ), 8 (T ₈ ) and 10 min (T ₁₀ ) of CO rebreathing. This protocol was tested at sea level and at 2320 m to investigate the altitude-related measurement error. At sea level, the mean difference (95% limits of agreement) in Hb _mass between CO _0.5 -1 and CO _0.5 -2 was 26 g (-26; 79 g) and between CO _1.5 -1 and CO _1.5 -2, it was 17 g (-18; 52 g) _. The respective typical error (TE) corresponded to 2.4% (CO _0.5 ) and 1.5% (CO _1.5 ), while it was 6.5% and 3.0% at 2320 m. With CO _0.5 , shortening the CO rebreathing resulted in a TE for Hb _mass of 4.4% (T ₈ vs. T ₁₀ ) and 14.1% (T ₆ vs T ₁₀ ) and with CO _1.5 , TE was 1.6% and 5.8%. In conclusion, the CO dose and rebreathing time for the CO rebreathing procedure can be decreased at the cost of a measurement error ranging from 1.5-14.1%.