Measured fraction of carboxyhaemoglobin depends on oxygen saturation of haemoglobin.

The use of the OSM3 oximeter for measurement of the fraction of carboxyhaemoglobin (FCOHb) in blood allows for estimation of total circulating haemoglobin mass (Hb(tot)) by using the carbon monoxide rebreathing method. To ensure high accuracy of Hb(tot) estimation, potential sources of analytical errors should be identified and adjusted for. Based on observed differences in results of measured FCOHb between simultaneously sampled, arterialized and venous blood samples we investigated the influence of haemoglobin oxygen saturation (sO2) on results of measured FCOHb. Blood from nine healthy non-smokers was tonometered with gas mixtures containing 94% N2 or air and 6% CO2. The resulting oxygenated and deoxygenated specimens were mixed in different proportions to obtain varying sO2 values in the same blood. sO2, fractions of dyshaemoglobins, pO2, pCO2 and pH were measured at each step. FCOHb was significantly (p<0.001) higher in oxygenated (median, range: 0.6%, 0.4-0.9%) compared to deoxygenated (-0.2%, -0.5-0.0%) blood. Regression analysis identified the sO2 as the most important factor explaining 86% of the variance in observed changes in FCOHb. The observed sO2 effect has important implications on calibration procedure of OSM3, accuracy of measured FCOHb, and FCOHb dependent calculations such as estimation of Hb(tot) and related quantities. If the highest accuracy of FCOHb measurement is needed, an sO2 effect on results of measured FCOHb has to be considered and adjusted for.