Accurate measurements of hemoglobin oxygen saturation, and fractions of carboxyhemoglobin and methemoglobin in fetal blood using Radiometer OSM3: corrections for fetal hemoglobin fraction and pH

Abstract

The differences in the visible absorption spectra between fetal and adult oxyhemoglobin and carboxyhemoglobin result in errors in the measurements og hemoglobin oxygen saturation (SO2) and carboxyhemoglobin fraction (FCOHb) in fetal blood, if not corrected for the actual fetal hemoglobin fraction (FHbF) in the sample. In 11 fully oxygenated umbilical cord blood samples (mean FHbF = 77%), we found a mean positive bias in SO2 of 4.7%, and in FCOHb of 2.7%, when measured with a dedicated spectrophotometer (OSM3, Radiometer A/S, Denmark), and using the matrix of absorption coefficients for adult hemoglobin. Accurate measurements were obtained by using OSM3's correction for FHbF in the blood specimen after measurement of FHbF by OSM3. The effects of plasma pH on the measurements of SO2 and FCOHb in fully oxygenated fetal blood were found to be similar to those found for adult blood. From plasma pH 7.05 to 8.02, measured SO2 increased 1.3% and FCOHb 0.6%. Correction for the pH of fetal blood samples should be considered when calibrating OSM3 and in connection with research studies. The effects of FHbF and pH on the measurement of methemoglobin fraction (FMetHb) were less than 0.2%, and can be ignored. FHbF measured by OSM3 at pH 7.4 is about 14% too high compared to alkali denaturation rate method. However, the presence of a metabolic acidemia, which is common in fetal blood specimens, decreases this bias, so that for example in our study, FHbF, measured by OSM3 and uncorrected for pH changes was on average only 6% too high. We recommend that OSM3's factor of 18.6 is reduced to 16.4, and that correction is made for pH.