Determination of the effect of pH and foetal haemoglobin fraction on haemoglobin oxygen saturation readings by the OSM3 hemoximeter

The visible absorption spectrum of oxyhaemoglobin depends on its content of foetal haemoglobin and on the pH of the blood. These variables are known to affect oxygen saturation readings measured by the OSM3 Hemoximeter. As the hemoximeter estimates foetal haemoglobin content from the absorption spectrum of oxyhaemoglobin, this estimation is affected by pH. We quantified the effect of pH on oxyhaemoglobin reading in order to calculate a correction factor to adjust for pH. We manipulated a pool of fresh heparinized cord blood samples to produce a range of pH values from 6.99-7.53 in fully oxygenated blood with a carbon dioxide tension of 5.3 kPa. We measured oxygen saturation and percent foetal haemoglobin using an OSM3 Hemoximeter and pH with a linked blood gas analyser. The effect of pH on readings of saturation and fractional foetal haemoglobin was confirmed and a correction factor was calculated. The value of K(HbF), the coefficient used in the OSM3 Hemoximeter to estimate the amount of fractional foetal haemoglobin present in a sample, was determined to be 18.3, which slightly but significantly differed (p=0.04) from the 18.6 used by the OSM3 Hemoximeter. An equation was derived to correct the errors made by pH on the saturation measurement of oxygenated blood.