Effect of pH on the absorption spectrum of human oxyhemoglobin: a potential source of error in measuring the oxygen saturation of hemoglobin
Wimberley, P.D.; Fogh-Andersen, N.; Siggaard-Andersen, O.; Lundsgaard, F.C.; Zijlstra, W.G.
Clinical Chemistry 34(4): 750-754
1988
ISSN/ISBN: 0009-9147 PMID: 3359614 Document Number: 305959
Small differences in the visible absorption spectrum of human oxyhemoglobin occur with changes in pH, CO2, and 2,3-diphosphoglycerate. This may bias the spectrophotometric measurement of oxygen saturation (SO2) in blood by as much as 3%. This negative bias in SO2 at low pH cannot be explained by a lowered hemoglobin-oxygen affinity, by interference from methemoglobin, or by an increased unknown dyshemoglobin fraction at low pH. For routine clinical work, we recommend calibrating the oxygen saturation meter to read 100% SO2 for fresh blood equilibrated to a pO2 > 95 kPa, pH 7.4, and pCO2 = 5.3 kPa. Subsequent measurement of SO2 in patients' blood samples may then reflect an inaccuracy of up to 1%, which is acceptable in routine clinical practice. For research purposes, particularly where greater variations in blood pH may be encountered, the measured SO2 values should be corrected for the pH effect on the oxyhemoglobin absorption spectrum.