Accuracy of Blood ph and Pco2 Determinations

The calculated Pco2 of tonometer equilibrated blood is 5–15% higher than the known tension when standard techniques are used. The major sources of error are: a) whole blood introduces a 'suspension effect' error in the (McInnes Belcher) glass electrode, so that blood reads 0.01 lower than its true plasma ph. b) Mean pk' with this glass electrode and present buffer standards measures 6.09 at 37.5° and ph 7.4, and pk' varies with ph. c) ph 7.0 buffers are quite inaccurate and require calibration against standard phthalate and borax buffers. d) Glycolysis in blood can maintain a CO2 tension gradient within a tonometer between blood and gas due to the falling ph. e) Accurate ph measurements (±.003 units) require extremely accurate glass electrode temperature control best obtained in a stirred oil bath. f) Sensitivity and linearity of ph meters require calibration. When these errors were corrected, the mean error in Pco2 was 0.0 ± 0.3 mm Hg in a series of 20 equilibrations of blood from 18 normal men. The standard deviation of pk' among them must have been less than 0.004 units. The CO2 content of plasma is independent of the temperature of centrifugation. Submitted on February 8, 1956