Comparison of p50 determination by siemens and radiometer blood gas analysers with tonometry

Abstract

<h3>Introduction</h3> The partial pressure of oxygen at half saturation (p50) is useful to detect altered oxygen-affinity states in isolated polycythaemia. Commercial gas analysers calculate this parameter from a single oxygen tension (PO₂) and saturation (sO₂) measurement utilising the Hill plot. The accuracy of these calculated p50s in patients with variant haemoglobin is not well documented. We compared results from two commercial analysers (Radiometer ABLFlex 800 and Siemens Rapidlab 1265) with tonometry for five patients with variant haemoglobin or red-cell enzyme deficiency and one control subject. <h3>Methods</h3> A reference interval for p50 was determined. Venous blood was analysed sequentially on the commercial analysers and by tonometry. Hill plots were prepared from the tonometry data and the Hill coefficient (<i>n</i>) derived, least-squares regression was used to calculate the standard error. <h3>Results</h3> The reference interval was 22–27mmHg. The p50 values showed good agreement between analysers, and between analysers and tonometry, except in the case of Haemoglobin Heathrow. <h3>Discussion</h3> Measuring p50 on a commercial gas analyser is convenient, readily available and shows good agreement with tonometry, except in haemoglobin variants with altered cooperativity (<i>n</i>), as in the case of Haemoglobin Heathrow. Gas-analyser derived numerical estimates of p50 for haemoglobin variants with altered cooperativity should be interpreted with caution; in this situation formal tonometry is more accurate.