title: The accuracy of in vivo P50 at high haemoglobin saturation.
abstract: The in vivo P50 (P50iv) provides a useful index of haemoglobin-oxygen affinity and is calculated according to software algorithms incorporated into commercial blood gas analysers. These algorithms are known to be inaccurate at high haemoglobin saturation (SpO2 > 97%) although just how inaccurate has not been documented. This study examines the arterial blood gas profiles of patients admitted to a busy secondary referral Intensive Care Unit and stratifies them according to haemoglobin saturation in order to quantify the accuracy and potential clinical utility of the Siggaard-Andersen algorithm (SAA) for assessing P50iv in blood with SpO2 > 90%. Sicker patients, as identified by plasma pH < 7.35 or [lactate] > 2.0 mmol/l, were substratified and the SAA assessed as before. In both groups, the results show not only that the SAA is completely unreliable above 97% saturation, a fact acknowledged by Siggaard-Andersen in 1984, but it is also inaccurate in the range 92% < or = SpO2 < or = 97%, thus rendering P50iv calculations suspect in 90% of the patients in each of the study groups.
authors.affiliation: Department of Intensive Care, Nambour General Hospital, Queensland, Australia.
authors.forename: C
authors.initials: C
authors.lastname: Anstey
mesh_terms: D000465:Algorithms; D001784:Blood Gas Analysis / Q000592:standards; D006801:Humans; D006863:Hydrogen-Ion Concentration; D010100:Oxygen / Q000097:blood*; D010108:Oxyhemoglobins / Q000378:metabolism*
chemical_list: D010108:Oxyhemoglobins; D010100:Oxygen