BACKGROUND: The P50 which is the oxygen tension corresponding to 50% saturation at pH 7.40, PCO2 40 mmHg and temperature 37oC is an accepted and useful reference to characterize the oxygen affinity for hemoglobin. This study evaluated the accuracy of P50 calculated from a single measurement of pH, PO2 and SO2 compared with linear regression and sigmoid Emax model in patients undergoing general anesthesia. This study also compared P50 of just after anesthetic induction to P50 of 90 min later. METHODS: This study was conducted in 10 patients undergoing total abdominal hysterectomy. Just after induction of anesthesia, 20G catheter was indwelled at cephalic vein of forearm and venous return was occluded with a tourniquet. After 4 min, 6 blood samples were obtained every one minute. After 90 min, 6 blood samples were obtained with same method. All blood samples were analyzed with blood gas analyzer within 30 min after withdrawal. P50 was calculated from three methods of calculation, linear regression and sigmoid Emax model. The difference between the actural P50 and calculated P50 was evaluated. RESULTS: P50's calculated from three methods were 25.8+/-1.1, 26.1+/-1.1, 26.0+/-1.2 mmHg respectively. P50's 90min after anesthetic induction (26.1+/-1.3, 26.3+/-1.2, 26.2+/-1.2 mmHg) were significantly greater than P50's just after anesthetic induction (25.5+/-0.8, 25.8+/-1.0, 25.9+/-1.1 mmHg)(P<0.05). Difference between calculated P50 and actual P50 of 120 blood samples from 10 patients according to SO2 were -5.7+/-4.5 (SO2<20%), 0.0+/-1.9 (20%90%). Sigmoidicity of oxygen dissociation curves from 10 patients with sigmoid Emax model was 2.78+/-0.13 (mean SD). CONCLUSIONS: The method calculating P50 from a single measurement of pH, PO2, SO2 was relatively reliable to estimate the change of oxygen dissociation curve of a patient. The affinity of hemoglobin to O2 was reduced during general anesthesia.