Does venous blood gas analysis provide accurate estimates of hemoglobin oxygen affinity?

Alterations in hemoglobin oxygen affinity can be detected by exposing blood to different PO₂ and recording oxygen saturation, a method termed tonometry. It is the gold standard to measure the PO₂ associated with 50 % oxygen saturation, the index used to quantify oxygen affinity (P₅₀Tono). P₅₀Tono is used in the evaluation of patients with erythrocytosis suspected to have hemoglobin with abnormal oxygen affinity. Since tonometry is labor intensive and not generally available, we investigated whether accurate estimates of P₅₀ could also be obtained by venous blood gas analysis, co-oximetry, and standard equations (P₅₀Ven). In 50 patients referred for evaluation of erythrocytosis, pH, PO₂, and oxygen saturation were measured in venous blood to estimate P₅₀Ven; P₅₀Tono was measured for comparison. Agreement among P₅₀Ven and P₅₀Tono was evaluated (Bland–Altman analysis). Mean P₅₀Tono was 25.8 (range 17.4–34.1) mmHg. The mean difference (bias) of P₅₀Tono-P₅₀Ven was 0.5 mmHg; limits of agreement (95 % confidence limits) were −5.2 to +6.1 mmHg. The sensitivity and specificity of P₅₀Ven to identify the 25 patients with P₅₀Tono outside the normal range of 22.9–26.8 mmHg were 5 and 77 %, respectively. We conclude that estimates of P₅₀ based on venous blood gas analysis and standard equations have a low bias compared to tonometry. However, the precision of P₅₀Ven is not sufficiently high to replace P₅₀Tono in the evaluation of individual patients with suspected disturbances of hemoglobin oxygen affinity.