The Ellis inversion10 of the Severinghaus equation11 provides a useful non-linear method for imputing PaO2 from SaO2. This technique has been used in multiple cohorts of patients with pneumonia12-14. This simple calculation can be improved by incorporating PaCO2 and pH values, which could be available from venous blood gases that may be obtained in patients without arterial catheters. Because Severinghaus/Ellis estimates SaO2, a method for estimating SaO2 from SpO2 is necessary. There is no current validated and reliable method for calculating SaO2 on the basis of a measured SpO2. Skin pigmentation affects accuracy of SpO2, as do sex and oximeter type15,16.