1. A method of converting venous blood values to arterial blood values, said method comprising the steps of: a) providing values of arterial oxygenation, b) measuring and estimating values of acid/base status and oxygenation status in a blood sample, the sample being obtained from venous blood, c) converting the venous blood values by applying a mathematical model for deriving blood acid/base status and oxygenation status into estimated arterial blood values.