The need for quality control in measurement of mixed venous oxygen saturation

We investigated the variability of arterial, mixed venous and peripheral venous oxygen saturation readings in three co-oximeters in regular use on two Intensive Care Units. Over a 2-week period, 96 readings were obtained from each of the following machines; two AVL 912 co-oxylite co-oximeters (Machines A and B, Biomedical Instruments, Graz, Austria) and one IL-282 (Machine C, Instrumentation Laboratories, Lexington, MA, USA). The coefficient of variation in the arterial oxygen saturations was extremely small on all three machines (< 0.43%). The coefficient of variation in the mixed venous samples, however, ranged from 1.96 to 4.61% on machine A, and from 2.73 to 4.71% on machine B, but only from 0.17 to 1.47% on machine C. The variation in mixed venous saturations obtained from a single blood sample repeatedly analysed on machines A and B was large enough to influence clinical management.