The benchtop blood analyzer is the gold standard for blood oxygen saturation (SO2) and hemoglobin (Hb) analysis. However, the benchtop analyzer only provides values at a given point in time. In the field of cardiovascular perfusion and the practice of cardiopulmonary bypass (CPB), continuous measurement of SvO2 and hemoglobin values have become commonplace. Two devices currently available which monitor these values are the Terumo CDI 500 and Spectrum Medical M4. A retrospective study was conducted to examine the accuracy of the M4 technology and the CDI 500 as they compare to each other and the ABL90 FLEX, a benchtop blood gas analyzer. The data revealed the magnitude of mean differences were small, even when significant. However, the 95% Limits of Agreement were too large for either device to allow substitution of the CDI 500 and M4 hemoglobin or SvO2 values for ABL90 values. As recommended by the manufacturers, the CDI 500 and M4 should only be used as a trending device.