Standard reference cassettes (SRCs) are used to check the electronics and the optics of the VetStat analyzer. To ensure optimal performance, IDEXX recommends that SRC measurements be confirmed within acceptable ranges once each day that the analyzer is in operation. Three SRC levels are available for the VetStat analyzer; each one tests a different optical range. A multi-level SRC is provided as part of the initial analyzer shipment because it measures the high, normal, and low optical ranges. These special test cassettes contain a stable optical sensor simulator that is measured by the analyzer in exactly the same manner as other cassettes, and provides assurance that all measured parameters are consistent. The results obtained should fall within the limits supplied with the SRCs.