An apparatus is provided for determining a calibration coefficient of a blood property sensor located in a blood system, wherein the blood system can include a vascular portion and an extracorporeal portion. The blood property sensor can be located in the extracorporeal portion to measure the change in a blood property corresponding to the passing of an indicator. The indicator can include a known amount of an introduced dilution indicator. The controller is provided for determining the calibration coefficient of the blood property sensor corresponding to the introduced dilution indicator in the measured change in the blood property.