A method for calibrating and subsequently operating an optical blood monitor during a dialysis session for determining hematocrit values, the method comprising: calibrating the optical blood monitor by (a) measuring the hematocrit value of whole blood from a supply of blood preserved in an anticoagulant using an in vitro method independent of the blood monitor; (b) baselining a hematocrit value of the supply of blood measured by the blood monitor against the value of the hematocrit measured by the in vitro method; and (c) determining a scaling factor to apply a baselined hematocrit value from the optical blood monitor taken during the clinical session to compensate for different anticoagulants in the whole blood used to calibrate the optical blood monitor and in blood taken from a patient during the clinical session; and operating the optical blood monitor during the dialysis session by (d) calculating in a non-invasive manner the hematocrit values of a patient's blood using a ratiometric model based on the detected intensity of multiple wavelengths of light directed through the patient's blood; (e) scaling the hematocrit values calculated by the ratiometric model in keeping with the scaling factor; and (f) displaying the scaled hematocrit values. calibrating the optical blood monitor by (a) measuring the hematocrit value of whole blood from a supply of blood preserved in an anticoagulant using an in vitro method independent of the blood monitor; (b) baselining a hematocrit value of the supply of blood measured by the blood monitor against the value of the hematocrit measured by the in vitro method; and (c) determining a scaling factor to apply a baselined hematocrit value from the optical blood monitor taken during the clinical session to compensate for different anticoagulants in the whole blood used to calibrate the optical blood monitor and in blood taken from a patient during the clinical session; and (a) measuring the hematocrit value of whole blood from a supply of blood preserved in an anticoagulant using an in vitro method independent of the blood monitor; (b) baselining a hematocrit value of the supply of blood measured by the blood monitor against the value of the hematocrit measured by the in vitro method; and (c) determining a scaling factor to apply a baselined hematocrit value from the optical blood monitor taken during the clinical session to compensate for different anticoagulants in the whole blood used to calibrate the optical blood monitor and in blood taken from a patient during the clinical session; and operating the optical blood monitor during the dialysis session by (d) calculating in a non-invasive manner the hematocrit values of a patient's blood using a ratiometric model based on the detected intensity of multiple wavelengths of light directed through the patient's blood; (e) scaling the hematocrit values calculated by the ratiometric model in keeping with the scaling factor; and (f) displaying the scaled hematocrit values. (d) calculating in a non-invasive manner the hematocrit values of a patient's blood using a ratiometric model based on the detected intensity of multiple wavelengths of light directed through the patient's blood; (e) scaling the hematocrit values calculated by the ratiometric model in keeping with the scaling factor; and (f) displaying the scaled hematocrit values.