Abstract: Method and apparatus for determining the flow rate of blood passing between a patient and an extracorporeal therapy instrument. The flow measurement apparatus includes a delivery unit for producing a saline bolus interface at a first position of a measurement channel having a predetermined volume between the first and second position. A sensor unit included in the apparatus detects the saline bolus interface as it passes the second position in the channel. An output signal from the sensor unit along with an indication of the production of the saline bolus interface is utilized by a control unit to determine the blood flow rate. The control unit of the apparatus utilizes instantaneous and average blood flow algorithms to calculate instantaneous and average blood flow rates based on the time interval associated with the production of the saline bolus interface and its progression to a predetermined position in the measurement channel.