We present an optical probe arrangement that surrounds blood in a receptacle. It comprises LED's and light detector arranged to overcome the variations when the receptacle is translucent medical tubing and the like. Also, a signal processing algorithm is used to average signals from a plurality of light detectors, to further enhance results when measuring hematocrit. The invention makes it possible to add the feature of hematocrit measurement into dialysis system without major alterations to the dialysis machine or transport tubing.