we present an optical probe arrangement that surrounds blood in a receptacle . it comprises led &# 39 ; 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 .