The present invention relates to a blood processing system (10)comprising a replacement fluid flow conduit (48), and a citrate detector (42) comprising a light source (58) configured to emit a light toward and into the replacement fluid flow conduit (48), the light having a wavelength absorbed by citrate, but at least partially transmitted by the replacement fluid flow conduit (48) and a light detector (46) configured to receive at least a portion of the light from the light source (44) exiting the replacement fluid flow conduit (48) and generate a signal indicative of the presence or absence of citrate in the replacement fluid flow conduit (48) based, at least in part, on the amount of light received by the light detector (46). The present invention also relates to a durable assembly for such blood processing system (10), and to a method of monitoring fluid within such blood processing system (10).
