a fluid sampling module is provided for utilization in on - line real time fluid monitoring systems . the fluid sampling module can be utilized as a urea input module in a hemodialysis treatment . the hemodialysis monitoring system quantitates the rate and amount of urea removed during the hemodialysis treatment by measuring the urea concentrations as a function of time in the spent dialysate effluent from a hemodialysis machine . the urea input module interfaces with the dialysate effluent waste line from the hemodialysis machine and periodically removes a quantity of the spent dialysate effluent for measurement . the urea input module detects low flow rates and eliminates low flow rate flutter . the urea input module includes a resealable make before break docking interface with the dialysate effluent line and preferably includes a filter between the urea input module and the urea sensor .