A method and apparatus is provided for measuring the content of a low-molecular weight compound in a complex medium which includes a dialyzer, conduits for conveying the complex medium through the dialyzer and conduits for supplying dialysis fluid to the dialyzer to receive by diffusion a portion of the low-molecular weight compound in the complex medium to produce a dialysate. Apparatus is also provided for conducting the dialysate produced from the dialyzer to a measuring unit, wherein the conducting apparatus includes a main flow line having an enzyme bed and a shunt flow line. A control device is provided for directing the flow of dialysate through the main flow line and the enzyme bed or the shunt flow line, so that the flow of dialysate bypasses the enzyme bed. Apparatus is also provided for comparing the dialysates in the main and shunt flow lines.