Abstract: A method and apparatus is provided for measuring the content of a compound in a liquid medium, and which is particularly useful for measuring the concentration of a low molecular weight compound in a complex medium in which there is provided a dialyzer in contact with the complex fluid medium and through which dialysis fluid is conducted to receive by diffusion a portion of the low molecular weight compound to produce a dialysate. The dialysate with the low molecular weight commpound to be examined is first conducted through a venting device for removal of gaseous particles therefrom before conduction through a reaction chamber containing enzymes for promoting the conversion of the low molecular weight compound in the dialysate into a converted reaction product.