Abstract: The present invention is concerned with a chemical sensor, more specifically with a chemical sensor which is capable of detecting the presence of, measuring a quantity of or monitoring the level of N-acetyl primary aromatic amines, such as paracetamol, in whole blood.The specification relates to a method of assay of the type in which an electrode poised at a suitable potential is contacted with a system comprising;(a) a sample suspected to contain paracetamol or a derivative thereof, and(b) an enzyme capable of catalyzing the hydrolysis of an N-acylated primary aromatic amine or a derivative thereof, and,wherein the current flowing in the electrode is a measure of the quantity of hydrolysis products formed and thereby of the concentration of N-acylated primary aromatic amine or derivatives thereof in the sample.