Disclosed herein is an assay for determining the concentration of p-aminophenol present in a sample. More particularly, the present invention relates to an improvement of an enzyme-based assay for determining the concentration of acetaminophen present in a sample.

An assay device for use with samples of biological fluid, the assay device comprising: an N-acetyl-p-aminophenol (APAP) detection unit for detecting and/or measuring APAP in a biological fluid sample; and an alanine aminotransaminase 1 (ALT1) detection unit for detecting and/or measuring ALT1 in said biological fluid sample, wherein the APAP detection unit and the ALT1 detection unit act together as part of an assay process.