Fouchet's reagent is a well-known component in medical diagnostic tests, composed of ferric chloride and trichloroacetic acid. It is primarily used in histology and scientific laboratories, especially in the analysis of bilirubin in biological specimens.

Chemically, Fouchet's reagent consists of ferric chloride, a Lewis acid, and trichloroacetic acid, a strong organic acid. It is used to oxidize bilirubin, which then reacts with ferric ions to form a greenish complex.

The reaction triggered by Fouchet's reagent allows visualization and semi-quantitative determination of bilirubin. It forms the cornerstone for diagnostic tests in medicine and biological research. In doing so, the application results in a color change that indicates the presence of bilirubin in the tested samples.