The detection of cholesterol uses enzyme reactions to generate hydrogen peroxide.
The H₂O₂ then reacts with a dye to form a highly-coloured product that you can measure by colorimetry or fluorimetry.
The cholesterol in blood consists of both cholesterol and cholesteryl esters.
The cholesteryl esters are hydrolyzed by cholesterol esterase into cholesterol.
The cholesterol is oxidized by cholesterol oxidase into H₂O₂ and a ketone (cholest-4-en-3-one).
In the presence of horseradish peroxidase (HRP), the H₂O₂ reacts quantitatively with a sensitive dye (Amplex ® Red) to produce a coloured product (resorufin)
Resorufin can be measured in a colorimeter by its absorption at 563 nm.
When resorufin is excited at 579 nm, it emits fluorescent radiation at 587 nm, so you can also use fluorimetry to detect cholesterol.