Compositions and methods for detecting β-hydroxybutyrate in a biological fluid from a subject in need thereof are disclosed. Also disclosed are compositions and methods of reducing an optical change in a composition for detecting the β-hydroxybutyrate during storage before the composition is exposed to the β-hydroxybutyrate.

The invention relates to a method for determining the activity of a component of the electron transport chain or any associated protein thereof and for determining the effect of a compound on the activity of a component of the electron transport chain or any associated protein thereof. The invention also relates to a kit comprising a redox probe, at least one electron donor compound of a component of the electron transport chain or a protein associated thereof and optionally comprising a sample comprising mitochondrial membranes and to the use of said kit in the methods of the invention.