1. General presentation

Either to measure a substance S (a hormone H, for example), in a sample of a given biological medium (serum); the choice of the best assay method to use will have to meet four imperatives:

• its detection limit or detectability must be as low as possible; as the concentration of S in the medium is generally very low, in the picomole range (10 ^–12 mole), the method chosen must have a sufficient detection limit, while remaining reliable (for example, normal values for thyroid hormones: [FT4] 10 to 25 pmol.L ^–1 ; [FT3] 4 to 8 pmol.L ^–1 );

• it must be specific; a biological medium is always extremely complex, and the technique used must specifically assay substance S alone and...