A number of immunoenzymatic or immunochemical methods have been developed to date for detecting the presence of ligands in media. The ELISA (Enzyme-Linked ImmunoSorbent Assay) method consists in detecting antigenic ligands by means of specific antibodies conjugated to an enzyme (phosphatase, peroxidase, etc.). The RIA (RadioImmuno Assay) method consists in analysing an antigen using a radioactive antigen, the antigen to be analysed displacing the bond between the radioactive antigen and the antibody in proportion to its concentration. The ARIS (Apoenzyme Reactivation Immunoassay System) method, described in FR 2 429 258 or U.S. Pat. No. 4,238,565, consists in detecting a ligand by using a ligand conjugated to a prosthetic group and using specific antibodies, the ligand to be analysed displacing the bond between the conjugate and the antibody in proportion to its concentration, the conjugate in free form then activating an apoenzyme which produces a colour reaction directly or indirectly.
Those methods are carried out especially for detecting molecules present in biological media. By way of example, methods have been developed for detecting progesterone concentration. European Patent Application EP 0 671 006 dated 26 Nov. 1993 relates to an ELISA method for detecting progesterone in cows' milk. British Patent Application GB 2 354 069 dated 8 Apr. 1999 relates to a method for detecting the progesterone concentration in cows' milk taken 4, 5 or 6 days after insemination, by carrying out ELISA or RIA methods. The ARIS method, on the other hand, has not hitherto been applied for detecting progesterone.
Those methods, as conducted at present, exhibit numerous disadvantages. They generally require solid supports with one or several reagents immobilized on them and several steps to be carried out, especially steps of incubation, washing and detection of the reaction (for example U.S. Pat. Nos. 4,461,829, 4,931,385, 4,496,658). This procedure frequently requires the use of specific equipment within the context of an analytical laboratory, especially for measuring the radioactivity, for measuring the enzymatic reaction or for measuring the colour reaction. Finally, those methods can be carried out only by the person skilled in the art. The way in which those methods are carried out at present is therefore not suitable for the simple, reliable and rapid detection of a ligand in a medium.