The most popular serological test method is ELISA (Enzyme-linked Immunosorbent Assay), which is used to demonstrate antibodies against micro-organisms (e.g. bacteria, viruses and parasites). ELISAs are based on the principle that the antibodies in question, or parts thereof, attach to a carrier (indirect ELISAs) or compete with a specific labelled antibody (blocking ELISAs), with attached antibodies then being made visible by staining. Averroes Laboratories (ARL) carries out millions of ELISA tests every year. In addition to ELISA, the ARL laboratory carries out other serological tests, such as various agglutination tests (HI tests), and serum (Plate) agglutination tests (SAT). Several commercially available ELISAs are used, but when kits are not available,ARL develops in-house ELISAs for routine testing.