The Coggins test, also known as the agar gel immunodiffusion test, is the gold standard for diagnosing Equine Infectious Anemia (EIA), a viral disease caused by a retrovirus. The test detects antibodies to the major group-specific antigen p26 of the EIA virus in the horse's serum. A negative test certificate is required for importing or exporting horses. The test can detect infected animals after the initial incubation period except during the first 4-6 weeks after exposure.