We have exported several horses to Europe and the UK. The USDA Vet said they had to have an AGID test for export and that Cornell University was the only lab that still did them at that time. He said they get a pretty high number of false positives with that test.
Unless for export, every state accepts the quicker Elisa test. Some states require one within 6 months while most require one within 12 months. We just shipped two horses to California and the Vet told me they needed a 6 month test so we had to re-do them before he could write Interstate Health Papers.