This test measures the level of antibodies against 2 Streptococcus equi ssp equi proteins referred to as Antigen A (N-terminal region of 2190 protein) and Antigen C (N-terminal region of the SEM protein). It is a more reliable serological test for identifying true antibody-positive specimens than the SEM test but is still prone to an inability to identify carrier of Strep equi ssp equi in their guttural pouches (see Durham et al (2020) Equine Vet J.; 00:1–6. . The test has to be referred to an external laboratory and so has a longer turnaround time than the SEM test.