Is there a utility in getting repeat antibody testing on GBS patients after PLEX?   

No, there is not, for a couple of reasons: Most patients with the most common variant of GBS (AIDP) are negative for ganglioside antibodies or other antibodies against nodal/paranodal proteins. Ganglioside antibodies are more often positive in GBS variants like AMAN, AMSAN, Miller Fisher syndrome, ...