A second blood sample has been taken from Grand Emperor after a pre-race screening at Caulfield showed an elevated TCO2 reading.

The horse finished fourth in the PFD Food Services Handicap and was allowed to start after trainer Dean Howard assured stewards the horse had no alkalising agents leading up to the race.

Senior steward Robert Cram said the horse was kept on course for two hours so a post-race sample could be taken.

The samples will be sent to Racing Analytical Services Limited (RASL) for confirmatory analysis.

"At this stage it's an irregularity of an elevated level in the screening only. So it's got to be confirmed," Cram said.