Racing NSW stewards have warned trainers of variations in withdrawal times after John Thompson was fined for a positive swab despite following the recommended guidelines.

A blood sample taken from Astronomos after his fourth in the Group Three JRA Plate on April 15 showed the presence of triamcinolone acetonide,a corticosteroid, which is banned on race day.

At an inquiry on Thursday, Thompson said he had authorised his vet to inject the horse's stifle joint eight clear days from his race as per the rules.

After fining Thompson $6000, stewards posted a notice to trainers to be aware that in the case of intra-articular injections, the withdrawal period will depend on the corticosteroid used as well as the joint(s) injected.

They said in certain situations, specifically for longer acting corticosteroid preparations such as methylprednisolone and/or for injections into joints other than the carpal (knee) and fetlock joints, the recommended "withdrawal" period may be much longer than the minimum stand down time prescribed by Australian Rule 64M.

"Trainers and their veterinarians must carefully consider the circumstances of each particular case before deciding upon the appropriate time of intra-articular injection before competition to ensure that the horse is presented without detectable residues of the corticosteroid administered," stewards said.

Astronomos was disqualified from his fourth place in the JRA Plate.