The British Horseracing Authority has confirmed that officials visited Philip Fenton's stables on Wednesday where the trainer was interviewed.
Tests were carried out on Fenton's Cheltenham entries, including Betfred Cheltenham Gold Cup hope Last Instalment.
Following consultation with the Irish Turf Club, it had been agreed that samples from Fenton's horses will be collected by the BHA and fast-track testing will take place at HFL Sport Science, Newmarket, with the results of the tests available next week.
The visit was part of an ongoing process of gathering all relevant information regarding the issue.
It is intended that blood and hair samples are taken for testing, with substances able to be detected in hair samples for a greater period of time than is the case with either blood or urine.
It is understood negative samples would not automatically mean the horses would be allowed to run at Cheltenham, should other issues arise as part of the BHA trip.