Bedrock missed the Ryanair Hurdle at Leopardstown on Saturday because of concerns his system was not clear of a medication permitted in training but prohibited on racedays.
The Iain Jardine-trained gelding shocked Samcro at Down Royal in November, and was due to face Gordon Elliott's star in a rematch at the Dublin track, having had to miss the Fighting Fifth Hurdle at Newcastle due to swelling on a hock.
In a statement issued through the National Trainers Federation on Monday, Jardine outlined the circumstances of Bedrock being taken out of the Leopardstown race.
The Scottish trainer said Bedrock had been injected with a corticosteroid - which is a legitimate treatment in training for joint injuries under the rules of racing, but must be clear of a horse's system on racedays.
Although the suggested withdrawal period had elapsed, when corticosteroids are administered by any route, particularly into joints, there is potential for considerable variation in excretion times.
As a result, Jardine asked the British Horseracing Authority to test Bedrock prior to running in Ireland to ensure the horse did not compete under the influence of the treatment.
The result of the BHA test came through on the night before the race, and as the result was positive, Bedrock was withdrawn.
Supporting Jardine, the BHA said in a statement: "Corticosteroids are legitimate substances and permitted for use on horses in training, but - as with all substances that might have an effect on a horse's body systems - cannot be present in a horse's system on raceday.
"As such the trainer took the correct course of action in ensuring the horse was electively tested prior to the race.
"Elective testing is available, subject to BHA approval, to trainers in circumstances such as this where a medication with a potentially unpredictable detection window which is permitted for use out of competition has been administered for legitimate therapeutic purposes."