Star steeplechaser Sprinter Sacre is in doubt for the Tingle Creek Chase at Sandown.
Trainer Nicky Henderson said Sprinter Sacre, who is unbeaten in 10 starts over fences, returned "unsatisfactory figures" from a tracheal wash.
Sprinter Sacre will be tested again on Thursday morning before final declarations are made for Saturday's Grade One to assess whether he can be given the go-ahead to run.
"We took what you can only consider to be a routine tracheal wash," Henderson said.
"Although it appeared to be visibly satisfactory, the cytology from the lab has returned some unsatisfactory figures, which obviously gives us cause for concern.
"We intend to repeat the procedure on Thursday morning ahead of declarations at 10am."
Last season's runaway Queen Mother Champion Chase winner has not been seen since his awesome victory at the Punchestown Festival in April.
Henderson is adamant he will do nothing to jeopardise the long-term prospects of Sprinter Sacre who will work again on Tuesday in the hope he can make the Tingle Creek line-up.
"This is devastating for us, but I wouldn't run any horse on these figures, but we're especially not prepared to take a chance with him," he said.
"There's very little we can do to make it better because medication is obviously not possible.
"Whatever happens, the long-term future for Sprinter Sacre is obviously paramount. To run him under unsatisfactory circumstances would be foolish considering his reputation."
Sprinter Sacre is one of 11 entries for the Tingle Creek.
Henderson has also entered Captain Conan, a winner of three Grade One novice chases last season but he also holds an entry over three miles at Sandown on Friday.