TRAINER Nicky Henderson said Sprinter Sacre is "not a certain participant" in the BetVictor Tingle Creek Chase at Sandown on Saturday.
The brilliant seven-year-old is the hot favourite to stretch his unbeaten record over fences to 11 races, but has returned "unsatisfactory figures" from a tracheal wash which was taken yesterday.
Sprinter Sacre will be tested on Thursday morning before final declarations are made for the Grade One to assess whether he can be given the go-ahead to run.
Henderson said: "We took what you can only consider to be a routine tracheal wash this morning. Although it appeared to be visibly satisfactory, the cytology from the lab has returned some unsatisfactory figures, which obviously gives us cause for concern.
"Although the horse is in fantastic form and his work on Saturday was, as ever, impeccable, I cannot ignore what these figures indicate.
"Although marginal, we have to issue a warning tonight, rather than any later, that he's not a certain participant on Saturday. I've spoken to Caroline Mould (owner), who, like all of us at Seven Barrows, would not wish to take any chances.
"Therefore, we intend to repeat the procedure on Thursday morning."
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 place in jeopardy the long-term prospects of Sprinter Sacre, who will work again today in the hope he can make the Tingle Creek line-up.
He said: "There is every chance this could change and go the right way, but, at the same time, if they did not improve, we would have to reluctantly decline this opportunity."