A posting on the Hong Kong Jockey Club's website on Tuesday said McDonald would not be riding on Wednesday night because of a virus, but that statement was overridden by a later posting that the New Zealand jockey was being stood down because of the prohibited substances.
On Thursday McDonald had to provide the Jockey Club with a further urine sample, which will be tested for hydrochlorothiazide and chlorothiazide.
Hong Kong's chief stipendiary steward Steve Railton told the Herald last night that those test results had not yet been made available.
The Jockey Club allowed McDonald to be declared for rides at Sha Tin tomorrow, pending the results of the testing of the Thursday urine sample. A date for the hearing into the substances found in McDonald has yet to be set.
Railton said he could remember only two previous cases of diuretic detection in Hong Kong.
"Quite a few years ago a local Chinese jockey was caught and copped something like a six meeting stand down.
"Australian jockey Mark De Montfort got two racedays and a $10,000 fine. The reason he was let off lightly was because he came here from Australia at a time when diuretics were legal in Australia, but not here.
"Now they are banned in almost every one of the world's racing jurisdictions.
"This one will be a bit different."
Diuretics have been a weight-reducing method for jockeys for decades. They were widely used in New Zealand through the 1960s, 70s and 80s - and known as 'piss-pills' - but following international trends became a prohibited substance two decades ago.
McDonald has had a tough time battling weight issues in recent months, which was quite noticeable when he was in New Zealand for several weeks before returning to Hong Kong a couple of weeks ago to ride out the last few meetings of the season there.
