For horses on lasix, a blood sample is taken as well. These are sent to a lab where they are analyzed. Purse money is not rewarded to the horse until these tests come back negative, usually within three to four days. This can be for a few days or permanently and is upheld at every racetrack in North America. Very few positive tests ever occur on The Ontario Jockey Club circuit because the loss of income or purse money is too significant.
Any problems are usually due to carelessness and a fine is levied.
There are many possible reasons a horse may be scratched. A lot depends on how big the field is and how close to the time of the running of the race it is. In harness racing, entries are submitted to the race office three days before the actual race will be run. Two days prior, the trainer has the option of scratching their horse. Such a request must be approved by the Ontario Racing Commission vets.