Consent is a record, not a habit
Canadian messaging rules reward the boring version
Reminding a patient that repeat bloodwork is due is useful and predictable. Sending that reminder electronically brings it inside a consent regime that is stricter than most, and the distinction between a clinical reminder and a commercial message is one your organisation draws for itself. Laboratories that treat this as a policy question answered once, and then recorded per patient, have a much easier time than those that treat it as a judgement call made per campaign.
Store the basis where the patient lives
The practical version is unremarkable: capture how and when the patient agreed to be contacted, keep clinical and commercial categories apart, and log every message against the record. If anyone later asks on what basis a particular reminder was sent, the answer is a lookup. Without that, the answer is an afternoon of reconstruction and a certain amount of hoping.

