When is your credit affected?

I received a billing notice from my health care provider saying I have a past due amount. It says that unless the office receives payment in full, the delinquent balance may be assigned to a professional collection agency. I was away and did not receive the bill until now. I am going to mail it today since it is due next week. I want to know if my credit score has already been affected by this past due amount or if it will only be affected if it is sent to collection. thanks