Can a doctor refuse treatment if a bill went to collection agency?

Laws that govern from state to state vary. However, most doctor's don't refuse treatment due to the Hippocratic Oath they undertake when they are sworn in to do no harm first. Dr's can refuse if it is a habitual matter, meaning, happening over and over again of not paying. Or, making small payments and never fullfilling the service obligations. They become indebt with services provided and cannot provide all services for those that are paying on time.