The only time a medical bill or debt will effect your credit score is if a collection agency gets involved. At this point, it is turned over and will be marked as an issue on your credit report.
Medical debt does not affect your credit score unless it's reported to a credit bureau, and virtually no hospital or medical provider will report the debt directly, according to the National Consumer Law Center (NCLC). However, they might turn it over to a collection agency, which might report it.
Once a collection agency has hold of it, it will impact your credit score and they don't give you much time to act on it before it is handed over. I had a $5,000 bill I couldn't afford from an ER trip and after 6 weeks, it was filed and turned over. I was mad.
Only if a debt collection agency has your bill. If this is the case, the damage will have been done already and even if you do pay it off, it is not very likely to fix or improve your scores.
My sister always told me it doesn't. She is a nurse and her husband is a banker. Both of them seem to think that nothing happens, even if it gets turned over to a collection agency so I honestly don't know!