How do I properly refer to a registered nurse in a formal letter?

I'm writing a letter to a registered nurse. While I know to refer to a Doctor as Dr.Joe Schmoe in a letter, do I refer to a Nurse as Nurse Joe Schmoe? Or is it Joe Schmoe, R.N.? And how would refer to them in the Salutation line?

Joe Schmoe R.N.
You almost always include an abbreviated title after the name.
( joe Schmoe C.P.A., joe schmoe M.D. etc)

I think the done thing is to simply address her as Miss or Mrs., but to acknowledge her position in the address line of your letter.

put the name first followed by a comma and the letter RN. As in "My immediate supervisor Laura Felix, RN has worked with Leadvalley Hospital for six years."

yeah, put RN at the end of her name

I would say Nurse Joe Schmoe, or just Joe Schmoe Saying R.N. Joe Schmoe or Joe Schmoe, R.N. is very formal and strange in my opinion.

My reasonining is that nurse is easier to get than doctor. It is like adressing yourself as Joe Schmoe, B.A. or Joe Schmoe, M.A. The Ph.d or M.D might mandate a "Dr." but I do not think that lesser degrees do.

Let's assume the nurse's name was Mary Johnson. In the salutation, you'd refer to her as Nurse Johnson.

And if you were writing her name in the address line, or for a signature, you'd write Mary Johnson, R.N.