Quote from platon20
All states require that testimony in malpractice cases come from a physician. Furthermore, it has to be in teh same specialty as the doctor. For example, if you are sueing a neurosurgeon you cant have a family practice doc testify against him.
That rule applies to cases where the defendant is a physician; e.g., just as a nurse would never be called as an expert witness to testify against a physician, physicians do not qualify as expert witnesses to testify against nurses. So, NPs could serve as expert witnesses in cases where an NP is the defendant (I have no idea how often or rarely NPs get sued ...), and might even be able to testify in cases involving "regular" RNs in the same specialty (I'm not sure about that).
Professionals who serve as expert witnesses, in addition to practicing in the same discipline and specialty as the defendant, typically have more than the "bare minimum" of recognized credentials in their field and significant clinical experience, but I'm not aware that there are any specific requirements. The term "expert witness" is somewhat intimidating; obviously, the more your credentials and CV would dazzle and impress a jury, the better (from the point of view of the attorney who is calling you as a witnesss), but you don't have to be THE TOP PERSON IN THE COUNTRY in your field, or anything like that -- just a competent, experienced, respected practitioner (with no professional skeletons in your own closet that could be used to challenge your testimony!