In a recent case, the Indiana Court of Appeals held that a nurse practitioner can provide expert medical testimony in areas previously reserved to only medical doctors.  In the decision from the Indiana Court of Appeals it allowed a Nurse Practitioner to testify as an expert witnesses in a soft-tissue case. See the link below:

http://publicaccess.courts.in.gov/Appellate/Document?id=186f4912-5b99-4061-8760-648e61c69cb5

In the decision,the Indiana Court of Appeals held that a nurse practitioner may testify that an injury was consistent with being injured in a particular way, but could go no further unless they witnesses the injury occur.

What is admissible and the admissibility standard applied are different than the quantum of evidence required to meet the necessary burden of proof and avoid a directed verdict. For example under our evidentiary rules relevance is determined by:

Rule 401. Test for Relevant Evidence

Evidence is relevant if: (a) it has any tendency to make a fact more or less probable than it would be without the evidence; and (b) the fact is of consequence in determining the action.

Admissibility for evidentiary purposes requires you to meet a very low threshold as seen above under IRE 401.  As a result, I would stick with a “reasonable degree of medical probability” in formulating medical/legal questions needed to establish an essential element of your claim. A particular medical finding could be consistent with multiple diagnoses. A mere possibility makes a finding consistent, but not necessarily probable.  So exercise discretion.