When if ever is a subpoena valid to obtain medical records?

Subpoena's to obtain medication records are necessary in a few instances:

• Postmortem patients often have records petitioned this way.
• Cases in which the patient is on trial, criminal or civil are another.
• Cases in which the patient requires their own records and the covered entity resists.
• Grand juries can also subpoena medical records.

Subpoenas for medical records occur pretty much any time a court or the grand jury needs to see designated record set and there's no other legal way to obtain it.