A Master Patient Index (MPI) is a core health information management function that supports accurate patient identity matching across an organization’s clinical and administrative systems. Its central purpose is to ensure that each patient’s records—encounters, lab results, imaging, medications, allergies, and billing information—are correctly linked to the right individual, even when the patient receives care at multiple locations or has multiple registrations. To accomplish this, an MPI assigns (and maintains) a unique person identifier for each patient. This identifier serves as the consistent “key” used to connect records from different systems and prevent duplicate charts or overlay errors (where one patient’s information is mistakenly filed under another’s record).

The other choices do not align with what an MPI does. A prescription number relates to a medication order/dispense transaction, not a person. “Master population index” is not something that is “assigned” to the patient; it is the system/process itself (sometimes called an enterprise master patient index). A medical provider identifier applies to clinicians, not patients. In practice, MPI integrity is supported by demographic attributes (name, DOB, address, phone), matching algorithms, and governance processes for duplicate resolution—built around the patient’s unique person identifier .