A Master Patient Index (MPI ) is a service used to manage patient identification in a context where multiple patient databases exist. Healthcare applications and middleware use the MPI to match patients between the databases, and as new patient details are encountered. MPIs are highly specialized applications, often tailored extensively to the institution's particular mix of patients. MPIs can also be run on a regional and national basis.\n\nTo ask an MPI to match a patient, clients use the "$match" operation, which accepts a patient resource which may be only partially complete. The data provided is interpreted as an MPI input and passed processed by an algorithm of some kind that uses the data to determine the most appropriate matches in the patient set. \n\nNote that different MPI matching algorithms have different required inputs. The generic $match operation does not specify any particular algorithm, nor a minimum set of information that must be provided when asking for an MPI match operation to be performed, but may implementations will have a set of minimum information, which may be declared in their definition of the $match operation by specifying a profile on the resource parameter, indicating which properties are required in the search.\n\nThe patient resource coming into the operation does not have to be complete, nor does it need to pass validation (i.e. Mandatory fields don't need to be populated), but it does have to be a valid instance. This is due to the resource being used for reference data to match from, and not being stored.