HCUP encrypted physician identifiers are referred to by different names in the HCUP data across years.

MDNUM1_R is specific to physicians. If the physician identifier is based on a state license number or Universal Physician Identification Number (UPIN), then MDNUM1_R can be used to track a physician across hospitals. If the physician identifier is based on hospital-specific identifiers, then it can only be used to track physicians within a hospital. Except in those data sources where physician license numbers are supplied, it is not known whether the physician identifier refers to individual physicians or to groups. Refer to state-specific notes for more information about the type of physician identifiers provided by each state.

Because of a change in the algorithm for creating a masked physician number, physicians cannot be tracked from before 2003 to after 2003. In HCUP data prior to 2003, a synthetic physician number (MDNUM1_S prior to 2003 and MDID_S prior to 2001), created using fixed-key encryption, was available. Starting in data year 2003, a reidentification number (MDNUM1_R) was used. MDNUM1_R includes an arbitrarily chosen, identifying number that is unique to the physician identifier provided to HCUP.