For HCUP data from 2001 to 2002, this data element is called MDNUM1_S. Beginning in 2003, this data element is called MDNUM1_R.

MDID_S contains a fixed-key (one-to-one) encryption of the supplied attending physician number (MDID), according to the following rules:

• All alphanumeric digits are used in the encryption.
• All symbols such as ".,:;'*@" are retained in the encrypted value, but not in the same location.
• Leading zeros are encrypted so that the two original physician identifiers "000A6" and "A6" are distinctly different.
• When the original attending physician and primary surgeon identifiers are the same, the synthetic identifiers, MDID_S and SURGID_S, are the same.
• When the MDID in the ambulatory surgery data and the inpatient data are the same, the synthetic identifier, MDID_S is the same.

Except in those data sources where physician license numbers are supplied, it is not known whether the physician identifier MDID_S refers to individual physicians or to groups. If the attending physician numbers supplied by the data source are not restricted to license numbers, the state-specific note includes available information about reporting practices, including whether MDID_S refers to individual physicians or to groups.

Beginning in the 1993 data, supplied physician identifiers were checked for null characters. If null characters were found, they were replaced by blanks before the identifier was encrypted. Since this conversion was not done in prior years of HCUP data, the encrypted physician identifiers from 1993 on may not match those in earlier years. However, null characters are rarely included.