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.

Beginning with the 1993 NIS, 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 inpatient data, the encrypted physician identifiers from 1993 on may not match those in earlier years. However, no null characters were found in the 1994 identifiers, and they were rare in prior years.

The attending physician identification number (MDID_S) may not accurately track physicians across hospitals for the following reasons:

• Some hospitals assign their own internal attending physician identification numbers rather than using the license numbers issued by the licensing agency of the physician or other health care practitioner. Information was not available about the prevalence of this practice.
• Some hospitals use one attending physician identification number for several physicians that are part of the same physician practice group. Information was not available about the prevalence of this practice.

The attending physician identification number includes license numbers from the following board of examiners: Medical, Osteopathic, Podiatrists, and Nurses. In addition, Arizona accepts licensing numbers from other health practitioner licensing boards, but these boards are unspecified.

The attending physician identification number (MDID_S) may not accurately track physicians across hospitals. The state encourages hospitals to use the Professional State License Number as an identifier, but some hospitals continue to use their own internal identification number. Also, some hospitals appear to pad the Professional State License Number (a 5-digit code). Information was not available from the data source about the prevalence of these practices.

Some hospitals may use one license number for all physicians in order to protect physician confidentiality. Information was not available from the data source about the prevalence of this practice.

Florida reports state license numbers for the attending physician identifiers. During HCUP processing, physician identifiers were encrypted (MDID_S).

In 1998, the Florida attending physician numbers (MDID and MDID_S) are truncated. Values that are normally 11 bytes in length, for 1998, are only 10 bytes long. Although it may be difficult to track attending physicians to those before and after 1998, it may still be possible. If 10 bytes match without the 11th byte, it is more than likely the same physician. Primary surgeon numbers (SURGID and SURGID _S) are not affected.

Iowa reports Universal Physician Identification Numbers (UPINs) as attending physician identification numbers.

The encrypted attending physician identifier (MDID_S) may not accurately track physicians across hospitals. Kentucky collects two different types of physician identifiers, Universal Physician Identification Numbers (UPINs) and state license numbers.

Maryland reports a state license number assigned by the Medical Chirurgical Faculty of Maryland (MED CHI) for the attending physician. Source documentation describes strict assignment and verification rules for this field.

Michigan reports hospital-specific physician identifiers for attending physicians. Coding of physician identifiers are not consistent across hospitals. During HCUP processing, physician identifiers were encrypted (MDID_S).

The coding of attending physician identification number (MDID_S) varies across years:

New York reports state license numbers as physician identifiers. Source documentation indicates that if the attending physician did not possess a valid New York state license number, the license number of the Chief of Service should have been reported.

New York does not limit this field to physicians; dentists, podiatrists, psychologists, nurse/midwives, and other licensed health care professionals may be included. It is impossible to identify the different types of providers in the HCUP data.

Source physician identifiers are encrypted during HCUP processing.

For the 1998 - 2000 data, physician identifiers are missing (" ") on discharges with an indication of an induced abortion. New York identifies an indication of induced abortion by ICD-9-CM diagnosis or procedure code:

• An admitting, principal, or secondary diagnosis of "6350" through "6399", or "7796".
• A principal or secondary procedure of "690", "695", "696", "6993", "738", "7491", "750", "751", or "9649".

Please note that the admitting diagnosis is not retained in the HCUP databases.

The Washington attending physician identifiers may not accurately track physicians across hospitals. Washington collects several different types of physician identifiers, depending on the type of identifier provided by the hospitals. Hospitals provide Medicaid, Universal Physician Identification Numbers (UPINs), and DOH/HPQAD license numbers as physician identifiers. During HCUP processing, physician identifiers were re-encrypted (MDID_S).