Beginning in 2003, this data element is called PNUM_R.

PNUM_S is a de-identified patient number provided by the HCUP Partner that is encrypted by HCUP and can be used to identify multiple admissions by the same patient within and across institutions.

PNUM_S should not be used for analyses without first consulting summary statistics on:

• Frequencies of the number of discharges and the number of different hospitals per nonmissing PNUM_S.
• State-level counts of the number of unique nonmissing PNUM_Ss, the number of discharges associated with these PNUM_Ss, the ratio of these two numbers (discharges/patients), and the number of discharges without a PNUM_S.

PNUM_S contains a fixed-key (one-to-one) encryption of the de-identified patient number (PNUM), 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 retained. If the data source codes the same de-identified patient number (PNUM) inconsistently (sometimes with leading zeros and sometimes with leading blanks), the PNUM_S values are different.
• When the PNUM in the ambulatory surgery data and the inpatient data are the same, the PNUM_S values are the same.

Beginning in the 1993 data, the de-identified patient numbers were checked for null characters. If null characters were found, they were replaced by blanks before the number was encrypted. Since this conversion was not done in prior years of HCUP data, PNUM_S values from 1993 on may not match those in earlier years. However, null characters are rarely included.