The day on which the procedure is performed (PRDAYn) is calculated from the procedure date (PRDATEn) and the admission date (ADATE) with the following exceptions:

• PRDAYn is set to the supplied day of principal procedure if the procedure day cannot be calculated (ADATE and/or PRDATEn is missing or invalid). Note: the supplied day of procedure is used only if it distinguishes between a procedure performed on the first day (procedure day = 0) and no procedure day (procedure day is missing).
• PRDAYn is missing (.) if the procedure day cannot be calculated and the supplied procedure day is missing.
• PRDAYn is invalid (.A) if the procedure day cannot be calculated and the supplied procedure day is nonnumeric.
• If the data source does not supply either admission date (ADATE) and procedure date (PRDATEn), or the day of procedure, then beginning in the 1998 data PRDAYn is not present on the HCUP files. In the 1988-1997 data, PRDAYn is retained on the HCUP files and is set to unavailable from source (.B).
• PRDAYn is inconsistent (.C) if
  • there is a day of procedure without a coded procedure (ED7nn), or
  • the day of procedure is not during the stay (EPRDAY01 beginning in the 1998 data and ED8nn in the 1988-1997 data).

Edit checks ED7nn are only performed on the 1988-1997 data. Beginning in the 1998 data, the procedure date without a coded procedure is discarded.

The procedure date vector (PRDATEn) is shifted with the ICD-9-CM procedure codes (PRn) when the procedure vector is packed.

Some sources do not require procedure dates/days for minor or diagnostic procedures which are considered UHDDS class 3 and class 4 procedures. The UHDDS system grouped ICD-9-CM procedure codes into four classes differentiated by impact on either the well-being of the patient or on the health care system. The criteria used to classify procedures included procedural risk, anesthetic risk, and the need for highly trained personnel, special facilities or special equipment. The classes are:

• Class 1: Surgical
• Class 2: Significant procedure (date required)
• Class 3: Significant procedure (date not required)
• Class 4: Minor procedures not normally coded on inpatient data.

PRDAY1 is present on the NIS from 1988 onward; secondary procedures (PRDAY2-15) are present on the NIS beginning in 1998.