The Wisconsin Department of Health and Family Services provides separate ambulatory surgery (AS) data and emergency department (ED) data to HCUP. The AS data are reported by Wisconsin hospitals, affiliated ambulatory surgery centers and freestanding ambulatory surgery centers. Wisconsin defines AS events as having one or more of the following surgical procedures: ICD-9-CM procedures 01.01-86.99 and CPT procedures 10000-69999. Each AS record contains items or aggregations of items from UB-92 or HCFA-1500 billing forms.

If a person has an outpatient surgical procedure and is then admitted as an inpatient, the hospitalization is represented with a record in the inpatient discharge data file. The principal procedure date may be prior to the admission date. There is no record in the ambulatory surgery data file.

If a patient has an outpatient surgical procedure and is subsequently admitted to a hospital as an inpatient, there will be a record in the ambulatory surgery data file and a separate record in the inpatient discharge file.

Using the standard HCUP criteria for assigning records to the 2003 SASD, the source records are separated as follows:

• 99.97% of source records are assigned to the SASD
• 0.03% of source records are excluded.

Identifying Freestanding Facilities. Starting in 1998, the data element FREESTANDING identifies hospital-based and freestanding ambulatory surgery facilities. Prior to 1998, there are two identifiers on the SASD that allow a researcher to identify hospital-based and freestanding ambulatory surgery centers.

• DSHOSPID is the facility identifier provided by the data source.
• AHAID is the facility identifier used by the American Hospital Association.

1997 Wisconsin SASD. If the DSHOSPID is less than 200, then the ambulatory surgery center is hospital-based. If the DSHOSPID is greater than equal 200, then the ambulatory surgery center is freestanding. The AHA identifier (AHAID) may be missing on a small number of hospital-based facilities because no inpatient data were reported for those sites.