The assignment of the data elements for the Elixhauser Comorbidity Software Refined for ICD-10-CM and the Clinical Classifications Software Refined (CCSR) for ICD-10-CM is incomplete for less than one percent (0.84 percent) of the records in the 2021 HCUP Nationwide Ambulatory Surgery Sample (NASS).

Data Issue

The 2021 NASS includes a maximum of 25 diagnosis codes. The assignment of comorbidity and diagnosis CCSR data elements used only 20 of the possible 25 diagnosis codes. This means that a small percentage of records (75,000 out of 8.9 million records, or 0.84 percent) in the 2021 NASS are missing an indication that the record is included in a comorbidity measure and/or CCSR category. The following provides two examples of how this issue affects the data:

• The number of records in the 2021 NASS that are identified with the comorbidity of AIDS (data element CMR_AIDS=1) is 20,746. If all diagnoses had been used in the assignment, an additional 45 records would be flagged as having this comorbidity.
• The number of records in the 2021 NASS that are identified with the comorbidity of alcohol abuse (data element CMR_ALCOHOL=1) is 31,696. If all diagnoses had been used in the assignment, an additional 161 records would be flagged as having this comorbidity.

Identification and Correction of the Affected Records

There are 113,560 records in the 2021 NASS with more than 20 diagnosis codes reported (identified by data element I10_NDX > 20). There are less than 75,000 records in the 2021 NASS that are affected by this issue. To be affected, a record needed to have more than 20 diagnoses reported and have a diagnosis that would trigger a comorbidity measure or CCSR category not flagged by the first 20 diagnoses.

The following lists the steps needed to correct these records:

1. Download the two software tools from the HCUP User Support (HCUP-US) website:
   1. Elixhauser Comorbidity Software Refined for ICD-10-CM, v2023.1
   2. Clinical Classifications Software Refined for ICD-10-CM, v2023.1
2. Run the Elixhauser comorbidity software on the NASS_2021_ENCOUNTER file specifying to use 25 diagnoses (I10_DX1-I10_DX25) and retaining the unique record number (data element KEY_NASS). This will create a file with the corrected set of comorbidity data elements (starting with CMR_).
3. Run the CCSR for ICD-10-CM software on the NASS_2021_ENCOUNTER file specifying to use 25 diagnoses (I10_DX1-I10_DX25) and retaining the unique record number (data element KEY_NASS). Specify the creation of a horizontal file (macro statement %LET HORZ=1). This will create a file with the CCSR data elements (starting with DXCCSR_).
4. Merge together the two resulting files (one with the CMR_ data elements and the other with the DXCCSR_ data elements) by the data element KEY. This new file will take the place of the current NASS_2021_DX_PR_GRPS file and have the corrected assignment of the comorbidity measures and CCSR categories.