Although NHSN HAI surveillance provides a standardized process to determine the occurrence of an HAI, implementing NHSN surveillance definitions is associated with interpretive variation independent of the quality of care (6, 11–12). First, some definition components are subjective, such as “purulent drainage from the deep incision” to determine the presence of a surgical site infection (13). Second, determining the presence of an HAI often relies on documentation of a provider's clinical assessment, and the variability between individual clinician determinations and documentation of those assessments can be considerable. Third, various data sources are required to apply surveillance definitions, and the ease of accessing this information can vary greatly among hospitals. Facilities with robust electronic medical record or electronic surveillance systems will be more likely to capture data used to determine the presence of an HAI and will thus report higher HAI rates than other facilities with limited access to a patient's record. Health care–associated infection surveillance is also resource-intensive, requiring trained reviewers to review many medical records, and the effort available for surveillance can vary substantially, affecting the completeness of case ascertainment. Finally, with limited patient-specific data included in the surveillance definitions, risk adjustment is incomplete. Efforts should be made to improve risk adjustment as necessary to prevent potentially misleading interfacility comparisons.