Background: National surveillance data should be validated to identify data quality issues. Denominator data can substantially affect the healthcare-associated infection (HAI) rate; however, they are relatively overlooked compared to numerator data. We analysed the status of and problems with collecting denominator data by the Korean National Healthcare-associated Infections Surveillance System (KONIS).
Methods: This study was conducted in 21 (10.8%) of the 193 hospitals that participated in the KONIS ICU module between July 2016 and June 2017. A survey examined denominator data collection methods, such as patient days and device days of central lines and urinary catheters, between November and December 2017. The knowledge of central lines and urinary catheters specifically for KONIS reporting purposes, and the correct answer rate to questionnaires regarding specific situations related to the calculation of device days, were also evaluated.
Results: Counting the denominator data, the respective proportions using manual or electronic methods were 52.4% vs. 42.8% for patient days, 66.7% vs. 33.3% for central line days, and 61.9% vs. 38.1% for urinary catheter days. The rate of accurately understanding and responding to the case questions on device days through a survey was 19.0–81.0% for the central lines and 33.3–95.2% for the urinary catheters.
Conclusion: It is important to collect accurate denominator data in addition to numerator data to maintain the reliability of the national HAI surveillance data. Continued education of the surveillance personnel on the definition of denominator data and accurate data collection methods is required.

Keywords: Denominator, Healthcare-associated infection, Surveillance