Severity of illness scoring systems to adjust nosocomial infection rates: A review and commentary

Background: Nosocomial infections (NI) are often used by hospitals for external comparisons. In the National Nosocomial Infection Surveillance system, NI rates from intensive care units (ICUs) are adjusted for extrinsic risk factors such as device use but would be enhanced if they were better adjusted with a direct measurement of patientsʹ severity of illness. Method: We performed a Medline search on the literature during 1991 to 1996 to identify a severity of illness scoring system (SISS) that would be useful for further adjusting ICU NI rates. We assessed the scoring system for objectivity, simplicity, discriminating power, and availability. Results: Eleven studies reported the use of SISS. Seven used scoring systems developed to predict mortality rates. Four correlated SISS with all sites of NI and, in general, did not meet with success. Six showed some predictive value between SISS and nosocomial pneumonia. The Acute Physiology and Chronic Health Evaluation score (version II or III), used in five studies, was the most commonly used SISS but performed inconsistently and may not be available in many ICUs. Conclusion: New approaches for measures of severity of illness need to be developed to adjust NI rates. Until such measures are available, comparative NI rates will be limited in their use as definitive indicators of quality of care.