The Impact of ICD-9-CM Code Rank Order on the Estimated Prevalence of Clostridium difficile Infections

The incidence and severity of Clostridium difficile infection (CDI) have been increasing in recent years [1–7], but national surveillance efforts and interhospital comparisons have been limited by the lack of a standard CDI surveillance system. As a result, the International Classification of Disease, Ninth Revision, Clinical Modification (ICD-9-CM) codes assigned at hospital discharge have been used as a proxy to estimate CDI prevalence in the United States [8–12]. Using administrative data (ICD-9-CM codes) to compare CDI rates between hospitals has several potential advantages. Administrative discharge data are inexpensive to obtain, are systematically collected, and utilize a single ICD-9-CM code to designate CDI (008.45), thus potentially providing a nationally representative method for tracking CDI rates [8, 13, 14]. Two administrative databases have been used to estimate CDI prevalence in the United States: the National Hospital Discharge Survey (NHDS) and the Nationwide Inpatient Sample (NIS). ICD-9-CM code data are collected differently in each database; neither data set collects all potential ICD-9-CM codes assigned at hospital discharge. The NHDS is collected annually by the National Center for Health Statistics at the Centers for Disease Control and Prevention (CDC), and about 90% of a panel of 500 hospitals participate [8]. The NIS, on the other hand, is collected from all states that participate in the Healthcare Cost and Utilization Project and includes information from about 1000 US hospitals [15]. The NHDS collects up to the first 7 ICD-9-CM codes assigned per patient, and the NIS collects up to the first 9–15 codes assigned per patient (the number of codes captured varies by state). Despite their advantages, a recent study suggests that ICD-9-CM codes may not be acceptable for hospital-onset CDI surveillance. A multicenter study performed at the CDC Prevention Epicenters hospitals compared the incidences of hospital-onset CDI as measured by ICD-9-CM codes with toxin assay results. ICD-9-CM codes overestimated the incidence of hospital-onset CDI, compared with toxin assay results, indicating that ICD-9-CM codes are not an acceptable surrogate for hospital-onset CDI surveillance [16]. The degree to which ICD-9-CM codes overreported the incidence of hospital-onset CDI varied by year and by hospital, indicating that ICD-9-CM codes would not have been useful for intrahospital or interhospital CDI surveillance. No previously published CDI prevalence study using ICD-9-CM codes has differentiated between the NHDS and NIS criteria. The data collection discrepancies between these 2 data sets may account for the conflicting results of previous studies of CDI prevalence. It is not known how the differences in the number of ICD-9-CM codes available for prevalence estimation affect the estimated CDI prevalence and how CDI burden estimates based on ICD-9-CM codes compare with toxin assay results. Therefore, we investigated how the NHDS and NIS criteria affect CDI prevalence and how these definitions compare with toxin assay results at multiple healthcare facilities during a 6-year study period.