The Nationwide Inpatient Sample (NIS) is a U.S. nationally representative administrative database of hospitalizations. The NIS is the largest all-payer publicly available national hospital database containing a 20% stratified sample of all short-term, nonfederal, nonrehabilitation hospitals. Hospitals are sampled according to geographic region, location (urban/rural), teaching status, ownership, and bedside; all discharges from sampled hospitalizations are included in the database. The NIS was developed as a part of the Healthcare Cost and Utilization Project, sponsored by the Agency for Healthcare Research and Quality. (www.hcup-us.ahrq.gov/hcupdatapartners.jsp)

Cases of dialysis-requiring AKI were identified using validated ICD9-CM codes (Waikar et al., 2006). Dialysis-requiring AKI was defined as the presence of any of the following diagnostic codes (in any position): 584.5, 584.6, 584.7, 584.8, or 584.9; and presence of any of the following procedure codes for dialysis (in any position): 39.95, V45.1, V56.0, V56.1. To avoid inclusion of patients admitted for initiation of chronic maintenance dialysis, cases with procedure codes for arteriovenous fistula creation or revision (39.27, 39.43, 39.43, 39.93) were excluded.

The total absolute number of cases of dialysis-requiring AKI for each calendar was estimated using appropriate discharge-level sampling weights to account for the NIS sampling scheme.

The population incidence of dialysis-requiring AKI for each calendar year was estimated by dividing the absolute number of cases by the total population (obtained from the U.S. Census Bureau).