Case mix, outcome and activity for patients with severe acute kidney injury during the first 24 hours after admission to an adult, general critical care unit: application of predictive models from a secondary analysis of the ICNARC Case Mix Programme database

Abstract

Introduction: This study pools data from the UK Intensive Care National Audit and Research Center (ICNARC) Case Mix Programme (CMP) to evaluate the case mix, outcome and activity for 17,326 patients with severe acute kidney injury (AKI) occurring during the first 24 hours of admission to intensive care units (ICU).

Methods: Severe AKI admissions (defined as serum creatinine >/=300 mumol/l and/or urea >/=40 mmol/l during the first 24 hours) were extracted from the ICNARC CMP database of 276,326 admissions to UK ICUs from 1995 to 2004. Subgroups of oliguric and nonoliguric AKI were identified by daily urine output. Data on surgical status, survival and length of stay were also collected. Severity of illness scores and mortality prediction models were compared (UK Acute Physiology and Chronic Health Evaluation [APACHE] II, Stuivenberg Hospital Acute Renal Failure [SHARF] T0, SHARF II0 and the Mehta model).

Results: Severe AKI occurred in 17,326 out of 276,731 admissions (6.3%). The source of admission was nonsurgical in 83.7%. Sepsis was present in 47.3% and AKI was nonoliguric in 63.9% of cases. Admission to ICU with severe AKI accounted for 9.3% of all ICU bed-days. Oliguric AKI was associated with longer length of stay for survivors and shorter length of stay for nonsurvivors compared with nonoliguric AKI. Oliguric AKI was associated with significantly greater ICU and hospital mortality (55.8% and 77.3%, respectively) compared with nonoliguric AKI (33.4% and 49.3%, respectively). Surgery during the 1 week before admission or during the first week in the CMP unit was associated with decreased odds of mortality. UK APACHE II and the Mehta scores under-predicted the number of deaths, whereas SHARF T0 and SHARF II0 over-predicted the number of deaths.

Conclusions: Severe AKI accounts for over 9% of all bed-days in adult, general ICUs, representing a considerable drain on resources. Although nonoliguric AKI continues to confer a survival benefit, overall survival from AKI in the ICU and survival to leave hospital remains poor. The use of APACHE II score measured during the first 24 hours of ICU stay performs well as compared with SHARF T0, SHARF II0 and the Mehta score, but it lacks perfect calibration.

Figure 1

Recoding of APACHE II/III categories for logistic regression model. Recoding performed to eliminate categories containing fewer than 50 admissions in the multiple logistic regression model. Grey cells denote reference categories. APACHE, Acute Physiology and Chronic Health Evaluation.

Figure 2

ROC curves for SHARF T0, SHARF II0, Mehta and UK APACHE II. APACHE, Acute Physiology and Chronic Health Evaluation; SHARF, Stuivenberg Hospital Acute Renal Failure.

Figure 3

Calibration plots for SHARF T0, SHARF II0, Mehta and UK APACHE II. Observed mortality plotted against deciles of predicted mortality. Diagonal line indicates perfect calibration. APACHE, Acute Physiology and Chronic Health Evaluation; SHARF, Stuivenberg Hospital Acute Renal Failure.