Paediatric Index of Mortality scores: An evaluation of function in the paediatric intensive care unit of the Red Cross War Memorial Children’s Hospital

Abstract

Background. Paediatric Index of Mortality (PIM) and PIM 2 scores have been shown to be valid predictors of outcome among paediatric intensive care unit populations in the UK, New Zealand, Australia and Europe, but have never been evaluated in the South African context.Objective. To evaluate the PIM and PIM 2 as mortality risk assessment models.Method. A retrospective audit of case records and prospectively collected patient data from all admissions to the Paediatric Intensive Care Unit (PICU) of Red Cross War Memorial Children’s Hospital, Cape Town, during the years 2000 (PIM) and 2006 (PIM 2), excluding premature infants, children who died within 2 hours of admission, or children transferred to other PICUs.Results. For PIM and PIM 2 there were 128/962 (13.3%) and 123/1113 (11.05%) PICU deaths with expected mean mortality rates of 12.14% and 12.39%, yielding standardised mortality risk ratios (SMRs) of 1.1 (95% confidence interval (CI) 0.93 - 1.34) and 0.9 (95% CI 0.74 - 1.06), respectively. Receiver operating characteristic analysis revealed area under the curve of 0.849 (PIM) and 0.841 (PIM 2). Hosmer-Lemeshow goodness of fit revealed poor calibration for PIM (χ2=19.74; p=0.02) and acceptable calibration for PIM 2 (χ2=10.06; p=0.35). SMR for age and diagnostic subgroups for both scores fell within wide confidence intervals.Conclusion. Both scores showed good overall discrimination. PIM showed poor calibration. For PIM 2 both discrimination and calibration were comparable to the score derivation units, at the time of data collection for each. Calibration in terms of age and diagnostic categories was not validated by this study.