John Langley Injury Prevention Research Unit, University of Otago

International Classification of Diseases Injury Severity Scale (ICISS) Presented at the Injury Statistics Meeting, 7-8 September 2006, Washington DC John Langley Injury Prevention Research Unit, University of Otago

What is it? what its not? • It’s a measure of threat to life • It is not a measure of threat of disability or anything else for that matter

Overview of talk • Description • Strengths and weaknesses • Some applications

How is it derived? • It involves estimating the probability of death directly from ICD injury diagnoses by examining a large set of cases for which the survival status is known • In NZ for example we have a dataset of all injury victims requiring hospital inpatient treatment, they have their diagnoses coded according to ICD and we know if they were discharged dead or alive.

Exactly how is it derived? • It involves calculating a survival risk ratio (SRR) (that is the probability of survival) for each individual injury diagnosis. • It’s the ratio of the number of patients with that injury code who died to the total number of patients diagnosed with that code. • Each patient’s ICISS score (survival probability) is the product of the probabilities of surviving each of their injuries individually.

Weaknesses • Its derivation is dependent on having a large number of injury cases which ideally include all injury events which result in death • It depends on the reliability and precision of the coding of the diagnostic data • To date limited number of studies deriving ICISS at population level • Probabilities may be influenced by co-morbidity SRR (thus ICISS) change over time-they change as the case fatality rate changes

Weaknesses ctd • Most studies deriving ICISS don’t include all injury deaths, e.g many deaths occur outside hospital so SRR’s are in some cases over estimated, there is thus potential of bias say in the selection of serious injury cases for indicators • Generalisability: Case fatality rates (and SRR’s for particular injuries) for developed vs developing are likely to be dramatically different

Strengths • It is empirically derived-unlike some other measures (e.g AIS is based on expert opinion) • Evidence suggests that it performs as well as ICD/AIS derived measures (e.g ICD/ISS) in terms of predicting survival • It uses existing diagnostic information • It does not involve any translation (e.g ICDMAP) and thus loss of predictive power

Strengths ctd • It is an interval scale (AIS is ordinal) • It’s not subject to observer bias, coding error • It is inexpensive -doesn’t involve record review by experts and coding • Currently it provides the only practical way forward for large administrative ICD-10 coded datasets whereas there is no easy AIS derived solution • It covers the entire spectrum of injury (unlike AIS)

Uses: Selecting main injury • The underlying approach to ICISS, determining probability of death for specific diagnoses, can be the basis for selecting main injury (NB: If your objective is to select the injury which is the greatest threat to life) • BUT SRRs derived for this purpose would have to come from a population of cases where there was only one injury (to avoid contamination of the estimate) and the question arises, at least at the serious end of the spectrum, as to how many such injuries exist by themselves

Uses: Indicators • Provides a means for developing valid indicators for measuring trends in the incidence of important injury over time where important refers to significant threat to life

% Deviation From 1988 Base in MVTC Hospitalisations by ICISS 1988 - 1999

Uses: International comparisons of non-fatal injury: (an ICE project?) • Potentially provides the means for under-taking valid international comparisons of non-fatal injury • An approach? • pool several countries comparable data (e.g. hospital inpatient data which includes fatality status on discharge) • Work out SRR’s and agree on a threshold for important injuries • Identify the injuries (codes) identified by this threshold • Select those injuries from each country as the basis for comparison

John Langley Injury Prevention Research Unit, University of Otago