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To use the hospital standardised mortality ratio (HSMR), as a tool for Dutch hospitals to analyse their death rates by comparing their risk-adjusted mortality with the national average.
The method uses routine administrative databases that are available nationally in The Netherlands—the National Medical Registration dataset for the years 2005–2007. Diagnostic groups that led to 80% of hospital deaths were included in the analysis. The method adjusts for a number of case-mix factors per diagnostic group determined through a logistic regression modelling process.
In The Netherlands, the case-mix factors are primary diagnosis, age, sex, urgency of admission, length of stay, comorbidity (Charlson Index), social deprivation, source of referral and month of admission. The Dutch HSMR model performs well at predicting a patient's risk of death as measured by a c statistic of the receiver operating characteristic curve of 0.91. The ratio of the HSMR of the Dutch hospital with the highest value in 2005–2007 is 2.3 times the HSMR of the hospital with the lowest value.
Overall hospital HSMRs and mortality at individual diagnostic group level can be monitored using statistical process control charts to give an early warning of possible problems with quality of care. The use of routine data in a standardised and robust model can be of value as a starting point for improvement of Dutch hospital outcomes. HSMRs have been calculated for several other countries.
In recent years, there has been an increasing interest in monitoring standards of clinical care in many countries. In the UK, the Bristol Royal Infirmary Inquiry into paediatric cardiac surgery deaths from 1999 to 20011 raised national awareness of the subject. In The Netherlands, an analysis of the death rates in cardiac surgery at the Radboud University by the Health Inspectorate led, in 2006, to a temporary six-month closure of the cardiac surgical department.
Mortality is a “hard” outcome with special relevance to the patient. Measuring death rates has the advantage that death is a definite unique event unlike morbidity, which often represents a spectrum of severity and can be difficult to record accurately. Death rates can, when adjusted for the factors that affect death rates, act as markers of a hard outcome of healthcare.
In England, the hospital standardised mortality ratio (HSMR), an overall measure of in-hospital mortality, has been used since 1999.2 About 67% of English acute hospital trusts nowadays use Real Time Monitoring (RTM)3 for monitoring and analysing HSMRs and their component diagnosis-level SMRs in order to deploy possible patient safety improvements. RTM makes the data available, updated monthly, to hospitals via the internet. HSMRs have also been calculated for the USA, Canada, Sweden, Wales, Australia (New South Wales), France, Japan, Hong Kong and Singapore and could be used to assess mortality, identify areas for possible improvement and monitor performance over time.
Until now, Dutch mortality figures, as measures of outcome of hospital care, were based on clinical databases and related to certain patient groups or procedures—for example, intensive care admissions,4 high-risk surgery5 and elderly patients.6 In the Dutch healthcare system, assessment of quality by calculating HSMRs has attracted considerable attention from government, patient organisations and the media. A study estimated that every year, more than 1700 avoidable deaths occur in Dutch hospitals.7 Following this study, a national patient safety programme was launched in 2007 by the associations of hospitals, medical specialists and nurses aimed at reducing the number of avoidable deaths. Monitoring the quality of hospitals within this programme by measuring HSMRs is one of the tools being used.
Two research organisations—Prismant and De Praktijk Index—developed, with Jarman and colleagues from Imperial College London, and Dr Foster Intelligence, a model to calculate HSMRs using data from the National Medical Registration (LMR) files, which contain all inpatient and day case admissions to hospitals. During the last years of calculating HSMRs for Dutch hospitals8 and using them for improving quality of care, several questions have been raised. The Dutch Minister of Health has announced that all Dutch hospitals should publish their HSMR in 2010.9 This article is intended to explain the current Dutch model and its statistical performance. It is hoped that it may be of assistance to hospitals by helping them to understand the method and that it may be of use for monitoring improvements in the quality of care for their patients.
The HSMR compares the actual number of hospital deaths with the expected number for those patients with a primary diagnosis within the set of diagnostic groups that account for 80% of all deaths in hospital nationally.
The national LMR dataset for 2005–2007 was used as the data source for the logistic regression calculations. The HSMRs were calculated for 2005–2007. In the LMR dataset, diagnoses are coded using the International Classification of Diseases, Ninth Revision (ICD-9), and these are converted to 259 Clinical Classification System (CCS) groups developed by the US Agency for Healthcare Research and Quality.10 From these CCS groups, those responsible for 80% of hospital deaths nationally were determined. Day cases (which have very few deaths) and inpatient admissions were included in the analysis. Logistic regression models were fitted for each of the CCS groups separately in order to generate an expected risk of death for each patient. The HSMR is derived from the sum of the observed deaths and expected risks across the CCS groups.
The statistical performance of the model was measured by the c statistic (area under the receiver operating characteristic curve) for each SMR and for the hospital level HSMR.11 The c statistic is the probability of assigning a greater risk of death to a randomly selected patient who died compared with a randomly selected patient who survived. A value of 0.5 suggests that the model is no better than random chance in predicting death. A value of 1 suggests perfect discrimination. In general, values above 0.75 suggest good discrimination.
In the 2005–2007 data and for the HSMR CCS groups only, there were 2363332 admissions and 90873 deaths (crude death rate 3.85%). The quality of the data of 15 hospitals did not fulfil the national registration standards in 2007, so we did not include them in a national comparison. Seven out of these 15 hospitals did not fulfil the standard for two or more criteria. Six of the 15 hospitals had more than 5% vague diagnosis, eight hospitals had less than 33% urgent admissions and ten hospitals had a ratio of comorbidity diagnosis to main diagnoses of <0.2. Another six hospitals were excluded because they had a patient population that differed too much from the national average. Four of these hospitals had less than 100 expected deaths in 2007, and finally two more hospitals were excluded because they are non-average hospitals in terms of their case mix. A funnel plot of the HSMRs of the remaining 65 hospitals is shown in figure 1.
Hospitals in figure 1 that lie within the control limits are said to exhibit common cause variation and those outside special cause variation unlikely to be due to natural random variation (in Shewhart's original terminology). Funnel plots provide a simple and easily understandable way to plot institutional comparisons.12 They have been used to plot anonymised mortality rates by surgeon for paediatric cardiac surgery13 and have been promoted as providing a strong visual indication of divergent performance, with the advantage of displaying actual event rates and allowing an informal check of a relationship between outcome and volume of cases.14
Dutch HSMRs differ widely among hospitals. According to this analysis, the chance of death in the hospital with the highest HSMR is 2.3 times the chance of dying in the hospital with the lowest HSMR, after adjusting for available case-mix factors.
The c statistic of the Dutch HSMR model is 0.91, similar to the values found for the other countries. Table 1 shows also the c statistics of all CCS groups: they vary from 0.68 to 0.96.
Significant factors determining the total hospital mortality were: primary diagnosis, age, sex, admission urgency (urgent/not-urgent, equivalent to emergency/elective (planned)), LOS, comorbidity (measured by the Charlson Index),15 area-level social deprivation (from the Dutch Central Office of Statistics), month of admission, type of organisation that made the referral and the CCS subgroup. These factors and their coefficients vary among each CCS group. Table 1 gives the significant factors (p<0.05) for every CCS group.
HSMRs have been calculated for The Netherlands in a manner similar to that used in several other countries. Currently, almost every Dutch hospital has asked for their HSMR without any pressure from the government or Healthcare Inspectorate. In addition, more than 50 hospitals have ordered a “Hospital Mortality Profile” over the last two years—a brief report giving the HSMR of a hospital broken down into its constituent diagnostic group SMRs and by age group, urgency and length of stay. The following applications of HSMRs are used in Dutch hospitals:
Our analysis of data completeness found no missing values of the date of admission, date of discharge, age, sex, urgency of admission or postal code (for social deprivation). However, for the recording of secondary diagnosis in particular, we cannot tell whether there is no comorbidity present or if comorbidity has simply not been recorded. Miscoding may also affect the HSMR.
The LMR data use a limited number of clinical variables but for the HSMRs examined in this study, the discrimination of the risk prediction model was very good. A recent UK study concluded that, at least for three common procedures, risk prediction with discrimination comparable with that obtained from clinical databases is possible using routinely collected administrative data.16
Although simplified models of risk prediction might be as effective in predicting outcome as some complex models currently in use,17 18 further improvements to the case-mix model are being evaluated. The numbers of previous admissions within a given time period, which requires the linking of admissions of the same patient, could be of potential use. Other features of the healthcare system that could potentially affect hospital mortality ratios include admission thresholds, the proportion of people in the area dying in hospital, discharge policies or underlying disease rates in the catchment population. It is unclear, however, whether and how one should measure and adjust for these factors.
A relevant discussion is also whether the length of stay and the procedure group are factors that are part of the case mix or determine quality. Both are related to the patient's illness but also to treatment.
Based on experience in other countries, the introduction of HSMRs raises various questions.19–22 Most recently, attention has been focused on the so-called “constant risk fallacy”23 in which some SMRs—for example, for some Charlson scores, differ from the overall HSMR. One paper suggests at least two mechanisms that might contribute: the first involves differential measurement error, and the second involves inconsistent proxy measures of risk.24 Measurement error, including poor coding, will have an impact on HSMRs, and this is the first thing that a hospital should check. The variation in SMRs can be interpreted in two ways, either as bias or as real differences in risk. Either way, further investigation using local data sources and case note reviews rather than more statistical analysis is suggested.
Another often heard query is that the methodology should correct for regional variation in health conditions or in the organisation and performance of healthcare facilities adjacent to the hospital. A multiple regression analysis has been developed for the Dutch HSMRs to find the factors that best explain the variation of HSMRs throughout The Netherlands.25 Depending on the extension of the dataset, further yearly refinements can be made to the models for the yearly releases of the HSMRs and SMRs.
The HSMR for The Netherlands appears to be a statistically robust model that can be used as an indicator for hospital deaths to help Dutch hospitals improve their quality of care. The statistical model is robust enough to include all hospitals with more than about 100 deaths per year, an average case mix and good quality data, varying in size and function, into one analysis. However, random variation and data quality issues need to be considered when interpreting the results. HSMRs can be used to highlight hospitals that have significantly high mortality, which may merit further investigation by the hospitals concerned. Furthermore, the impact of interventions designed to reduce mortality can be tracked using this measure.
The Dutch Ministry of Health26 27 has put HSMR high on its quality agenda and commissioned RIVM (the National Institute for Public Health and the Environment) to use HSMRs as one of the performance indicators in the Dutch Health Care Performance report. In the future, international comparisons might also be possible.
We would like to thank Laurens Touwen who has been immensely helpful in developing the Dutch HSMRs since the start of the project in 2004. We also would like to thank the Dutch Hospital Association (NVZ) and the Federation of Medical Specialists (de Orde) for granting permission to use the Dutch hospital data.
Contributors: All the authors have been involved in writing, reading and commenting on the manuscript. BJ, DP, PA, AB and SJ were involved in developing the model. BJ is the guarantor of the content.
Funding: There was no separate funding for this study. BJ, AB and PA are employed within the Dr Foster Unit at Imperial College London (BJ, part-time), which is partly funded by a grant from Dr Foster Intelligence (an independent health service research organisation). The unit is also partly funded for its HSMR work by the Rx Foundation of Cambridge, Massachusetts, USA, and is affiliated with the Centre for Patient Safety and Service Quality at Imperial College Healthcare NHS Trust, which is funded by the National Institute of Health Research. The Department of Primary Care and Social Medicine is grateful for support from the National Institute for Health Research Biomedical Research Centre Funding Scheme.
Competing interests: None.
Provenance and peer review: Not commissioned; externally peer reviewed.