Applied use of CUSUMs in surveillance
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This document discusses using statistical process control (CUSUM) charts to monitor mortality rates at the level of individual general practitioners and health authorities. It describes how CUSUM charts could potentially have detected Harold Shipman, a GP who murdered over 200 patients, by spotting outliers in the routine mortality data. The document also discusses challenges in risk adjusting outcomes to account for differences in patient characteristics and casemix between providers. Accurately adjusting for factors like age, comorbidities, and emergency status is important for fair comparisons but difficult using only administrative data.
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Applied use of CUSUMs in surveillance
- 1. Applied use of CUSUMs in surveillance Paul Aylin, Co-Director of the Dr Foster Unit, Imperial College London Chair: Chris Sherlaw-Johnson, Senior Research Analyst, Nuffield Trust
- 2. Applied use of CUSUMs in surveillance Professor Paul Aylin Dr Foster Unit at Imperial College London [email protected]
- 3. Measuring Performance in Institutions • Background • Risk adjustment • Analysis • Alerting system
- 4. In an attempt to arrive at the truth, I have applied everywhere for information but in scarcely an instance have I been able to obtain hospital records fit for any purpose of comparison. If they could be obtained they would enable us to answer many questions. They would show subscribers how their money was being spent, what amount of good was really being done with it or whether the money was not doing mischief rather than good. Florence Nightingale, 1863 Where healthcare measurement began
- 5. • Heart operations at the Bristol Royal Infirmary “Inadequate care for one third of children” • Harold Shipman Murdered more than 200 patients Key events
- 10. Could we detect Shipman by looking at the data? • Provided with data for over 1000 GPs in five health authorities • One GP was Shipman, but we were blinded as to which one. • Investigate methods for routine surveillance of mortality data at the level of General Practitioner (GP), Practice and Health Authority (HA).
- 11. Prospective surveillance and multiple testing • Different to Bristol • No prior hypothesis • Statistical process control charts (SPC) among the most widely used methods for sequential analysis
- 12. CUSUM charts for practice level mortality rates
- 14. Key lessons • Routine hospital admissions data are good enough for performance monitoring (Bristol) • Statistical methods exist for spotting outliers and detecting improvement (Shipman) • Make use of multiple data sources • Risk adjustment important
- 15. Casemix • Not all patients are alike: age, sex, illness severity, comorbidity, frailty… • Many of these factors affect your risk of clinical events e.g. death, complications • The distribution of these casemix factors varies by hospital • Comparing hospitals’ crude death rates will therefore be misleading -> risk adjust
- 16. Tricky. Why not compare processes of care instead? • Patients care about outcomes • Process measures describe what was done to patient (drugs given, guidelines followed, scans done, advice given) • A large number of these comprise quality of care, but which ones really affect the outcome? Which ones should we monitor? • Less available in electronic data
- 17. Challenges - Case mix adjustment Limited within administrative data? • Age • Sex • Emergency/Elective
- 18. Data used in our UK risk modelling • National routine hospital admissions data (HES), updated monthly • 15m records annually, 300+ fields • Dx, ops, age, sex, emergency status, area code, deaths. No lab or drugs • ICD10 and OPCS coding systems • Easy(ish) access, cheap, comp • Quality much improved but varies
- 19. Casemix adjustment methods: evolving science/art • Indirect standardisation by age and sex -> SMR (19th Century to date) • Computing and stats algorithms -> regression methods (1970s onwards) • Machine learning methods (1980s onwards) • We need i) right list of casemix factors ii) defined and blended the best way…
- 20. How does risk adjustment work for mortality? • For each hospital, get actual (‘observed’) number of deaths and divide by predicted (or ‘expected’) number of deaths: this is the SMR • Derive expected number from risk model where each patient’s probability of death is estimated depending on their set of casemix factors: sum these probabilities of death for each hospital to give the expected number of deaths
- 21. What might go into these risk models? • age • sex • elective status • socio-economic deprivation • diagnosis subgroups or procedure subgroups • comorbidity – e.g. Charlson, Elixhauser, Holman, DRG… • number of prior emergency admissions • palliative care • year • month of admission • ethnic group • source of admission (own home, care home, other hospital)
- 22. Our approach to risk modelling • Build one model per outcome and per patient (dx/op) group • This allows for e.g. age to affect post-op mortality differently from readmission for stroke • Try all the candidate variables and key interactions and drop the unimportant ones • Automate age grouping and technical issues to allow industrial scale • Update models at least yearly: things change
- 23. Comorbidity adjustment • MANY approaches tried: count ICD codes; count ‘common’ or ‘impt’ ICD codes; count comorbs; weight subset of comorbs (which ones??? What weights???) • Common indices include Charlson (1987), Elixhauser (1998). Which is better? Depends on outcome, pt group
- 24. How successful is the casemix adjustment?
- 25. ROC curve areas comparing ‘simple’, ‘intermediate’ and ‘complex’ models derived from HES with models derived from clinical databases 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 CABG AAA - unruptured AAA - ruptured Colorectal excision for cancer Index procedure ROC HES Simple model (Year, age, sex) HES Intermediate model (including method of admission) HES Full model Best model derived from clinical dataset Aylin P; Bottle A; Majeed A. Use of administrative data or clinical databases as predictors of risk of death in hospital: comparison of models. BMJ 2007;334: 1044
- 26. Comparison of HES vs clinical databases Vascular surgery • HES = 32,242 • National Vascular Database = 8,462 Aylin P; Lees T; Baker S; Prytherch D; Ashley S. (2007) Descriptive study comparing routine hospital administrative data with the Vascular Society of Great Britain and Ireland's National Vascular Database. Eur J Vasc Endovasc Surg 2007;33:461-465 Bowel resection for colorectal cancer • HES 2001/2 = 16,346 • ACPGBI 2001/2 = 7,635 • ACPGBI database, 39% of patients had missing data for the risk factors Garout M, Tilney H, Aylin, P. Comparison of administrative data with the Association of Coloproctology of Great Britain and Ireland (ACPGBI) colorectal cancer database. International Journal of Colorectal Disease 2008;23(2):155-63
- 27. How to identify outliers? “Even if all surgeons are equally good, about half will have below average results, one will have the worst results, and the worst results will be a long way below average” • Poloniecki J. BMJ 1998;316:1734-1736
- 29. Funnel plot for surgeon-level adjusted return to theatre (RTT) rates for hip replacement.
- 32. Pyramid Model Of Investigation To Find Credible Cause Lilford et al. Lancet 2004; 363: 1147-54 1st Step: Does the coding reflect what happened to the patient 4th Step: Examine when other issues have occurred 2nd Step: Has something occurred locally to affect your casemix 3rd Step: The Local Health Economy may treat patients differently than the rest of the country/region e.g. provision of hospices, etc Finally: An individual is rarely the cause of an alert. A Consultant name may be coded against the primary diagnosis but many individuals and teams are involved in the patient’s care