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Measures of Disease Occurrence Kitty J. Jager¹, Carmine Zoccali², Reinhard Kramar³ and Friedo W. Dekker 1,4 1 ERA–EDTA Registry, Dept. of Medical Informatics, Academic Medical Center, Amsterdam, The Netherlands 2 CNR–IBIM Clinical Epidemiology and Pathophysiology of Renal Diseases and Hypertension, Renal and Transplantation Unit, Ospedali Riuniti, 89125 Reggio Cal., Italy 3 Austrian Dialysis and Transplant Registry (OEDTR), General Hospital of Wels, Wels, Austria 4 Department of Clinical Epidemiology, Leiden University Medical Centre, Leiden, The Netherlands Kidney International: Series on epidemiology

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Epidemiology The study of the occurrence of disease Centers of Disease Control and Prevention (CDC): Epidemiology is the study of the distribution and determinants of health- related states or events in specified populations, and the application of this study to the control of health problems 1 Measures of disease occurrence to be used for analysis depend on the aim of the study Prevention, etiology, risk factors new cases Incidence Burden of disease, planning of health care existing cases Prevalence 1 http://www.cdc.gov/reproductivehealth/EpiGlossary/glossary.htm

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Incidence - Definition of Terms ConceptDefinitionFormula RiskProbability of developing disease No of subjects developing disease during a time period No of subjects followed for the time period Incidence rateRatio of the number of cases to the time at risk for disease No of subjects developing disease Total time experienced for the subjects followed

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Incidence - Risk Risk (synonyms: cumulative incidence, incidence proportion) always necessary to define a time period to which the risk applies assumes that subjects are followed for the entire time period 2 Puliyanda DP, Stablein DM, Dharnidharka VR. American Journal of Transplantation 2007; 7: 1–5 Example 1 – Risk of hospitalization for bacterial infection The paper of Puliyanda et al. 2 describes a cohort of 3106 children during the first 2 years post-renal transplantation. One of the purposes of the study was to determine the risk of hospitalization for bacterial infection in the first two years after renal transplantation. 164 children lost their grafts in the first 6 months after transplantation. 687 patients were hospitalized for bacterial infection.

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Incidence - Risk Risk = 687 / 3106 = 22.1% but …. 164 lost their grafts for other reasons and therefore were not able anymore to develop the event of interest! In case the follow-up time is long, there are problems with risk as a measure of disease occurrence, due to loss to follow-up and competing risk 2 Puliyanda DP, Stablein DM, Dharnidharka VR. American Journal of Transplantation 2007; 7: 1–5 Example 1 – Risk of hospitalization for bacterial infection The paper of Puliyanda et al. 2 describes a cohort of 3106 children during the first 2 years post-renal transplantation. One of the purposes of the study was to determine the risk of hospitalization for bacterial infection in the first two years after renal transplantation. 164 children lost their grafts in the first 6 months after transplantation. 687 patients were hospitalized for bacterial infection.

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Incidence – Incidence rate Incidence rate (synonyms: incidence density, hazard, force of morbidity/ mortality) not required for every study subject to complete the entire risk period, as only time at risk is taken into account this makes the incidence rate very useful in cases where subjects may or may not be at risk for the event of interest for particular periods of time Example 2 – Incidence rate of peritonitis requiring hospitalization in CAPD patients Suppose we would have four episodes in 17 patients. We can only calculate the incidence rate if we know the total time at risk (= total time on CAPD).

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Incidence – Incidence rate In total 144 months at risk 4 peritonitis episodes Incidence rate = 4/144 = 0.028/patient month = 4/12 = 0.33 /patient year Incidence rate = reciprocal of the waiting time, i.e. the average time until an event occurs Average waiting time for a peritonitis requiring hospitalization to occur in a CAPD patient is 1/ 0.33 = 3 years

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Incidence – Risk vs Incidence rate Over short periods of time risk and incidence rate will be similar, as there will be little loss to follow-up and competing risk will only play a minor role Therefore, in-hospital mortality for acute renal failure is commonly expressed as a risk, whereas mortality on dialysis for end-stage renal disease is usually expressed as a rate A comparison of properties of risk and incidence rate (adapted from [3]) PropertyRiskIncidence Rate Range0 – 1 (0 – 100%)0 - infinity UnitsNone1 / time InterpretationProbabilityReciprocal of waiting time 3 Rothman KJ. Epidemiology: an introduction. Oxford University Press 2002

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Prevalence - Definition of Terms Prevalence is a measure of disease status: it deals with existing cases of disease ConceptDefinitionFormula Point prevalenceProportion of people in a population having disease at a particular point in time No of subjects having disease at a particular point in time Total no of subjects in the population Period prevalenceProportion of people in a population having disease over a period of time No of subjects with disease at the start of the period + no of subjects developing disease over the time period Total no of subjects in the population

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Prevalence – Point Prevalence vs Period Prevalence Prevalence is a measure of disease status: it deals with existing cases of disease Example 3 – Prevalence of ESRD due to glomerulonephritis At the beginning of 2005, 20 patients out of the 80 patients in our dialysis centre had ESRD due to glomerulonephritis (GN). During 2005 there were 2 new patients with the same condition taken into dialysis. Point prevalence of ESRD due to GN at January 1, 2005: 20 / 80 = 0.25 Period prevalence in 2005 = 22 / 80 = 0.275

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Prevalence – Dependence on Incidence rate and Disease Duration Prevalence = Incidence rate x Average disease duration Incidence rate Survival = Incidence rate = Survival Incidence rate Survival Prevalence

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Burden of disease in a population (e.g. costs, life expectancy, morbidity) Helps to determine where investment in health should be targeted Example 4 – Prevalence: economic burden of mineral regulating therapy in dialysis patients Lorenzo et al. did a study among 1312 haemodialysis patients from 6 centres in Spain 4, which was estimated to represent almost 10% of all haemodialysis patients in Spain. They performed a cost analysis to evaluate the economic burden of mineral regulating therapy in this patient group. It turned out that on average the cost of this specific therapy was 1,68 Euro per patient per day. Costs of mineral regulating therapy: 10 x 1,312 x 365 x 1.68 = 8,045,184 Euro 4 Lorenzo V, Martin-Malo A, Perez-Garcia R, et al. Nephrol Dial Transplant 2006;21:459-465

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