(Stroke. 2004;35:2045.)
© 2004 American Heart Association, Inc.
Original Contributions |
Editorial Comment—Stroke Incidence Studies One Step Closer to the Elusive Gold Standard?
Clinical Trials Research Unit, Department of Medicine & School of Population Health, University of Auckland, New Zealand
Stroke should be studied in a population-wide context because a large proportion of the burden of care for stroke is borne by health services outside the hospital sector and by families of affected patients.1,2 Assessing the need for stroke-related prevention strategies and health services, and geographical and secular trends in stroke burden, is best achieved with standardized population-based registers. Analyses limited to hospital cases, incomplete mortality data, or cases with varying criteria and definitions may distort results because of nonstandardized measures and nonrepresentative study populations. However, identifying all new stroke events in a population is particularly challenging, so that such epidemiological studies are relatively rare compared with studies using mortality data, hospital-based stroke registers, or incidence studies in younger age groups only.2,3 Moreover, even among published population-based stroke incidence studies, there are differences in the methodologies used to ensure completeness of case ascertainment.
Until recently, assessing completeness of case ascertainment in stroke incidence studies has been performed directly (eg, repeated cross-sectional surveys of the study population) and/or indirectly (eg, quality-control procedures, statistical modeling).3 Although repeated surveys are very expensive, other indirect methods may carry a considerable potential for error,3 and there is still no standard method of assessment of the completeness of stroke case ascertainment. Although capture–recapture method of case ascertainment is cost-efficient, there is much debate regarding its usefulness given the necessary assumptions that the population is closed; the sources of notification (or lists) are independent; the probability of being on a list should be the same. More extensive techniques for capture–recapture have been proposed to overcome some of the deviations from the assumptions, such as adding covariates to the model.4 However, these have not been validated in epidemiological studies.
In this issue of Stroke, Coull et al report 2 direct methods of stroke case ascertainment used in the Oxford Vascular Study (OXVASC).5 This article measures the marginal benefits of some new types of direct assessment of accuracy of case ascertainment in a population-based stroke incidence study. In addition to the core methods of case ascertainment used in the first Oxfordshire Community Stroke Project, conducted 20 years ago,6 the authors used supplementary methods to ascertain all patients with transient ischemic attack (TIA), recurrent strokes, and those referred for neuroimaging studies. The use of these supplementary methods yielded an additional 15% of incident strokes detected over and above the core ascertainment methods only. Direct assessment of the completeness of case ascertainment by regular searching of databases of general practitioners and reviewing of all cardiovascular patients admitted to hospital highlighted only 2 cases that had been missed by both the core and supplementary methods. Direct methods to assess the completeness of case ascertainment can be used when a well-established electronic patient record system is in place. However, it has been shown in this study that interviewing a population at high risk for stroke does not identify any more cases. Coull et al acknowledge that the generalizability of these findings and effectiveness of particular methods of case ascertainment are likely to be limited by community-specific differences in health care systems. Unfortunately, the authors did not conduct capture–recapture analyses to compare these results with indirect assessments of completeness of ascertainment. Such comparisons would yield important information on how best to use indirect methods in the many instances when only incomplete direct data are available.
The study by Coull et al5 justifies an expansion of the current criteria for "ideal" stroke incidence studies by including supplementary methods of case ascertainment and direct assessment of underascertainment. In 1987, Malmgren et al7 published a list of 12 core criteria for "ideal" stroke incidence studies that were related to definitions, methods, and mode of data presentation, by which the quality of population-based studies of stroke could be judged. These criteria have been updated by Bonita (1995),8 Sudlow and Warlow (1996),3 and Feigin et al (2003, 2004).2,9 Taken together, we propose updated criteria (gold standards) for "ideal" population-based stroke incidence study (Table). We believe that separating these criteria on core and supplementary allows some methodological flexibility in studying stroke incidence based on local resources and health care system while ensuring internal and external validity of the study.
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In summary, "ideal" stroke incidence studies based on both core and supplementary criteria are the most valuable source of information for developing evidence-based strategies for stroke prevention and health services and, therefore, should be used whenever possible. Further advancements in computerized medical record-linkage systems are likely to facilitate such studies. However, these "ideal" criteria are not practical for stroke incidence studies undertaken in most settings, particularly in developing countries, where most strokes occur but resources are limited. To address the problem of accurate and comparable stroke incidence studies in less affluent countries and those with scarce research funding, a WHO stepwise stroke surveillance approach10 can be recommended. The question of validity of capture–recapture methods of case ascertainment in stroke incidence studies remains to be answered. However, refinement of both direct and indirect methods will take us closer to the elusive gold standard.
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1. Bonita R, Beaglehole R. Monitoring stroke. An international challenge [editorial]. Stroke. 1995; 26: 541–542.
2. Feigin VL, Lawes CMM, Bennett DA, Anderson CS. Stroke epidemiology: a review of population-based studies of incidence, prevalence, and case-fatality in the late 20th century. Lancet Neurol. 2003; 2: 43–53.[CrossRef][Medline] [Order article via Infotrieve]
3. Sudlow CLM, Warlow CP. Comparing stroke incidence worldwide: what makes studies comparable? Stroke. 1996; 27: 550–558.
4. Tilling K, Sterne JA, Wolfe CD. Estimation of the incidence of stroke using a capture-recapture model including covariates. Int J Epidemiol. 2001; 30: 1351–1359.
5. Coull AJ, Silver LE, Bull LM, Giles MF, Rothwell PM. Direct assessment of completeness of ascertainment in a stroke incidence study. Stroke. 2004; 35: 2041–2047.
6. Bamford J, Sandercock P, Dennis M, Burn J, Warlow C. A prospective study of acute cerebrovascular disease in the community: the Oxfordshire Community Stroke Project, 1981–86. 2. Incidence, case fatality rates and overall outcome at one year of cerebral infarction, primary intracerebral and subarachnoid haemorrhage. J Neurol Neurosurg Psychiatry. 1990; 53: 16–22.
7. Malmgren R, Warlow C, Bamford J, Sandercock P. Geographical and secular trends in stroke incidence. Lancet. 1987; 2: 1196–1200.[Medline] [Order article via Infotrieve]
8. Bonita R, Broad JB, Anderson NE, Beaglehole R. Approaches to the problems of measuring the incidence of stroke: the Auckland Stroke Study, 1991–1992. Int J Epidemiol. 1995; 24: 535–542.
9. Feigin V, Vander HS. How to study stroke incidence. Lancet. 2004; 363: 1920–1921.[CrossRef][Medline] [Order article via Infotrieve]
10. Truelsen T, Bonita R, Jamrozik K. Surveillance of stroke: a global perspective. Int J Epidemiol. 2001; 30: S11–S16.
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