Interobserver Agreement in the Classification of Stroke in the Physicians’ Health Study

Abstract

The evaluation of cerebrovascular end points in prospective studies is often based exclusively on medical record examination and may be made by more than one observer over time. To address the issues of adequacy of medical record information and consistency in diagnosis over time, we evaluated interobserver agreement for the main items of the stroke classification system used in the Physicians' Health Study. This trial included 22,071 physicians randomly assigned in 1982 to receive either aspirin or placebo to assess the subsequent risk of cardiovascular events, including stroke. Stroke subtype, stroke severity, and certainty of diagnosis were first classified from medical records from the years 1982 through 1988. The 216 stroke events reported in this period were independently reclassified in 1994 and compared with the initial classification using kappa statistics. Overall agreement in major stroke types (hemorrhagic, ischemic, undetermined stroke) as well as in hemorrhagic stroke subtypes was excellent (kappa = 0.81 and kappa = 0.95, respectively). A wide range of values for the ischemic stroke subtypes (kappa = 0.13 to kappa = 0.96) was obtained. Agreement was substantial in assessment of stroke severity (kappa = 0.71), and it was fair (kappa = 0.33) for certainty of diagnosis. Interobserver agreement is high for major stroke types as well as for categories of hemorrhagic stroke on the basis of review of medical records and results of imaging data. The classification of ischemic stroke subtypes, however, is subject to substantial interobserver disagreement. Periodic reclassification of random samples of end points might be considered in long-term prospective studies to assess potential misclassification of events by different observers.