Development and validation of an epidemiologic case definition of epilepsy for use with routinely collected Australian health data
Introduction
Data linkage is an emerging powerful tool, particularly for ascertaining low incidence events, enabling medical diseases and health outcomes to be connected using routinely collected centralized databases. For the study of epilepsy, it has the advantage of efficiently identifying large samples of patients with epilepsy [1], [2], [3]; however, misclassification of cases with epilepsy in administrative databases is a major issue, limiting its utility as an epidemiologic instrument for disease surveillance and research.
International Classification of Diseases (ICD)-coded data are utilized internationally as an epidemiological tool for collection of national health statistics [4]; however, their use requires validation. Previous studies that utilize this classification system have used ICD-9 and ICD-10 versions [5], [6], [7], [8], [9], [10]. As ICD-10 revisions are country-specific and coding practices may vary between regions, it has been recommended that specificity and predictive values be evaluated for each population studied [11]. Current epidemiological guidelines suggest that a probable diagnosis of epilepsy can be made if 1 of the following 3 conditions is met: one medical encounter with a 3-digit code of G40.x (epilepsy); ≥ 2 medical encounters on separate days coded with G41 (status epilepticus) or with a 4-digit code R56.8 (other and unspecified convulsions); and a single medical encounter coded as other and unspecified convulsions (R56.8) and an antiepileptic drug prescription for three or more months [11]. A suspected diagnosis of epilepsy can be made with single episodes coded with R56.8 or G41 [11].
The coding of Australian Modification (AM) version of ICD-10 (ICD-10AM) clinical data in Australia exists for diagnoses and procedures of acute admitted patient episodes only [12]. At present, epilepsy coding for the ICD-10AM has not been validated. We sought to estimate the diagnostic accuracy of different ICD-10AM coding algorithms to identify patients with epilepsy in an Australian hospital setting and to develop an algorithm combining other routinely nationally collected data to maximize its precision for potential epidemiologic surveillance and research.
Section snippets
Methodology
Following a discharge from a public or a private hospital in Australia, all principal diagnoses and additional diagnoses (up to 99) in medical records are coded with ICD-10AM codes and submitted to the National Hospital Morbidity Database for statistical reporting [13], [14]. We retrospectively recruited a validation cohort by identifying cases with epilepsy and their common differential diagnoses in a metropolitan adult hospital setting with a large neurology unit including epilepsy
Results
During the study period, 42,760 patients were seen at our hospital in the ED and inpatient units. The records of 600 case episodes were reviewed, with 158 confirmed to have epilepsy (72/300 in the ED sample, and 86/300 in the inpatient sample, Χ2 (1) = 1.46, p < 0.23). This represents 52.3% of cases coded with epilepsy (G40), status epilepticus (G41), or ‘other and unspecified convulsions’ (R56.8) having confirmed epilepsy. There were 20 first unprovoked seizures and 5 incident epilepsy cases
Summary of findings
The linking of multiple sources of data has been recommended by the ILAE Commission on Epidemiology to improve epilepsy case ascertainment from administrative datasets [11]. This study, the first to be conducted in Australia, found that the identification of epilepsy in an adult hospital setting can be reasonably accurate and comparable with other countries of similar economic development [6], [7], [27] when based on a combination of data elements: ICD-10AM coding for epilepsy (G40), status
Conclusion
Our results demonstrated that the combination of administrative and prescription databases is necessary when designing privacy preserving data linkage studies in an Australian population. While this method provides considerable potential for future epilepsy epidemiological studies, limitations in the collection of Australian administrative data to hospital inpatient populations restrict its use. Incidence and prevalence studies are likely to be affected by selection bias given the lack of
Acknowledgments
We also acknowledge Ms. Esther Taylor from the data integrity and application support department at St Vincent's Hospital, Melbourne for her assistance in data acquisition.
Disclosure
Assoc. Prof. D'Souza has received travel, investigator-initiated, and speaker honoraria and has served on scientific advisory boards from UCB Pharma; has received educational grants from Novartis Pharmaceuticals, Pfizer Pharmaceuticals, and Sanofi-Synthelabo; has received educational, travel, and fellowship grants
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