International Classification of Diseases, Ninth Revision (ICD-9) is a legacy diagnostic coding system that remains relevant for historical datasets and long-term longitudinal analyses.
Clinical research programs may encounter ICD-9 in older claims data, archived registries, or studies spanning coding transitions. Accurate mapping to newer code sets is important to preserve consistency and avoid misclassification bias. Clear documentation of coding versions supports defensible analysis and transparent reporting. Proper handling of ICD-9 data strengthens comparability across time periods and data sources.