Questions on validity of International Classification of Diseases-coded diagnoses

International Classification of Diseases (ICD) codes are used for indexing medical diagnoses for various purposes and in various contexts. According to the literature and our personal experience, the validity of the coded information is unsatisfactory in general, however the 'correctness' is purpose and environment dependent. For detecting potential error sources, this paper gives a general framework of the coding process. The key elements of this framework are: (1) the formulation of the established diagnoses in medical language; (2) the induction from diagnoses to diseases; (3) indexing the diseases to ICD categories; (4) labelling of the coded entries (e.g. principal disease, complications, etc.). Each step is a potential source of errors. The most typical types of error are: (1) overlooking of diagnoses; (2) incorrect or skipped induction; (3) indexing errors; (4) violation of ICD rules and external regulations. The main reasons of the errors are the physician's errors in the primary documentation, the insufficient knowledge of the encoders (different steps of the coding process require different kind of knowledge), the internal inconsistency of the ICD, and some psychological factors. Computer systems can facilitate the coding process, but attention has to be paid to the entire coding process, not only to the indexing phase.