The conversion between ICPC and ICD-10. Requirements for a family of classification systems in the next decade.

The International Classification of Primary Care (ICPC) was developed to order medical concepts into classes that have been chosen for their relevance for family medicine. Family physicians use this to label the most prevalent conditions in their practice as well as their patients' symptoms and complaints. At the same time they do not want to be divorced from the needs of the medical community at large as these are reflected in the most recent medical nomenclature: the Tenth Revision of the International Classification of Diseases (ICD-10). A full conversion between all classes in the first and seventh component of ICPC (n = 646) with those of ICD-10 (n = 1983), with the exception of the chapter on external causes, has been prepared. It was concluded that ICD-10 at the three-digit level cannot function as a core classification for an international primary care system. Of the three-digit ICD-10 rubrics only 120 are compatible on a one to one basis with an ICPC rubric. A total of 114 three-digit ICD-10 rubrics have to be broken open into four-digit rubrics to allow at least one compatible conversion to one or more ICPC rubrics. On this basis only 25% of the diagnostic classes in ICPC can be converted to a single three- or four-digit ICD-10 rubric without lumping. The rest of ICD-10, either on the three- or on the four-digit level, has to be grouped into combinations of classes (lumping) to allow compatible conversion to the remaining rubrics of ICPC. Even though ICD-10 cannot serve as a core classification for primary care, a technical conversion between ICPC and ICD-10 is practically always possible which allows primary care physicians to implement ICD-10 as a contemporary nomenclature within the classification structure of ICPC.