To better understand the development of primary care classifications over the past 15 years, 10 primary care databases have been retrospectively analysed using the structure of the International Classification of Primary Care (ICPC) as the basis. All datasets were based on routine data collection using different classification systems by several family physicians during all encounters with their patients over considerable periods of time, in most cases one year. The prevalences or the rates of the available diagnostic--and reason for encounter--classes were distributed over four frequencies. With a few exceptions the distribution of diagnostic labels referring to common diseases is surprisingly similar. The use of ICPC however results in a quantum leap in the use of symptom and complaint diagnoses. Because of this shift primary care physicians now have available a classification with 400 diagnostic classes used with a prevalence of > or = 1/1000 patient-years or per 1000 visiting patients per year. The classification of reasons for encounter allows the physician to identify over 300 reasons for encounter used > or = 1/1000 patient years or per 1000 visiting patients per year. Family physicians have been successful in the development of new primary care classifications. Rag bag rubrics which are the result of the structure of ICPC are used relatively often and deserve more attention from primary care taxonomers.

The international Classification of Primary Care (ICPC) has now been available to the family medicine community for a decade as the main ordering principle of its domain. Research data and practical experiences with ICPC, as well as the development of new concepts in family medicine, have resulted in new applications. The structure of episodes of care to be included in a computer-based patient record has been further developed and refined. ICPC as the ordering principle of patient data is now available in 19 languages. Its conversion structure with the International Classification of Diseases (ICD-10) allows the highest possible level of specificity in a patient's problem list necessary in patient care, while the compatibility of the ICPC drug codes with the Anatomic Therapeutic Chemical Classification Index allows the systematic inclusion of data on prescription.

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.