[Uncovering rare diseases in medical data-coding]

The ICD-10-GM coding system used in the German healthcare system only captures a minority of rare disease diagnoses. Therefore, information on the incidence and prevalence of rare diseases as well as necessary (financial) resources for the expert care required for evidence-based decisions by health insurers, care providers, and politicians are lacking. Furthermore, the missing information complicates and sometimes even precludes the generation of scientific knowledge on rare diseases. Therefore, starting in 2023, all in-patient cases in Germany with a rare disease diagnosis must be coded by an ORPHAcode using the Alpha-ID-SE file.The file Alpha-ID-SE links the ICD-10-GM codes to the internationally established ORPHAcodes for rare diseases. Commercially available software tools progressively support the coding of rare diseases. In several centers for rare diseases linked to university hospitals, IT tools and procedures were established to realize a complete coding of rare diseases. These include financial incentives for the institutions providing rare disease codes, systematic queries asking for rare disease codes during the coding process, and a semi-automated coding process for all patients with a rare disease previously seen at the institution. A combination of the different approaches probably results in the most complete coding.To get the complete picture of rare disease epidemiology and care requirements, a specific and unique coding of out-patient cases is also desirable. Furthermore, a structured reporting of phenotype is required, especially for complex rare diseases and for yet undiagnosed cases.