A unidirectional mapping of ICD-8 to ICD-10 codes, for harmonized longitudinal analysis of diseases

• DOI: 10.1007/s10654-023-01027-y
• Publication Date: 2023-08-09T15:06:25Z
• Authors: Mette Krogh Pedersen; Robert Eriksson; Roc Reguant; Catherine Collin; Helle Krogh Pedersen; Freja Karuna Hemm...; Christian Simon; Anna Marie Birch; Michael Larsen; Anna Pors Nielsen; Kirstine Belling; Søren Brunak
• Keywords: Disease codes; ICD-10; Mapping; International classification of diseases; Denmark; Diagnosis; Methods; Data harmonization; Conversion table; ICD-8; Crosswalk

AbstractPeriodic revisions of the international classification of diseases (ICD) ensure that the classification reflects new practices and knowledge; however, this complicates retrospective research as diagnoses are coded in different versions. For longitudinal disease trajectory studies, a crosswalk is an essential tool and a comprehensive mapping between ICD-8 and ICD-10 has until now been lacking. In this study, we map all ICD-8 morbidity codes to ICD-10 in the expanded Danish ICD version. We mapped ICD-8 codes to ICD-10, using a many-to-one system inspired by general equivalence mappings such that each ICD-8 code maps to a single ICD-10 code. Each ICD-8 code was manually and unidirectionally mapped to a single ICD-10 code based on medical setting and context. Each match was assigned a score (1 of 4 levels) reflecting the quality of the match and, if applicable, a “flag” signalling choices made in the mapping. We provide the first complete mapping of the 8596 ICD-8 morbidity codes to ICD-10 codes. All Danish ICD-8 codes representing diseases were mapped and 5106 (59.4%) achieved the highest consistency score. Only 334 (3.9%) of the ICD-8 codes received the lowest mapping consistency score. The mapping provides a scaffold for translation of ICD-8 to ICD-10, which enable longitudinal disease studies back to and 1969 in Denmark and to 1965 internationally with further adaption.