Background Revisions of the International Classification of Diseases (ICD) have previously been shown to cause dramatic effects with regard to injury mortality data when implemented. However, limited knowledge exists on the effects on the coding of external causes of injury morbidity, despite this being an important aspect with regard to injury prevention.

Method Hospitalised injuries in Sweden were studied using time series intervention analysis to observe the effect of the ICD change from ICD-9 to ICD-10 in 1997 on external cause coding.

Results The results would suggest considerable coding issues with a large spike in the proportion of injury admissions registered without an external cause code in 1997, with continuing, although gradually diminishing, problems up to 2002. The coding change seems to have had an immediate effect on all external cause of injury categories, although the categories that were not directly convertible from ICD-9 to ICD-10 were seemingly more greatly affected.

Discussion The study illustrates the potential issues associated with changes between ICD revisions and the importance of data quality control both during surveillance and collection of data, but also when presenting injury trends across ICD versions.