Comparing ICD-9 and ICD-10: the impact on intentional and unintentional injury mortality statistics in Italy and Norway.

Background: The international classification of diseases (ICD) provides guidelines for the collection, classification and dissemination of official cause-of-death statistics. New revisions of the ICD can potentially disrupt time trends of cause-of-death statistics and affect between-country comparisons. The aim of this study was to measure how switching from ICD-9 to ICD-10 affected mortality statistics for external causes of death, i.e. intentional and unintentional injuries, in Italy and Norway.

Methods: A sample of death certificates (N=454,897) were selected in Italy from the first year the ICD-10 was implemented (2003) and reclassified from ICD-10 to ICD-9 by the Italian National Institute of Statistics. A sample of death certificates was also selected in Norway (N=10,706) from the last year the ICD-9 was used (1995) and reclassified according to ICD-10 by Statistics Norway. The reclassification (double-coding) was performed by special trained personal in governmental offices responsible for official mortality statistics. Although the reclassification covered all causes of death (diseases and injuries) in the sample, our analysis focused on just one ICD chapter XX. This was external causes of mortality (injury deaths), and covered 15 selected categories of injuries.

Results: The switch from ICD-9 to ICD-10 had a significant net impact on 8 of the 15 selected categories. In Italy, accidental falls decreased by 76%; traffic accidents decreased by 9%; suicide by hanging decreased by 3%; events of undetermined intent decreased by 69%; and overall injury deaths decreased by 4%. These net decreases reflect the moving of death records from injury categories in ICD-9 to other injury or disease categories in ICD-10. In Norway, the number of records in three categories decreased significantly: transport accidents, 9%; traffic accidents, 13%; and suicide by self-poisoning, 18%. No statistically significant differences (net changes) were observed in the total number of accidents, suicides and homicides in either country.

Conclusions: Switching to ICD-10 did not change the overall trends for accidents, homicides and suicides in either country. However, the number of records in some injury subcategories e.g. accidental falls and traffic accidents, decreased. Changing classification can thus affect the ranking of causes of injury mortality, with consequences for public health policy.