Coding ill-defined and unknown cause of death is 13 times more frequent in Denmark than in Finland.

Exact cause and manner of death determination improves legislative safety for the individual and for society and guides aspects of national public health. In the International Classification of Diseases, codes R00-R99 are used for "symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified" designated as "ill-defined" or "with unknown etiology". The World Health Organisation recommends avoiding the use of ill-defined and unknown causes of death in the death certificate as this terminology does not give any information concerning the possible conditions that led to the death. Thus, the aim of the study was, firstly, to analyse the frequencies of R00-R99-coded deaths in mortality statistics in Finland and in Denmark and, secondly, to compare these and the methods used to investigate the cause of death. To do so, we extracted a random 90% sample of the Finnish death certificates and 100% of the Danish certificates from the national mortality registries for 2000, 2005 and 2010. Subsequently, we analysed the frequencies of forensic and medical autopsies and external clinical examinations of the bodies in R00-R99-coded deaths. The use of R00-R99 codes was significantly higher in Denmark than in Finland; OR 18.6 (95% CI 15.3-22.4; p<0.001) for 2000, OR 9.5 (95% CI 8.0-11.3; p<0.001) for 2005 and OR 13.2 (95% CI 11.1-15.7; p<0.001) for 2010. More than 80% of Danish deaths with R00-R99 codes were over 70 years of age at the time of death. Forensic autopsy was performed in 88.3% of Finnish R00-R99-coded deaths, whereas only 3.5% of Danish R00-R99-coded deaths were investigated with forensic or medical autopsy. The codes that were most used in both countries were R96-R99, meaning "unknown cause of death". In Finland, all of these deaths were investigated with a forensic autopsy. Our study suggests that if all deaths in all age groups with unclear cause of death were systematically investigated with a forensic autopsy, only 2-3/1000 deaths per year would be coded as an ill-defined and unknown cause of death in national mortality statistics. At the same time the risk to overlook unnatural deaths is decreased to a minimum. To achieve this in Denmark requires that the existing legislation on cause of death investigation would need to be changed to ensure that all deaths with unknown cause of death are investigated with a forensic autopsy.