Ill-Defined Deaths in Slovenia: Implications for Mortality Data Quality and Burden of Disease Studies.

Introduction: Ill-defined deaths (IDDs) are deaths assigned non-specific or insufficiently informative causes that do not identify a distinct underlying disease or injury. They reduce the accuracy of mortality statistics. In Slovenia, the overall proportion, structure and distribution of IDDs have not yet been comprehensively evaluated. This study examined the most frequent IDDs and their demographic and regional patterns in Slovenia from 2010 to 2021. Methods: Individual-level mortality data from 2010 to 2021 were analysed. Deaths assigned ICD-10 codes denoting vague/non-specific conditions were classified as IDDs using the GBD 2019 cause list. Frequencies and distributions were examined overall and by sex, age group, and regions. Results: Of 243,166 deaths, 53,020 (21.8%) were IDDs. The proportion was higher in females and increased with age. Heart failure was the most frequent IDD and showed a distinctly higher share among females. Other common IDDs included unspecified stroke, unspecified lower respiratory infections, unspecified cancers, and R99-coded (Ill-defined and unknown cause of mortality) deaths. Clear variation in age groups was observed: poisoning-related IDDs were concentrated in young adults, R99 predominated in working age groups, and cardiovascular-related IDDs dominated in older adults. Regional distributions of the top categories of IDDs resembled the national one, except for unspecified cardiomyopathy, which appeared prominently in two north-eastern regions. Conclusions: Heart failure is the major contributor to IDDs in Slovenia, with demographic and regional variation. The findings highlight the need to strengthen death certification and improve the quality of mortality data to support burden-of-disease analyses.