Do Death Certificates Underestimate the Burden of Rare Diseases? The Example of Systemic Lupus Erythematosus Mortality, Sweden, 2001-2013

Objectives: Mortality due to rare diseases, which are substantial sources of premature mortality, is underreported in mortality studies. The objective of this study was to determine the completeness of reporting systemic lupus erythematosus (SLE) as a cause of death. Methods: In 2017, we linked data on a Swedish population-based cohort (the Swedish Lupus Linkage, 2001-2013) comprising people with SLE (n = 8560) and their matched general population comparators (n = 37717) to data from the Cause of Death Register. We reviewed death records of deceased people from the cohort (n = 5110) and extracted data on patient demographic characteristics and causes of death. We estimated odds ratios (ORs) and 95% confidence intervals (CIs) for not reporting SLE as a cause of death by using multivariable-adjusted logistic regression models. Results: Of 1802 deaths among SLE patients in the study, 1071 (59%) did not have SLE reported on their death records. Most SLE decedents were aged 75-84 at death (n = 584, 32%), female (n = 1462, 81%), and born in Nordic countries (n = 1730, 96%). Decedents aged >= 85 at death were more likely to have SLE not reported on their death records than were decedents aged <50 (OR = 2.34; 95% CI, 1.48-3.68). Having renal failure listed as a cause of death decreased the likelihood of SLE not being reported on the death record (OR = 0.54; 95% CI, 0.40-0.73), whereas having cancer listed as a cause of death increased this likelihood (OR = 2.39; 95% CI, 1.85-3.07). Conclusions: SLE was greatly underreported as a cause of mortality on death records of SLE patients, particularly in older decedents and those with cancer, thereby underestimating the true burden of this disease. Public health resources need to focus on improving the recording of rare diseases in order to enhance the epidemiological utility of mortality data.