Supplementary Material for: Unveiling the burden of miscoding and misclassification in stroke mortality: analysis of multiple cause-of-death data in Mexico

Background. The prevalence of risk factors for cerebrovascular diseases in Mexico is increasing although stroke mortality declined from 1990 to 2010, without meaningful changes afterwards. While improving access to adequate prevention and care could explain this trend, miscoding and misclassification in death certificates need to be assessed to unveil the true burden of stroke in Mexico. Practices in death certification along with the presence of multi-morbidity could contribute to this distortion. Analyses of multiple causes of death could reveal ill-defined stroke deaths, providing a glimpse of this bias. Methods. Cause of death information from 4,262,666 deaths certificates in Mexico from 2009 to 2015, was examined to determine the extent of miscoding and misclassification on the true burden of stroke. Age-standardized mortality rates per 100000 inhabitants (ASMR) were calculated for stroke as underlying and multiple causes of death, by sex and state. Deaths were classified following international standards as ischemic stroke, intracerebral haemorrhage, subarachnoid haemorrhage and unspecified, which were kept as an independent category to measure miscoding. To approximate misclassification, we compared ASMR under three misclassification scenarios: 1) Current (the status-quo); 2) moderate, which includes deaths from selected causes mentioning stroke; and 3) high which includes all deaths mentioning stroke. National and subnational data was analysed to search for geographical patterns. Results. The burden of stroke in Mexico is underreported due to miscoding and misclassification. Miscoding is an important issue, since almost 60% of all stroke deaths are registered as unspecified. Multiple cause analysis indicates that stroke ASMR could increase 39.9% to 52.9% of the current ASMR under moderate and high misclassification scenarios, respectively. Both problems indicate the need to improve death codification procedures and cause of death classification. Conclusions. Miscoding and misclassification lead to underestimation on the burden of stroke in Mexico. Stroke deaths are underreported when other important causes coexist, being diabetes the most frequent.