Death by mental retardation? The influence of ambiguity on death certificate coding error for adults with intellectual disability

Background: Although the coding of mental retardation as underlying cause of death has been recognised by previous researchers as erroneous, factors influencing this inaccuracy have not been sufficiently analysed. This study explores the effects of diagnostic ambiguity on risk of mental retardation being coded as underlying cause of death on US death certificates from 2004. Methods: Utilising all US death certificates from 2004 that included a cause of death code for mental retardation, logistic regression analysis provided estimates of the likelihood of having mental retardation erroneously coded as the underlying cause of death. Estimators used to measure ambiguity included the number of multiple causes of death, the place of death, and ICD-10 diagnostic Chapter codes. Results: A total of 2278 US death certificates from 2004 included a cause of death code for mental retardation. Of these death certificates, 20% erroneously coded mental retardation as the underlying cause of death. Reflecting the negative impact of diagnostic ambiguity on death certificate coding accuracy, mental retardation was more likely to be coded as underlying cause of death for decedents who: (1) had a death certificate that provided less information on co-morbid disease processes; (2) died in an outpatient or emergency room setting; or (3) had either abnormal symptomatology or death by injury, accident or other external cause. Conclusions: Findings from this study, as well as prior research, demonstrate that attempts to understand mortality trends for this population must attend to frequent underlying cause of death coding errors which threaten accuracy of cause of death data. Furthermore, inquiry is warranted into the impetus behind US death certificate coding policy that continues to allow the erroneous coding of 'death by mental retardation'.