Mortality among prisoners: how accurate is the Australian National Death Index?

Objective: The effectiveness of the National Death Index (NDI) in identifying deaths among different populations has been studied before. Prisoners represent a segment of the population that might be expected to be difficult to link. The objective of this research was to evaluate the accuracy of the Australian NDI to ascertain the mortality status of prisoners in New South Wales, Australia. Methods: Data on 311 inmates who had died in custody between January 1988 and March 2003, and on 7,558 inmates known to be alive at the end of May 2003, were matched to the NDI database. Matching was undertaken using a probabilistic linkage method followed by a clerical review. The underlying causes of death reported by the NDI were compared against those reported by the NSW Coroner and held at the Legal Branch of the Department of Corrective Services. Results: Linkage correctly identified 275 of the 311 known deaths (sensitivity: 88.4%). Of the 7,558 prisoners known to be alive in custody at the end of May 2003, linkage to the NDI incorrectly identified 23 as dead (specificity: 99.7%). Cause of death was accurately reported by the NDI for 95.5% of deaths identified by the linkage. Conclusion: The results of this study show that the NDI is an accurate and feasible means for evaluating mortality among prisoners. The nature of the prisoner population makes the linkage a difficult task for the NDI. The success of the data linkage depends on the extent and accuracy of information contained in the index file and type and quality of information available.