The use of the Spanish National Death Index (S-NDI) is increasing as it is free and easily accessible. Prompted by unexpected findings when linking two cohorts of patients with the S-NDI, this study was performed to evaluate the quality of mortality data from this source.
Individual records of two cohorts of persons admitted to drug abuse treatment in Barcelona (n = 19,974) and Madrid (n = 17,557) were linked with the S-NDI and then with the National Statistics Institute (NSI) to assign vital status. The sensitivity and specificity of the S-NDI in relation to the NSI and estimates of mortality with both systems were obtained. Underestimation of mortality by the S-NDI for the same period in the whole of Spain was also calculated.
For Madrid, from 1997-2008, independent searches in the S-NDI underestimated mortality by 18.9%compared with the NSI. A subsequent individualized search limited to deaths detected by the NSI but not by the S-NDI reduced underestimation to 13.9%. For Barcelona, underestimation with a combined search (independent plus limited) was 3.3%. From 2001-2002, underestimation with combined searches reached 32.3%in Madrid and 7.3%in Barcelona. Underestimation was also seen in the general Spanish population (1.8%in 1997-2008 and 9.3%in 2001-2002). From 1997-2008, the specificity of independent searches in the S-NDI was 100%, but its sensitivity in Madrid was only 81%.
The quality of the S-NDI is good enough for many uses in the health sector, but this index underestimates mortality with substantial spatiotemporal variations. The quality of the S-NDI could be increased by improving its search algorithms, enabling on-line tabulations of the main results, and introducing quality controls and routine evaluations.