Déficit de calidad del índice Nacional de Defunciones: lecciones aprendidas al estudiar la mortalidad de dos cohortes de admitidos a tratamiento por drogas

objective: 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. methods: 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. results: 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%. conclusions: 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.