title: [Quality deficits in the Spanish National Death Index: lessons learned from studying the mortality of two cohorts of people admitted to drug abuse treatment].
abstract: 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.
authors: Centro Nacional de Epidemiología, Instituto de Salud Carlos III, Madrid, España.
authors: Gemma
authors: G
authors: Molist
authors: Gregorio
authors: G
authors: Barrio
authors: Sara
authors: S
authors: Santos
authors: M Teresa
authors: MT
authors: Brugal
authors: Fernando
authors: F
authors: Rodríguez-Artalejo
authors: José
authors: J
authors: Pulido
authors: Anna
authors: A
authors: Guitart
authors: Luis
authors: L
authors: de la Fuente
mesh_terms: D003644:Death Certificates; D006801:Humans; D011786:Quality Control; D013030:Spain; D019966:Substance-Related Disorders / Q000401:mortality*