[Formation of a cohort of opiate addicts through linkage of confidential records]

The need to combine several files in order to create a cohort of opiate addicts, sufficiently large for the analysis of its mortality, requires use of the probabilistic method of record linkage. This study is a preliminary phase in which two sub-files of the Hospital del Mar (Barcelona) Register of toxicological emergencies are linked. This phase served to adapt probabilistic record linkage to our files, develop computer programs, define agreement criteria, and evaluate the validity and performance of the method. In order to safeguard confidentiality, identification variables were limited to sex, birth date and three initial letters from each surname. The automated probabilistic linkage was seen to be feasible, valid and efficient; in contrast to deterministic approaches, sensitivities and specificities above 95% were obtained with visual reviewing of under 5% of the records. Difficulties encountered during the process are discussed.