The PHARMO medical record linkage scheme started in 1985 and includes the drug-dispensing records from community pharmacies and hospital discharge records of all 650 000 community-dwelling inhabitants of nine medium-sized cities in defined demogeographic areas in The Netherlands (Herings, 1993). The underlying population has been defined by using information for each city from the Bureau of Statistics (CBS). Patients that are registered within the pharmacy files are regarded as non-residents and are eliminated from the patient registers if they are not recorded as having a family practitioner residential in one of the cities. Patients are assumed to be present in the source population between the first and last encounter in the pharmacy. For all residents, the drug-dispensing histories are linked on a yearly basis to the national hospital discharge records of the same patient, using a probabilistic algorithm, based on characteristics such as date of birth, gender and a code for the general practitioner since no unique patient identifier is present. Validation showed that these registries are linked with a sensitivity and specificity exceeding 95%, which is comparable to record linkage systems based on unique personal identifiers (Herings et al., 1990, 1992; Herings, 1993). The computerised drug-dispensing histories contain outpatient prescription data concerning the dispensed drug, type of prescriber, dispensing date, dispensed amount, prescribed dose regimens, and the legend duration (prescription length). The hospital records include detailed information concerning the primary and secondary diagnoses, procedures, and dates of hospital admission and discharge. Al diagnoses are coded according to the ICD-9-CM. To date, the PHARMO system has provided data for several doctorate theses and pharmacoepidemio-logical publications in major medical journals (Herings et al., 1993, 1995a, 1995b, 1995c, 1996, 1999, 2000; van der Klauw et al., 1993, 1999; Herings and Stricker, 1995; Heerdink et al., 1998;
Gerrits et al, 1999; Bouvy et al, 2000; Burger et al, 2000). The PHARMO data are available for research purposes, after approval of the protocol by the Scientific Board. Research projects can be conducted only through the PHARMO Institute. Inspection of discharge letters is possible after approval by the Administrative Boards of the individual hospitals.
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