We conducted a case-control study nested within a small cohort identified by using a hospital information system in Japan to examine the relationship of the use of calcium channel blockers (CCBs) to myocardial infarction. Although the crude odds ratio of myocardial infarction associated with CCBs was high, the ratio was reduced when adjusted by known confounding factors, suggesting a mechanism of confounding by indication. In addition, the results obtained in this study using records from a single hospital should not be generalized. Quality Indicators (QIs) are a tool for assessing healthcare quality, and QIs monitoring works to improve the quality of medical care. When QIs would be analyzed by using a hospital information, confounding factors should be removed if we measure QIs using the same method in the same hospital in chronological order. QIs analyzed by using a hospital information system are valuable and applicable to patients. It would be a driving force of the medical quality improvement that the number of the pharmaco-epidemiology experts would increase more and more. Because they can make the drug information conformed to Evidence-based Medicine using a medical database. Then, the medical quality improvement will decrease medical cost in Japan.

Objective : To know how to conduct good pharmacoepidemiology studies using hospital-based database in Japan.
Methods : Medical records during 15 months January 1996 and March 1997 in the University of Tokyo Hospital Information System (HIS) are examined know whether it is possible to conduct pharmacoepidemiology studies similar to previous studies on asthma drugs (Spitzer et al, 1992) and calcium antagonists (Psaty et al, 1995). To know the stability of population covered by HIS, the following two intervals are calculated for ambulatory patients with asthma and hypertension ; 1) average intervals of successive two outpatient visits and 2) intervals between the last day of outpatient visit and the last day of observation.
Results : The size of possible pharmacoepidemiology studies attainable using HIS is judged to be more than 5% of previous studies in Canada and America. Average intervals of successive two outpatient visits are estimated to be 30 days or less for 59% of 693 asthmatics and 77% of 2842 hypertensives. For 48% of asthmatics and 71% of hypertensives, intervals between the last day of outpatient visit and the last day of observation are estimated to be 30 days or less.
Discussion : To attain a size appropriate for pharmacoepidemiology study, researchers must cooperate across hospitals. Although a patient can visit any hospital anywhere under Japanese comprehensive medical care plan, it seems that patients tend to become to visit one particular hospital. However, additional information on medical care in other hospitals is needed for each study subject.
Conclusion : Japanese hospital-based database is suitable for pharmacoepidemiology studies as a record during a long time period is usually available for a large fraction of patients with a particular disease. The study may be free from some of biases closely associated with referral processes known to occur in hospital case-control studies. A design of case-control study selecting patients with long medical records across 5-10 hospitals is probably the most promising when using Japanese hospital-based databases.