Cheng et al. BMC Public Health 2011, 11:380 http://www.biomedcentral.com/1471-2458/11/380 between geography and drug use could be useful for planning drug prevention programs. Cardiovascular disease (CVD) remains the leading cause of death and disability, representing a major bur- den for health systems worldwide [15,16], and impress- ive literature has documented geographic variation in the use of medication and procedures for treating car- diovascular disease in Western countries [17-21]. The cardiovascular medication-use patterns observed reflect the influence of a complex combination of demo- graphic, social, economic, cultural, and environmental factors. Quality improvement programs based on geo- graphic data may help to reduce the variation in qual- ity of care [22]. A few previous studies have specifically examined the cardiovascular medication utilization in Taiwan [23,24]; however, seldom is the spatial pattern of drug use addressed. Therefore, the objective of this study was to investigate prescribing patterns of cardiovascular medi- cations based on the variations of geo-space in Taiwan. We hypothesized there would be significant geographic differences in prescribing cardiovascular medications among townships. Methods Data source We conducted a cross-sectional study by using the non- sampled National Health Insurance Research Database (NHIRD) in Taiwan. This database is population-based and derived from the claims data of the National Health Insurance program, a mandatory-enrollment and single- payer system implemented in 1995 [25]. Patients who had at least one outpatient prescription for CVD during the year 2004 were enrolled in this study. The dataset contained 22.13 million enrollees (about 97.5% of the total population in Taiwan) in 2004. The data elements available for each patient in NHIRD contains minimal demographic characteristics, medical diagnoses (up to 5 for each admission, up to 3 for each ambulatory visit), procedures, expenditures, and all detailed prescriptions [26]. Cardiovascular medications The prescriptions were coded according to the Anato- mical Therapeutic Chemical coding system of the NHI Pharmaceutical subsidy, which was used as the interface for retrieving the pharmaceutical claims data. Five drug classes (and ATC numbers) were recorded, including agents acting on the rennin-angiotensin system (C09), called Group A hereafter; beta blocking agents (C07), called Group B hereafter; calcium channel blockers (C08), called Group C hereafter; diuretics (C03), called Group D hereafter; and antihypertensives (C02), called Group O hereafter. We presented the proportion of use Page 2 of 9 of these cardiovascular drug classes among all patients based on geographic location. We used the “defined daily dose” (DDD) as a measure unit for drug consump- tion, which was developed by the World Health Organi- zation (WHO) Collaborating Center for Drug Statistics Methodology (WHO, 2004). The DDD of a pharmaceu- tical substance represents the assumed average mainte- nance dose per day for a drug, when used for its main indication in adults. For example, the recommended maintenance dose of captopril, an angiotensin convert- ing enzyme inhibitor, for hypertension is 50 mg to 75 mg per day, the DDD of this drug was define as 50 mg from WHO. The consumption of drugs was expressed as DDDs/1000 inhabitants per day, which may serve as an estimation of the proportion of the population receiving the drug treatment. The information on resi- dents in each township in Taiwan was based on annual statistics of the Ministry of Interior [27]. Geographic distribution Taiwan became one of East Asia’s economic “Tigers” in the twentieth century. Its population was more than 22 million in the year 2000, with a combined area of approximately 36,000 square kilometers. The study area focuses on the main island of Taiwan; the offshore islands are not considered. In total, there are 352 town- ships included in this study, which can be further classi- fied into 5 metropolitan areas, 2 secondary metropolitan areas, 206 rural townships, and 55 aboriginal townships (Figure 1)[28]. The metropolitan classification was revised in 1993 by the Directorate General of Budget, Accounting, and Statistics of Taiwan. According to this classification, there are 5 major metropolitan areas (Tai- pei-Keelung, Taoyuan-Chungli, Taichung-Changhwa, Tainan, and Kaohsiung) and 2 secondary metropolitan areas (Hsinchu and Chiayi), which consist of 66.87% of the population. Each metropolitan area usually has at least one core city with satellite cities of similar socio- economic attributes. A metropolitan area has at least 0.3 million people, with population above 1 million consid- ered a major metropolitan area and below 1 million a secondary metropolitan area. The Council of Indigenous Peoples of Taiwan defines 55 aboriginal townships, including 30 indigenous mountain and 25 plain town- ships. Although Taiwan has experienced rapid economic growth, there has been a south-north (using Taichung- Changhwa as the division) inequity. That is, the region north of the Taichung-Changhwa area has a more highly developed service economy, technology complex, and financial sector than the southern region. Statistical Analysis We analyzed consumption of all classes of cardiovascu- lar medications by DDD/1000 inhabitants per day across