[How many patients are on oral anticoagulant therapy in Denmark? Methods to estimate the number]

The aim of this study was to estimate the extension of oral anticoagulant therapy (OAT) in Denmark between 1997 and 1999.

1) Data from persons who were registered at the Danish Medicines Agency with at least one prescription of a vitamin K-antagonist, and 2) persons registered in the Laboratory Information System, Aarhus County Hospital, Denmark, with three or more consecutive INR tests, with no more than four months between each test, and with at least one INR test > 1.9 were examined.

The prevalence of OAT increased from 663 patients per 100,000 inhabitants in 1997 to 784 patients in 1999 (method 1). The prevalence increased significantly from 1997 to 1998 and from 1998 to 1999 and had a mean increase of 73.1 patients per 100,000 inhabitants (95% CI: 61.3-84.8) and a mean increase of 52.8 patients per 100,000 inhabitants (95% CI: 44.3-61.2), respectively. The prevalence of OAT was highest within the Country of Copenhagen and lowest within the County of Ringkobing. The difference between these two counties in 1997 was 0.0024 (p < 0.0001; 95% CI: 0.0017-0.0031). Within the admission area of Aarhus County Hospital (method 2), the prevalence increased from 518 patients per 100,000 inhabitants in 1997 to 630 patients in 1999. There was no statistical difference in the prevalence identified by the two methods.

The study shows: 1) a significant increase in the prevalence throughout the study period, 2) a significant variation between the counties. A precise estimate of prevalence is identified by method 1. A more detailed survey is, however, provided by method 2. We recommend further investigations on the identified regional variation.