Base cohort and data sources
A base cohort was established by linking different Danish health registries. The base cohort comprised all patients registered with at least one INR measurement in the Copenhagen Primary Care Laboratory (CopLab) database. The database includes laboratory test results from primary health care patients in the Copenhagen area of Denmark from 2000 to 2015.16,17 During this period the primary health care doctors in the Copenhagen area were served by the Elective Laboratory of the Capital Region (ELCR). ELCR covered approximately 1.2 million inhabitants and provided a wide range of biochemical, physiological, and cardiac tests. The CopLab database does not include INR point of care testing (POCT) results from general practice. The ELCR was accredited for International Organization for Standardization (ISO) standards ISO17025 and ISO15189. Appendix A provides a detailed description of the INR assay.
For the cohort identified via CopLab, we retrieved data about drug use from the Danish National Prescription Registry18 as well as hospital diagnoses from the Danish National Patient Registry (NPR).19 Data linkage was done using the unique Danish Civil Registration Number assigned to all Danish residents.20