Index date and number of patients in the National Institute for Health Research (NIHR) Health Informatics Collaborative (HIC) hepatitis B virus dataset. (A) The time line and index date definition for an exemplar patient. (B) Cumulative numbers of chronic hepatitis B virus patients entering the cohort over time. In panel A, all data collected after the index date are added into the dataset, and those data before index date are also included if they are available in EPR systems. The second positive test could be within 365 days after the index date (within baseline period) or could be >365 days after the index date (outside of the baseline period). Note that the second positive test is not required for a contributing site to include patients for data submission after June 2021. In panel B, for the six sites which had submitted data, the earliest year of the included data at each site is marked. CUH, Cambridge University Hospitals NHS Foundation Trust; ICHT, Imperial College Healthcare NHS Trust; LUH, Liverpool University Hospitals NHS Foundation Trust; OUH, Oxford University Hospitals NHS Foundation Trust; UCLH, University College London Hospitals NHS Foundation Trust; UHS, University Hospital Southampton NHS Foundation Trust; EPR, electronic patient record; HBsAg, hepatitis B surface antigen; HBV, hepatitis B virus; DNA, deoxyribonucleic acid; NHS, National Health Service