Data linkage reduces loss to follow-up in an observational HIV cohort study.
To ascertain the degree of loss to follow-up in a cohort and to identify its predictors. Human immunodeficiency virus (HIV)-infected individuals without CD4 cell counts for a year or more were defined as potentially lost to follow-up (LFU). Multivariable Poisson regression models identified the risk factors for potential LFU. Multivariable logistic regression models compared demographic and clinical characteristics of those who returned for care and those permanently LFU. Of 16,595 patients under follow-up, 43.6% were potentially LFU at least once. Of these, 39.8% were considered permanently LFU and 60.2% were seen again after 1 year. Of 9,766 episodes when patients were potentially LFU, 59% resulted in the patient returning for care at the same clinic or at a different clinic. Compared with those permanently LFU, patients returning were more likely to have started highly active antiretroviral therapy, to have higher CD4 counts and viral loads, to be younger, and to have had more CD4 tests before LFU. They were less likely to have had a previous episode of potential LFU. A substantial proportion of patients in the UK Collaborative HIV Cohort study are potentially LFU. Data linkage identifies patients returning for care at different centers. Recognition of factors associated with LFU may help reduce this important source of bias in observational databases.