Retention in care is essential to the health of people living with HIV and also for their communities. We sought to quantify the value of integrating HIV surveillance data with clinical records for improving the accuracy of retention-in-care estimates and the efficiency of efforts to re-engage out-of-care patients. Electronic medical records (EMRs) of HIV+ patients ≥18 years old from a public, hospital-based clinic in Minneapolis, MN between 2008 and 2014 were merged with state surveillance data on HIV-related laboratory tests, out-of-state relocation, and mortality. We calculated levels of retention and estimated the number of required case investigations to re-engage patients who appeared to be out of care over the study period with and without surveillance data integration. Retention was measured as the proportion of years in compliance with Health Resources and Services Administration (HRSA) guidelines (two clinical encounters >90 days apart annually) and the proportion of patients experiencing a gap in care >12 months. With data integration, retention estimates improved from an average HRSA compliance of 70.3% using EMR data alone to 77.5% with surveillance data, whereas the proportion of patients experiencing a >12-month gap in care decreased from 45.0% to 34.4%. If case investigations to re-engage patients were initiated after a 12-month gap in care, surveillance data would avoid 330 (29.3%) investigations over the study period. Surveillance data integration improves the accuracy of retention-in-care estimates and would avert a substantial number of unnecessary case investigations for patients who appear to be out of care but, in fact, receive care elsewhere or have died.