OBJECTIVES: Attendance at biannual medical visits has been proposed as a minimum U.S. standard for adequate engagement in human immunodeficiency virus (HIV) care. The U.S. National HIV Surveillance System collects dates of HIV laboratory tests but not medical visits, the current metric for determining if a patient is engaged in medical care. Using data from the HIV Outpatient Study (HOPS), we analyzed how reported laboratory data correlated with actual engagement in care.  METHODS: The HOPS is an open prospective study of HIV-infected patients receiving outpatient care. The dataset included dates for laboratory measurements and medical encounters.  We included patients with at least one HIV laboratory test and one medical visit during 2010-2011.  An HIV laboratory test was associated with a medical visit if it occurred within 3 weeks of the visit. We assessed the predictive value of HIV laboratory tests as a proxy for adequate engagement in clinical care, defined as having had ≥2 HIV laboratory tests within 1 year where the tests were performed ≥90 days apart.  RESULTS: A total of 10,301 HIV laboratory tests were recorded from among 2,895 patients. Most (75%) laboratory tests were measured on the same day as a clinical visit; 90% were within ±3 weeks. The prevalence of adequate engagement in clinical care in HOPS based on medical visits was 89%. Using HIV laboratory tests to measure engagement had sensitivity of 85%, specificity of 87%, and positive and negative predictive values of 98% and 42%.  Of the 21.4% of persons classified as not engaged in care by the proxy measure, 58% were actually engaged. CONCLUSIONS: Using ≥2 documented HIV laboratory tests measured ≥90 days apart reliably classified persons as engaged in care. However, more than half of persons not meeting the proxy definition were misclassified as not adequately engaged in care.