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Background Single-center comparative effectiveness studies evaluating outcomes that can occur posthospitalization may become biased if outcomes diagnosed at other facilities are not ascertained. Administrative datasets that link patients' records across facilities may improve outcome ascertainment. Objective To determine whether use of linked administrative data significantly augments thromboembolic outcome ascertainment. Research design Retrospective cohort study. Subjects Patients with an acute isolated calf deep vein thrombosis (DVT) diagnosed at 1 Californian center during 2010-2013. Measures Proximal DVT or pulmonary embolism (PE) within 180 days. We ascertained outcomes from linked California hospitalization, emergency department, and ambulatory surgery data and compared this information to outcomes previously identified from review of the center's medical records. Results Among 384 patients with an isolated calf DVT, 333 could be linked to longitudinal administrative data records. Ten patients had a possible proximal DVT or PE (4 more clearly so) from administrative data; all were unknown from medical record review. Eleven patients with known outcomes from medical record review had no outcome from administrative data. The adjusted odds ratio of proximal DVT or PE with therapeutic anticoagulation attenuated from 0.33 [95% confidence interval (CI), 0.12-0.87] using only medical record review to 0.64 (95% CI, 0.29-1.40) using both medical record review and possible outcomes from administrative data. Restricting the outcome to diagnoses clearly involving proximal DVT or PE, the adjusted odds ratio was 0.46 (95% CI, 0.19-1.10). Conclusions Use of linked hospital administrative data augmented detection of outcomes but imperfect linkage, nonspecific diagnoses, and documentation/coding errors introduced uncertainty regarding the accuracy of outcome ascertainment.