Ruta Tuckuviene1, Soeren Risom Kristensen1, Jon Helgestad2, Anette Luther Christensen1, Soeren Paaske Johnsen31Department of Clinical Biochemistry, Center for Cardiovascular Research, 2Department of Pediatrics, 3Department of Clinical Epidemiology, Aarhus University Hospital, Aalborg and Aarhus, DenmarkAbstract: Data on the validity of pediatric thrombosis diagnoses are missing. We aimed to examine the predictive value of a diagnosis of venous and arterial thrombosis using the Danish National Patient Registry (DNPR). We identified all first-time diagnoses among children and adolescents (aged 0–18 years) between 1994 and 2006 in DNPR. In total, 1138 potential cases of thrombosis were identified; the medical records were retrieved for 1112 (97.7%) and the positive predictive value (PPV) computed. Overall, the diagnosis of thrombosis was verified in 598 of the 1112 cases, corresponding to a PPV of 53.7% (95% confidence interval [CI]: 50.8–56.7). Diagnoses from wards had the PPV of 62.5% (95% CI: 59.4–65.6). The predictive value of a thrombosis diagnosis from wards was age-dependent, with a higher PPV (77.4%, 95% CI: 68.7–84.7) in neonates (<28 days) and adolescents (15–18 years) (68.2%; 95% CI: 63.2–72.5)) than in children (28 days–14 years) (51.2%; (95% CI: 46.0–56.4)). The PPV of a thrombosis diagnosis was improved by restricting the analysis to diagnoses from wards, primary diagnoses, and admissions with a length of stay of three or more days. The results indicate that an interpretation of nonvalidated hospital discharge data for pediatric thrombosis in a registry like DNPR should be made with caution.Keywords: pediatric thrombosis, discharge diagnosis, registry, positive predictive value