Bleeding Disorder of Unknown Cause in the Netherlands

Inclusion Criteria: * Referred to a (pediatric) hemostasis specialist for evaluation of bleeding tendency. * Increased bleeding tendency based on: Abnormal International Society on Thrombosis and Haemostasis Bleeding Assessment Tool (ISTH-BAT) score (≥ 5 in women age 18-30; ≥ 6 in women age 31-51, ≥ 7 in women age 52 or older; ≥4 in men and ≥ 3 in children) OR Clinical gestalt according to the investigating physician * Absence of diagnostic test results for a bleeding disorder in standard laboratory hemostasis tests: * Complete blood count: Hemoglobin \> 6.0 mmol/L; thrombocyte count \> 100 x10\^9/L * Prothrombin (PT) and activated Partial Thromboplastin Time (aPTT): within local reference range, or prolonged without explanatory factor deficiency * Fibrinogen activity, von Willebrand Factor (VWF) antigen \& activity, Factor VIII, IX, XI and XIII: within local reference range or abnormal but not explaining bleeding phenotype * Light transmission aggregometry (LTA): Not diagnostic for a platelet function * Kidney function: eGFR \> 45 ml/min * Liver function: ALAT, bilirubin \< 3 x upper limit of normal Exclusion Criteria: * Use of medication interfering with laboratory hemostasis tests which cannot be stopped before blood withdrawal * Pregnancy or lactation at moment of inclusion * Presence of an established bleeding disorder * Presence of an acquired cause or another explanation for the increased bleeding tendency * Inability to provide informed consent