How I Investigate Bleeding Disorders of Unknown Cause: Current Diagnostic Pathways and Gaps in Laboratory Investigation.

Bleeding disorder of unknown cause (BDUC) constitutes the largest group of patients presenting with a mild-to-moderate bleeding tendency in tertiary care settings. Their clinical bleeding phenotype is characterized by mucocutaneous bleeding, as well as bleeding following surgical challenges or childbirth, and is associated with impaired health-related quality of life. Iron deficiency, with or without anemia, is common, particularly among women, who comprise up to 80% of BDUC cohorts and frequently report heavy menstrual bleeding. Diagnosing BDUC requires a rigorous exclusion of established hemostatic and non-hemostatic causes of bleeding. Common inherited bleeding disorders, including coagulation factor deficiencies (CFD), von Willebrand disease (VWD), and platelet function disorders (PFD), must be systematically excluded. CFD evaluation should extend beyond routine assays (prothrombin time, activated thromboplastin time, thrombin time), as clinically relevant mild reductions in factors VIII, IX, and XI may occur despite normal screening tests; and factor XIII deficiency is not detected by global assays. VWD assessment mandates measurement of VWF antigen and activity, with additional studies to define subtype when indicated. For PFD, light transmission aggregometry remains the reference gold standard. Substantial diagnostic overlap exists among these entities and BDUC, and repeated testing is often required. Investigations for rare causes such as hyperfibrinolysis or excess natural anticoagulants are typically limited to patients with distinctive phenotypes or strong family histories. Although the pathogenesis of BDUC remains incompletely understood, continued investigation into platelet biology, global hemostasis, and vascular contributions holds promise for uncovering therapeutic targets, ultimately improving management for this prevalent yet understudied condition.