Hemostasis and Thrombosis: Coagulation, Platelets, and Antithrombotic Therapy
Screen first
The PT measures the extrinsic and common pathway (factors VII, X, V, II, fibrinogen); the aPTT measures the intrinsic and common pathway (VIII, IX, XI, XII). A prolonged aPTT with a normal PT in a bleeding boy points to hemophilia A or B or von Willebrand disease. A mixing study that corrects indicates a factor deficiency; one that fails to correct indicates an inhibitor, such as a factor VIII inhibitor or a lupus anticoagulant.
von Willebrand disease
The commonest inherited bleeding disorder. Type 1 is a partial quantitative deficiency and responds to desmopressin; type 2A has abnormal multimers; type 2B causes thrombocytopenia that worsens with desmopressin, so it is contraindicated; type 3 is severe and needs factor concentrate. Mucocutaneous bleeding with a prolonged closure time and a normal platelet count is typical.
Hemophilia
X-linked, with bleeding into joints and muscles. Treat with factor replacement; prophylaxis prevents arthropathy. Roughly 30% of boys with severe hemophilia A develop inhibitors, suspected when bleeding stops responding to factor. Emicizumab, a bispecific antibody, is used for prophylaxis.
Immune thrombocytopenia
Isolated thrombocytopenia with an otherwise normal blood count and no splenomegaly; a diagnosis of exclusion. Observation suits most children; IVIG or anti-D raises the platelet count quickly when bleeding. Platelet transfusion is reserved for life-threatening bleeding because transfused platelets are consumed.
Thrombosis
Increasingly recognized in children and usually catheter-related. Enoxaparin is monitored with anti-Xa levels. Heparin-induced thrombocytopenia is immune-mediated and requires stopping all heparin.
Rule of thumb
Bleeding that starts immediately after a cut, involves mucous membranes, and stops with pressure suggests a platelet or vessel problem; delayed, deep, or joint bleeding suggests a coagulation factor problem.
Trap
Mild hemophilia can have a normal aPTT - do not exclude it on a single normal screen when the history is convincing.