Factor XIII cross-links fibrin after the clot has formed. PT and aPTT only measure the time to clot formation, so in factor XIII deficiency both are normal, despite deep tissue and joint bleeding. Normal PT, aPTT, fibrinogen and platelets are seen in factor 13 deficiency, platelet disorders and von Willebrand disease. Isolated aPTT prolongation points instead to factor VIII, IX or XI deficiency.