factor XIIIa
Summary
Factor XIIIa is the activated form of factor XIII, a transglutaminase enzyme that cross-links fibrin monomers to stabilize the clot. It acts as the final step in the coagulation cascade, converting soluble fibrin into an insoluble, mechanically stable clot. Deficiency causes delayed bleeding and poor wound healing despite a normal PT/PTT.
Detail
Factor XIII is a pro-transglutaminase that circulates as a heterotetramer (A2B2) and is activated by thrombin (in the presence of calcium) after fibrin formation, cleaving an activation peptide to yield factor XIIIa. Factor XIIIa catalyzes the formation of covalent ε-(γ-glutamyl)lysine cross-links between adjacent fibrin monomers (particularly the α and γ chains), converting the loosely associated fibrin polymer into a mechanically stable, protease-resistant clot. It also cross-links α2-antiplasmin to fibrin, which inhibits plasmin-mediated fibrinolysis, and cross-links fibronectin to collagen, aiding wound healing and tissue repair. Because factor XIII acts downstream of fibrin formation, screening coagulation tests (PT and PTT) are normal in factor XIII deficiency; diagnosis requires a specific clot solubility test (clot dissolves in 5M urea or monochloroacetic acid) or factor XIII activity assay. Clinically, factor XIII deficiency (congenital, autosomal recessive, or acquired) presents with delayed umbilical cord bleeding, poor wound healing, recurrent miscarriages, and delayed-onset bleeding after trauma or surgery (since initial clot forms but is unstable and lyses early). Treatment involves factor XIII concentrate or cryoprecipitate/FFP replacement. This factor is a key high-yield topic linking the coagulation cascade to clot stability and distinguishing it from factors affecting PT/PTT.
Sources
- First Aid for the USMLE Step 1
- Robbins Basic Pathology
- Harrison's Principles of Internal Medicine
- UpToDate: Factor XIII deficiency
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