Which test would best screen for Factor XIII deficiency?

• A. Clot stability testing (urea clot dissolution).
• B. Prothrombin time.
• C. APTT.
• D. Factor XIII activity assay.

Answer: (D)

Factor XIII’s job is to crosslink and stabilize the fibrin clot, so standard tests that measure how quickly or how much fibrin forms (like PT and aPTT) don’t reveal a deficiency in FXIII because those steps happen before crosslinking. To detect FXIII deficiency, you need a test that specifically assesses the enzyme’s activity in plasma. The Factor XIII activity assay directly measures how well FXIII is functioning; a low result confirms deficiency, making it the most sensitive and specific screen.

A clot stability test that uses urea dissolution can indicate a problem with clot stabilization, but it isn’t as specific or standardized as directly measuring FXIII activity, and results can be affected by technical factors. Therefore, directly assessing FXIII activity provides the clearest, most reliable screen for this deficiency.