What is the difference between D-dimer and fibrinogen degradation products (FDP)?

• A. D-dimer is a specific product of cross-linked fibrin degradation; FDP includes both cross-linked and non-cross-linked fibrin degradation products.
• B. D-dimer detects cross-linked fibrin; FDP measures platelet function
• C. D-dimer measures fibrinogen; FDP measures platelet function
• D. D-dimer and FDP are the same test

Answer: (A)

The main idea is that D-dimer is a specific degradation product that only comes from cross-linked fibrin, while fibrinogen degradation products are a broader set of fragments from both fibrinogen and fibrin. When a clot forms, fibrin is cross-linked by factor XIII, and when it’s later dissolved, plasmin breaks that cross-linked fibrin into D-dimer fragments. If the fibrin is not cross-linked or if the breakdown involves fibrinogen/fibrin without cross-linking, those specific D-d fragments aren’t produced. FDP tests, on the other hand, detect a mixture of degradation products from both fibrinogen and fibrin, including non-cross-linked fibrin breakdown products, so they’re less specific for clot turnover. Clinically, this means D-dimer is a more specific marker of clot formation and breakdown, whereas FDP reflects broader fibrinogen/fibrin degradation and can be elevated in more conditions.