June 2025 Quick Question Summary: Fibrinolysis

Our June 2025 Quick Question was a quiz that asked, “Which fibrinolysis component directly digests fibrin?” Our 51 respondents answered…

1. Plasminogen: 12 (23%)
2. Antiplasmin: 0
3. D-dimer: 1 (2%)
4. Plasmin: 37 (73%)
5. TPA: 1 (2%)

The correct answer is #4, plasmin. In brief, plasminogen is a zymogen. Plasmin, the activated form of plasminogen, binds and digests fibrin.

TPA that is bound to fibrin cleaves the adjacent bound plasminogen to form active plasmin.

We use D-dimer as a marker for thrombosis, and a markedly raised D-dimer is a cardinal sign for DIC. Click here for my March 6, 2023 review of D-dimer’s properties and applications.

For a complete review of fibrinolysis, go to our Coagulation Overview Educational Module, and skip to slides 13–17. Here’s a transcription of the audio for the fibrinolysis slides:

“Tissue plasminogen activator (TPA) is secreted by endothelial cells and cleaves plasminogen to form plasmin. Circulating plasmin is neutralized by plasma alpha-2-plasmin inhibitor (antiplasmin). Free tissue plasminogen activator (TPA) is neutralized by plasminogen activator inhibitor (PAI-1), also secreted by endothelial cells. Plasmin that is bound to fibrin digests the fibrin at lysine and arginine locations, aided by bound TPA, which is also suppressed by PAI-1. A separate control protease, thrombin-activatable fibrinolysis inhibitor (TAFI), is cleaved by thrombin and binds fibrin, where it digests C-terminal lysine residues and thus slows plasmin-mediated digestion.”

Thank you for your participation in our Quick Question, and check out our July 2025 question in the right-hand column.