FibrinogenFactor I
The clotting protein that also rises with inflammation.
Fibrinogen is a protein made by the liver and the raw material of a blood clot: when clotting is triggered it is converted into fibrin, the mesh that holds a clot together.
It matters twice over. It is essential to normal clotting, and it is also an acute-phase protein, meaning the liver produces more of it during inflammation, so a raised level can reflect either.
In practice it is measured when a clotting problem is suspected, and it is the protein largely responsible for driving the ESR, which is why the two move together.