D-dimer

D-dimer is a tiny fragment left behind when the body dissolves a blood clot, like sawdust left after sawing wood. Because forming and breaking down clots happens together, a blood D-dimer level tells you whether clotting has been unusually active somewhere in the body.

When a clot forms, strands of fibrin are cross-linked to hold it together; when the body later breaks the clot down, it releases these cross-linked D-dimer pieces into the blood. The test is used mainly to help evaluate suspected deep vein thrombosis and pulmonary embolism — clots in the leg veins or lungs.

Its real strength is ruling out clots, not confirming them. In a patient judged to be at low or moderate risk, a normal D-dimer makes a dangerous clot very unlikely and can spare them further imaging. But many harmless conditions — infection, recent surgery, pregnancy, cancer, advanced age — also raise it, so a high D-dimer alone is not proof of a clot.