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A protein fragment released when blood clots break down, used to rule out dangerous clots in your veins and lungs.
When your body forms a blood clot and then dissolves it, the cleanup process leaves behind fragments. One of those fragments is D-dimer (a fibrin degradation product). Measuring its level in your blood tells you whether your clotting system has been unusually active recently. The test's greatest strength is its ability to rule things out: if your D-dimer is normal, it is very unlikely that you have a dangerous clot forming in your legs or lungs.
That makes this test most valuable in a specific moment: when you or a clinician suspect a blood clot but the suspicion is low to moderate. A normal result can spare you from imaging scans and the anxiety that comes with them. An elevated result, on the other hand, does not confirm a clot. It simply means further investigation is warranted.
Your blood maintains a constant balance between forming clots and dissolving them. When you cut yourself, proteins in your blood weave together into a mesh of fibrin, the structural scaffold of every clot. Once healing is underway, another protein called plasmin cuts that mesh apart. D-dimer is one of the fragments plasmin produces when it breaks down fibrin that has been cross-linked, meaning fully stabilized.
Healthy people have low circulating levels of D-dimer because the normal rate of clot formation and breakdown is modest. When levels rise, it signals that somewhere in your body, more clotting and clot breakdown than usual is happening. The challenge is that many things can trigger this process, not just the dangerous clots known as venous thromboembolism, or VTE.
D-dimer is extremely good at catching clots when they exist. Highly sensitive assays detect more than 95% of VTE cases. But the test has low specificity, meaning many conditions besides clots will push your level up. In one study of unselected emergency department visitors, nearly half had elevated D-dimer, and many of those elevations were linked to infection, cancer, heart failure, or anemia rather than clots.
The wide range of non-clot causes has even inspired an informal clinical term, "D-dimeritis," to describe the pattern of elevated D-dimer driven by inflammation rather than thrombosis. This is why the test works best as a rule-out tool rather than a rule-in tool. A negative result is powerfully reassuring. A positive result simply opens the door to further testing.
Conditions that commonly raise D-dimer fall into a few broad categories:
What this means for you: if your D-dimer comes back elevated and you do not have symptoms of a blood clot, the elevation very likely reflects one of these other causes. Context matters enormously with this test.
While ruling out VTE is the most common reason to check D-dimer, the test has other established applications.