D-dimer

When the body forms a clot, it also begins dismantling it, and that demolition releases small protein fragments into the blood. D-dimer is one such fragment, a leftover crumb of broken-down clot. Measuring it in a blood sample tells us whether clot formation and breakdown have been unusually active somewhere in the body.

Its great strength is as a rule-out test. In a patient judged unlikely to have a clot, a normal D-dimer makes pulmonary embolism or deep vein thrombosis very improbable, often sparing them the radiation and contrast of further imaging. A negative result is reassuring; that is its main clinical job.

D-dimer is far less useful for ruling clots in, because it rises in many situations: infection, inflammation, cancer, recent surgery, pregnancy, and simply advancing age. A raised value is non-specific and usually leads to imaging rather than a diagnosis. For this reason D-dimer is interpreted alongside clinical probability, such as the Wells score, not in isolation.