D dimer is a protein fragment that is found in the blood after
someone has had a blood clot. Fibrinolysis breaks down the clot,
leaving D dimer in the blood. The D dimer blood test is usually
performed when doctors suspect that a patient has a pulmonary
embolism (blood clot in the lung) or deep venous thrombosis. While
a negative D dimer blood test usually means that a blood clot is
not present, a positive D dimer blood test does not necessarily
indicate a blood clot or deep venous thrombosis