A d-dimer test is a blood test that measures a substance (fibrin) that is released as a blood clot breaks up. D-dimer levels are often higher than normal in people who have a blood clot; however, other conditions, such as cancer, also may cause high d-dimer levels.

A low d-dimer test result means that a blood clot in the leg or the lung (pulmonary embolism) is unlikely.