D-dimers are fibrin fragments, the main component of blood clots. D-dimers appear in the blood when clots dissolve, for example. D-dimers are normally present in very small quantities (less than 230 nanograms per millilitre of blood [ng/mL]). Several conditions can lead to elevated levels of fibrin and D-dimers: phlebitis (inflammation of a vein, generally in the legs) and arterial thromboses (formation of clots in an artery) likely to lead to a pulmonary embolism, stroke or kidney damage; disseminated intravascular coagulation (abnormal formation of a large number of small clots); surgery; trauma; recent infection or myocardial infarction (heart attack); pregnancy; liver disease, etc. The main clinical purpose of a D-dimer test is to look for phlebitis and pulmonary embolisms in an outpatient (non-hospitalized).
A negative D-dimer result indicates less than a 3% probability of an acute condition (phlebitis or pulmonary embolism). A positive result indicates an abnormally high quantity of fibrin products, regardless of the cause. More than 600 ng/mL in an outpatient is cause for concern, and the patient needs to be treated quickly, especially if they have recently experienced difficulty breathing with shortness of breath, coughing, chest pain or accelerated heart rate.