Brother Can You Spare a Dime...er!
In recent years, many physicians have begun to rely heavily on a blood test that will enable them to monitor diseases and conditions that cause hypercoagulability. Originally used to help diagnose disseminated intravascular coagulation (DIC), it was later found out that when elevated, this test more than likely indicates the presence of an abnormally high level of fibrin degradation products (FDP) in your body.
Commonly known as D-dimer, this test used in conjunction with other laboratory tests and imaging scans, help rule out, diagnose, and monitor diseases and conditions that cause hypercoagulability or to help diagnose DIC.
Description
But what exactly is a d-dimer; where does it come from? A blood clot, you will recall, is composed of a mesh of
fibrin polpeptides
which will eventually dissolve by a process known as
fibrinolysis
. This process is initiated by a protein called tissue
plasminogen
activator that is released from blood vessel cells adjacent to an injured site. This plasminogen activator then converts plasminogen to an enzyme called
plasmin
. It is this plasmin that splits the polypeptides from the fibrin clot into fragments. These fragments are known as fibrin degradation products (FDP).
There are four principal fibrin degradation products called X, Y, D, and E that are liberated in various combinations; and each of these fragments will also undergo some degree of degradation. The last fragment to be degraded is one consisting of two D and one E subunits. The two D subunits are covalently linked together and is referred to as the D-dimer.
What does the test result mean?
A positive D-dimer* indicates the presence of an abnormally high level of fibrin degradation products in your body. It tells us that there has been significant thrombus(clot) formation and breakdown in the body, but it does not tell the location or cause. An elevated D-dimer may be due to a venous thrombus embolism (VTE) or DIC but it may also be due to a recent surgery, trauma, or infection. Elevated levels are also seen with liver disease, pregnancy, eclampsia, heart disease, and some cancers.
Is there anything else I should know?
Anticoagulant therapy can cause a false negative D-dimer because it prevents thrombus extension. False negative readings can also occur if the blood sample is taken either too early after thrombus formation or if testing is delayed for several days.
On the other hand D-dimer concentrations may rise falsely in the elderly, an false positive may be seen with high levels of rheumatoid factor (a protein seen in patients with rheumatoid arthritis) inflammation , malignancy , trauma , pregnancy , recent surgery , liver disease as well as advanced age. Substances such as high triglycerides , lipemia (a large amount of fats in the blood that can be caused by the patient consuming a greasy meal prior to testing), and bilirubin can also cause false positives as can hemolysis caused by improper collection and handling. Thrombolytic drugs that may increase D-dimer levels are barbiturates, heparin, streptokinase, and urokinase.
*NOTE: A standard reference range is not available for this test. Because reference values are dependent on many factors, including patient age, gender, sample population, and test method, numeric test results have different meanings in different labs. (Most sampling kits have 0-300 ng/ml as normal range. Values exceeding 250, 300 or 500 ng/ml (different for various kits) are considered positive.
References
• Schrecengost JE, LeGallo RD, Boyd JC, Moons KG, Gonias SL, Rose CE Jr, Bruns DE. Comparison of diagnostic accuracies in outpatients and hospitalized patients of D-dimer testing for the evaluation of suspected pulmonary embolism. Clin Chem 2003;49:1483-90. PMID 12928229 .
• Wells PS, Anderson DR, Rodger M, Forgie M, Kearon C, Dreyer J, Kovacs G, Mitchell M, Lewandowski B, Kovacs MJ. Evaluation of D-dimer in the diagnosis of suspected deep-vein thrombosis. N Engl J Med 2003;349:1227-35. PMID 14507948
