Fibrin degradation products (FDP): the clot-breakdown marker behind D-dimer

When a vessel is injured, the coagulation cascade builds a mesh of fibrin threads that catches platelets and forms a clot. Lab Tests Online UK explains that these threads are cross-linked by thrombin to form a net, and that once the area has healed the body uses plasmin to break the clot into small pieces so that it can be removed. The fragments of the disintegrating clot are called fibrin degradation products (FDP).

Plasmin can also break down fibrinogen itself, so FDP tests may detect fragments from both fibrinogen and fibrin. D-dimer, in contrast, comes only from cross-linked fibrin, which is why it is more specific for clot formation.

Lab Tests Online UK states that one of the FDPs is D-dimer, consisting of variously sized pieces of cross-linked fibrin, normally undetectable in the blood and produced only after a clot has formed and is being broken down. Because it is specific, D-dimer has become the standard test in clinical practice, and many laboratories no longer offer a separate FDP assay.

The main use of D-dimer is ruling out venous blood clots as a cause for symptoms in the legs or lungs: if the result is negative it is extremely unlikely you have a significant venous clot. If it is positive this does not mean you have a clot, as it is raised in many conditions, so D-dimer is not useful for confirming a diagnosis.

They are requested for symptoms of deep vein thrombosis (leg pain, tenderness, swelling, discolouration) or pulmonary embolism (breathlessness, cough, lung-related chest pain), where a doctor first uses a Wells score to judge whether a clot is likely. They are also requested in suspected disseminated intravascular coagulation (DIC), a complex, rapidly developing condition in which clotting factors are activated and used up throughout the body, which can arise after some surgical procedures, burns, infections, cancer, liver disease, snake bites and childbirth. For DIC the test is requested with PT, aPTT, fibrinogen and a platelet count, and may be repeated to monitor treatment.

Sheffield Laboratory Medicine notes that high concentrations of D-dimers (FDPs) can prolong the thrombin time, which is one reason a prolonged thrombin time prompts a look at clot breakdown.

A positive result indicates abnormally high levels of cross-linked fibrin degradation products, telling the doctor that significant clot formation and breakdown has occurred but not where or why. Lab Tests Online UK lists venous thromboembolism and DIC but also recent surgery, trauma, infection, liver or kidney disease, cancers, normal pregnancy and some diseases of pregnancy such as eclampsia, and says levels may rise in the elderly.

A normal D-dimer means an acute clot or disease causing abnormal clot formation and breakdown is unlikely. It is most valid and useful in low-risk patients and should be an additional test, not the only test used for diagnosis. Anticoagulant therapy can cause a false negative.

No preparation is needed. Blood is taken from a vein, or by finger-prick for some bedside tests. Quantitative results are generally done in a hospital laboratory, whereas qualitative ones can be done at the bedside. Methods differ between laboratories, so reference limits are laboratory-specific, and your report prints the limit used.

A positive result leads to imaging such as a leg ultrasound or CT pulmonary angiogram, guided by symptoms. Do not delay care while waiting for results: the NHS advises urgent attention for chest pain, breathlessness or coughing blood; call 999. A swollen, painful leg needs same-day medical assessment.