D-dimer

D-dimer is a specific plasmin-mediated breakdown product of crosslinked fibrin. Thrombin converts fibrinogen into soluble fibrin monomer. This monomer then spontaneously polymerizes to form the soluble fibrin polymer. Thrombin also activates factor XIII, which in the presence of calcium, crosslinks the fibrin polymer, producing crosslinked fibrin. Plasmin cleavage of fibrinogen or soluble fibrin produces the "traditional" FDPs, fragments X, Y, D and E. Plasmin cleavage of crosslinked fibrin produces different degradation products, that vary in molecular weight and are called X-oligomers. D-dimer is a specific neoantigen produced by the factor XIIIa-mediated crosslinking of fibrin and is exposed after plasmin degrades crosslinked fibrin, allowing it to be detected by immunologic-based assays. Note that although plasmin is the main fibrinolytic enzyme, proteolytic enzymes released by neutrophils can also degrade crosslinked fibrin exposing D-dimer.


Thus, D-dimer is more specific for fibrinolysis than FDPs, as its formation requires the action of thrombin (to activate factor XIII) to produce crosslinked fibrin and the cleavage of this fibrin by plasmin. In contrast, traditional FDP assays cannot distinguish between plasmin action on fibrinogen (fibrinogenolysis) and fibrin (fibrinolysis), therefore FDPs can be elevated when there is no clot present (and plasmin is just cleaving fibrinogen).

D-dimer can be detected in human patients with assays using monoclonal antibodies specific for the human D-dimer epitope. Some of these monoclonal antibodies crossreact with some animal species and can be used for veterinary patients. Certain D-dimer latex agglutination assay have been validated in the dog, cat and horse. The test is run similarly to the FDP assays, but the sample can be assayed undiluted (to obtain a positive or negative result) or can be serially diluted to obtain a semi-quantitative D-dimer value. It is far better to get a semi-quantitative D-dimer result, because higher values may be more specific for thrombembolic conditions (see below).


Normal dogs and cats have D-dimer values < 250 ng/ml, whereas most healthy horses have D-dimer values < 500 ng/ml (but D-dimer can be as high as 1000 ng/ml in this species).

High D-dimer
D-dimer will be elevated whenever there is activation of thrombin, to form crosslinked fibrin AND fibrinolysis, i.e. thrombosis and fibrinolysis. The prototypical thromboembolic disease is disseminated intravascular coagulation (DIC) and D-dimer is often very high in this disorder (indeed, D-dimer is quite sensitive for DIC and values may increase in early DIC before any other coagulation assays, such as the PT and aPTT, become abnormal). However, any disorder resulting in crosslinked fibrin formation and breakdown can potentially elevate D-dimer (i.e. high D-dimer is not specific for DIC). This includes physiologic (e.g. associated with surgical wound-healing) and pathologic fibrinolysis (associated with thrombosis of any cause, e.g. pulmonary thromboembolism).

Note that coagulation and fibrinolysis are not restricted to the intravascular space. If thrombin is activated in extravascular tissues in which fibrinogen is also present, e.g. protein-rich or hemorrhagic effusions into body cavities (e.g. joints, thoracic cavity, central nervous system), crosslinked fibrin could conceivably form in these extravascular sites. Activation of plasmin or release of proteolytic enzymes from neutrophils within these sites could degrade this crosslinked fibrin, releasing D-dimer into the fluid. This could potentially be reaborbed into blood, elevating plasma D-dimer levels.
Illustration of a latex agglutination D-dimer assay: Positive and negative controls are in wells 1 and 2. Agglutination is evident in well 1 but not 2, indicating correct functioning of the test. Two canine patient samples are being tested concurrently; the first patient plasma is in well 3 (undiluted) and the second in well 4 (undiluted). Agglutination is observed in wells 3 and 4. This is compatible with a positive D-dimer result (>250 ng/ml) for both patients, which were dogs with DIC.
 

Dogs: D-dimer values are often very high (> 1000 ng/ml) in dogs with documented thrombo-embolic disease, including DIC. Indeed, D-dimer appears to be a very sensitive indicator of fibrinolysis in dogs with DIC (100% sensitive in one study). However, D-dimer is not specific for DIC or other thrombotic disorders. High values have been reported in dogs secondary to neoplasia, inflammatory disease and hemorrhagic effusions, e.g. hemoperitoneum (although most dogs with hemorrhagic effusions have concurrent disease processes that could independently initiate DIC). Thus, D-dimer indicates fibrinolysis, regardless of whether this is physiologic (i.e. associated with surgical wound healing) or pathologic (associated with disease) and is not specific for DIC. D-dimer has been evaluated in the cerebrospinal fluid (CSF) as a marker of prior hemorrhage into the central nervous system. One study showed that D-dimer values were increased in the CSF of dogs with evidence of hemorrhage, however values were still below the limit of detection of the latex agglutination assays (< 250 ng/ml) limiting the usefulness of this test.

Cats: D-dimer has been evaluated in cats with cardiac disease, who are predisposed to aortic thromboembolism. However, D-dimer values were similar between healthy control cats and cats with cardiac disease, arguing against its usefulness to predict thrombosis in cats with cardiac disease. We have seen high D-dimer levels in cats with conditions associated with DIC, e.g. feline infectious peritonitis virus infection, suggesting that D-dimer may still be useful to detect thrombosis in some feline diseases.

Horses: D-dimer is increased in plasma in horses with severe colic and is a sensitive diagnostic test for the presence of underlying DIC. In some studies, a high D-dimer was a negative prognostic indicator for outcome.