Increased Fibrin and Fibrinogen Degradation Products
FDPs can be composed of fragments of fibrin and/or fibrinogen. Assays are either serum or plasma specific, as fibrinogen present in plasma can react to the polyclonal antibody of the serum assays, and are relatively species specific.
A study in horses found that two FDP assays, one serum-based and one plasma-based, had low sensitivity for FDPs in horses with colic.41 At the same time, a plasma D-dimer assay was a useful adjunct test. Commercial D-dimer assays have also been used in cattle; in one study peritoneal fluid D-dimer was a useful indicator of peritonitis.42 D-dimer (one form of FDP) is a cross-linked fibrin fragment and is therefore specific for fibrinolysis, whereas FDPs as a group are less specific. When measurable, increased levels of FDPs and/or D-dimer generally indicate increased fibrinolysis in response to excessive coagulation (Box 27.7). In addition to DIC, localized intravascular fibrin deposition following surgery, internal hemorrhage, sepsis, and other severe inflammatory processes may cause increased FDP levels. Plasmin perturbations may occur with snake envenomation, heatstroke, surgery, or administration of plasminogen activators and can lead to fibrinogenolysis and increased FDPs.1 Decreased FDP clearance due to hepatic or renal disease can also lead to increased FDP levels.Increased serum FDPs can contribute in turn to hemorrhagic clinical manifestations of DIC by interfering with thrombin activity, fibrin polymerization, and platelet aggregation. Interpretation of FDP or D-dimer tests depends on evaluation of the other components of the hemostasis profile (platelet count, PT, PTT) in concert with the patient's history and clinical signs. Serum FDP levels greater than 40 pg/mL or plasma levels greater than 20 pg/mL most often occur with DIC; lower values do not exclude DIC. Horses have measurable D-dimer concentrations in health, so an upper cutoff value must be used to report abnormal values.41 In one study horses with clinical evidence of DIC secondary to colic had decreased D-dimer levels consistent with hypofibrinolysis during hypercoagulation.17