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Plasma d-dimers are fibrin derivatives generated by the action of plasmin on cross-linked fibrin fragments D, indicating that the clotting mechanism had been activated and thrombin generated. Although it is a direct marker of active fibrinolysis, it is an indirect, but very useful, marker of ongoing coagulation.
Normal range: <0.2 μg/mL for the latex assay; <1.1 mg/L for the ultrasensitive immunoturbidimetric test.
Two d-dimer assays are available, each with a different use.
The latex agglutination d-dimer has relatively low sensitivity; hence, it is not positive in single clots but elevated when multiple clots are generated. For this reason, it had been proven to be the most specific and sensitive test in the diagnosis of DIC.
The ultrasensitive d-dimer is performed by ELISA or immunoturbidimetric techniques that allow its precise quantitation. Because of its exquisite sensitivity, it becomes elevated in the presence of single clots.
Its main value is its high negative predictive ability, because a negative ultrasensitive d-dimer excludes thromboembolic events, with approaching 100% certitude (depending on methodology and equipment used). Although POC methods are available, they have slightly lower negative predictive values.
Elevated values are less useful, although persistent elevations after 3–6 months of anticoagulation following a thromboembolic event suggest a high probability of recurrent events.
The cutoff value for the ultrasensitive d-dimer is <1.1 mg/L (it varies with methods and equipment used). Any values below 1.1 mg/L are considered negative and are used in most diagnostic algorithms for the exclusion of deep vein thrombosis (DVT) or pulmonary embolism (PE) in low probability situations.
The latex d-dimer is elevated in all situations with multiple clots, the prototype being DIC. The higher the titer, the more severe the DIC may be
The ultrasensitive d-dimer is elevated in the following conditions:
DVT and PE
Renal, liver, or cardiac failure
Disseminated cancer and monoclonal gammopathies
Major injury and surgery
The ultrasensitive d-dimer may be falsely elevated or decreased in hyperlipidemic or very turbid blood samples and in patients treated with mouse monoclonal antibodies.
RF may give false-positive results.