Laboratory Evaluation

Laboratory Evaluation

The normal plasma fibrinogen concentration is approximately 150 to 350 mg dL –1  with a half-life of about 3.5 days.38 Fibrinogen assay methods are based on one of the following principles:

  • a functional activity assay, which measures clottable protein on the basis of either coagulation time or velocity (Clauss or thrombin clotting method)39
  • a radial-immunodiffusion assay, utilized by most laboratories to measure antigen level.40

In general, functional fibrinogen clotting assays are less sensitive and the threshold of detection can be as high as 50 mg dL-1. With more sensitive ELISA methods, levels as low as 0.4 mg/dL can be detected.38

 Afibrinogenemia
The key diagnostic criterion for afibrinogenemia is the absence of immunoreactive fibrinogen. All coagulation tests that depend on fibrin as the endpoint, including prothrombin time (PT), activated partial thromboplastin time (aPTT), thrombin time (TT) and reptilase time are infinitely prolonged.

Fibrinogen is undetectable by functional (Clauss) assays in persons with afibrinogenemia. Sensitive ELISA assays may determine trace amounts of fibrinogen (< 10 mg/dL).  On coagulation screening panels, a normal platelet count and negative D-dimer are useful to discriminate afibrinogenemia from disseminated intravascular coagulation and severe liver disease. 

Hypofibrinogenemia
Since hypofibrinogenemia is a proportional decrease of functional and immunoreactive fibrinogen, most fibrin-based coagulation tests are variably prolonged, the most sensitive test being the thrombin time, which is usually prolonged at fibrinogen activity < 100 mg dL –1. Both total clot-based and immunologic fibrinogen levels are reduced to similar levels.

Tests should be interpreted with regard to the possibility of acquired hypofibrinogenemia, including consumptive coagulopathy, hepatic failure, and L-asparaginase therapy. Normal platelet counts and negative D-dimers as well as family studies may be helpful in differentiating acquired fibrinogen deficiency from CFD.

Dysfibrinogenemia
Both the thrombin time and reptilase time are sensitive screening tests. A prolonged reptilase time in the presence of a normal functional fibrinogen provides strong evidence of dysfibrinogenemia. A normal or increased antigen with a lowered functional level (Clauss method) resulting in a low functional-antigen ratio (most commonly 1:2) is usually diagnostic.41

The sensitivity of coagulation tests to dysfibrinogenemia are dependent on the specific mutation, and laboratory reagents and techniques.42 A definitive diagnosis can be established by demonstrating the molecular defect. However, as these disorders are dominantly inherited, family studies may be helpful to differentiate congenital from acquired dysfibrinogenemia. In the small percentage of patients that present with thrombosis, a thrombophilia work up to exclude other co-existing prothrombotic defects may be useful.

Given the thrombotic risk in afibrinogenemia and dysfibrinogenemia, both with and without fibrinogen replacement therapy (FRT), a role may exist for monitoring hypercoagulability with global assays, including thrombin-antithrombin complexes for either condition and potentially thromboelastography (TEG or ROTEM) for dysfibrinogenemia.43  It is important to note that individuals without detectable fibrinogen or with fibrinolytic defects are unable to generate D-dimers or other fibrin degradation products; therefore these assays are insensitive markers of hypercoagulability in these individuals with CFD.

Prenatal Diagnosis
Afibrinogenemia is an autosomal recessive disorder while hypofibrinogenemia and dysfibrinogenemia are autosomal dominant. Hence, individuals with a family history, especially those with a history of consanguinity, should be appropriately counseled as to the risks of a child with the disease. If the mutation is known, genetic testing can be planned during pregnancy using amniotic fluid testing of fetal fibroblasts to aid in planning an appropriate and safe delivery for an affected child.

In infants born to parents who are known or suspected carriers, cord blood testing for fibrinogen activity and antigen should be offered to facilitate prompt and accurate diagnosis and management of neonatal bleeding. Avoidance of arterial punctures, intramuscular injections, and invasive interventions is recommended. Routine screening for intracranial hemorrhage in neonates is currently under debate as ultrasound is insensitive to parenchymal bleeding and computed tomography (CT) is associated with radiation exposure. However, performance of magnetic resonance imaging (MRI) in neonates without the need for sedation is increasing in tertiary care hospitals with large neonatal services.