Quantitative measurement of fibrinogen can be carried out using various immunological assays and the clottable protein assay (Mackie et al., 2003). It is important to note that whilst these assays give an indication of how much fibrinogen is present, they do not give an indication of the functional activity of fibrinogen. They are therefore used alongside functional assays to determine fibrinogen levels and activity, and are most often used to confirm suspected cases of congenital fibrinogen deficiency alongside genetic analysis.
Immunological assays allow for quantitative measurements since they detect antigen, however it is not possible to distinguish between functional and non-functional protein. They are also time consuming and can take many hours to complete. They are therefore not regularly used to diagnose fibrinogen deficiency in acute settings; however, they are often used to confirm congenital fibrinogen deficiency. The enzyme-linked immunosorbent assay (ELISA) is the most accurate and widely used immunological assay compared to other techniques such as electrophoretic techniques, radial immunodiffusion, rapid latex agglutination, immune turbidimetry and nephelometry (Mackie et al., 2003; Chen et al., 2010). Polyclonal antibodies are generally utilised for these assays to ensure full coverage of fibrinogen protein; however, monoclonal antibodies that are specific to non-proteolysed fibrinogen are also available. The latter allows for the identification of fibrinogen that has not been processed through plasmin digestion and gives a better indication of functional fibrinogen levels.
A clottable protein assay can be used to determine clot weight. In this assay, thrombin is added to patient sample plasma in the absence of calcium ions. The resulting clot is washed and treated with alkaline urea for protein measurement by spectrophotometry (Ratnoff & Menzie, 1951; Jacobsson et al., 1955, Blomback & Blomback, 1956). Since fibrin is the only protein in the clot structure, the protein concentration achieved is related directly to the amount of fibrin present in clots. This diagnostic test is therefore highly accurate and is often used to confirm congenital fibrinogen deficiencies; however, it is not used as a standard diagnostic test since it is a time-consuming and laborious technique (Mackie et al., 2003).
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