An abnormal fibrinogen was discovered in the plasma of a clinically asymptomatic woman. Laboratory evaluation of five members of the affected family showed low fibrinogen values in kinetic assays whereas the fibrinogen levels, tested by immunological procedures were normal. The patient's plasma had an inhibitory effect on the thrombin time of normal plasma. The calcium ions totally corrected the thrombin and reptilase times. Either low or high ionic strength prolonged the thrombin time of the proposita's purified fibrinogen. Kinetic analysis of clotting by monitoring transmission at 350 nm showed abnormally slow clotting with thrombin and reptilase. Assays were preformed in whole plasma as well as in purified fibrinogen. A delay in the rate of polymerization was evident when purified patient monomers were compared with those of normals. Immunoelectrophoretic, chromatofocusing, and isoelectrofusing experiments detected neither structural nor immunological abnormalities of fibrinogen. The rate of release of fibrinopeptide A by thrombin, measured by a specific immunoenzymatk method was also normal. HPLC analysis showed normal liberation of fibrinopeptides after prolonged thrombin action. Cross-linking of fibrin by factor XIII and lysis of fibrinogen by plasmin were normal. In view of these results, the defect of this dysfibrinogenemia, designated as Fibrinogen Oviedo I, probably could be due to conformational modifications in the D section of the molecule
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