Automated nephelometry of fibrinogen: analytical performance and observations during thrombolytic therapy

We evaluated the performance of an automated nephelometric determination of fibrinogen, which is an integral part of the prothrombin time assay, in a new coagulation analyzer, the ACL-810 (Instrumentation Laboratory). Results were compared with those by a total clottable protein assay and with the thrombin clotting time assay for fibrinogen. In normal and slightly abnormal plasma, the performance of the ACL method was quite satisfactory (CV 3-10%). However, in abnormal plasma (prolonged prothrombin times because of heparin or oral anticoagulants) the accuracy of the ACL method was poor. Nor could the instrument determine fibrinogen in clearly lipemic plasma. In plasma containing high concentrations of fibrin(ogen) degradation products (FDP), collected during thrombolytic therapy with streptokinase-containing drugs, the ACL method gave spuriously high values for fibrinogen concentration. We determined that this was mainly because of interference by intermediate FDP (fragment Y). Finally, we demonstrated that early FDP (fragment X) increased the ACL results for fibrinogen to the same extent as in the total clottable protein method and that late FDP (fragments D and E) affected the thrombin clotting time method, but not the ACL fibrinogen determination.