MEASUREMENT OF PLASMA-FIBRINOGEN CONCENTRATION BY THE PROTHROMBIN-TIME-DERIVED METHOD - APPLICABILITY AND LIMITATIONS

A prothrombin-time-derived method was used to measure plasma fibrinoge n concentration (PFC) in 286 samples from 242 normal and 44 orally ant icoagulated subjects. Absorbance changes at 405 nm (Delta OD) during t he clotting process were obtained by an automatic coagulometer and the ir relationship with plasma fibrinogen concentration (range 90-1090 mg /dl), measured by the Clauss method, was investigated. A weighted line ar regression between the Delta OD and the Clauss-derived PFC values p rovided the best fit of the experimental data. The fitting equation wa s found to be reliable and accurate for PFC determination in normal su bjects, whereas a systematic overestimate of fibrinogen level was demo nstrated in plasma with high fibrinogen concentrations (> 400 mg/dl) a nd in plasma from anticoagulated patients. The systematic overestimate in the latter samples could be a result of an increased fibrin gel tu rbidity, as shown by in-vitro experiments using purified fibrinogen cl otted by different thrombin concentrations. The PFC overestimate by th e prothrombin-time-derived method could also be experimentally reprodu ced by competitively inhibiting thrombin-fibrinogen interaction by hir udin 54-65 peptide and the fibrinogen fragment E. A similar qualitativ e result was also found for the prothrombin-time-derived method in the presence of the Gly-Pro-Arg-Pro peptide, which competitively inhibits the end-to-end fibrin aggregation process. Notably, under both the ab ove experimental conditions, the Clauss method underestimated the PFC. On the other hand, the 'clot recovery' method was minimally affected by the above inhibitors. These results indicate that the prothrombin-t ime-derived method is accurate and precise for most routine purposes. Its precision seems inadequate, however, under those conditions where the prothrombin time is prolonged (such as anticoagulant therapy) and in the presence of high fibrinogen levels. Blood Coag Fibrinol 9:251-2 59 (C) 1998 Lippincott-Raven Publishers.