Fibrinogen assay (Clauss method)

Principle

• Diluted plasma is clotted with a high concentration of thrombin. With thrombin in excess, clotting time depends on fibrinogen concentration and is inversely proportional to it.
• Results are read from a calibration curve prepared from a reference plasma with known fibrinogen.

Specimen

• 3.2% sodium citrate, 9:1 ratio; platelet-poor plasma.

Reagents and equipment

• Thrombin reagent (about 100 NIH U/mL), Owren's veronal (imidazole) buffer, calibrated reference plasma, coagulation analyser.

Procedure

• Prepare a calibration curve using serial dilutions of reference plasma (e.g. 1:5 to 1:40); plot clotting time against fibrinogen on log–log paper.
• Dilute patient plasma 1:10 in buffer, warm to 37 °C, add thrombin and record clotting time.
• Read the result from the curve; re-test at 1:5 for very low values or 1:20 for high values and correct for dilution.

Interpretation

• Low: DIC, liver disease, massive transfusion, thrombolytic therapy, L-asparaginase, congenital afibrinogenaemia or hypofibrinogenaemia.
• High: acute phase response (infection, inflammation, malignancy), pregnancy, smoking; a cardiovascular risk marker.
• Dysfibrinogenaemia: low Clauss activity with normal or higher antigen (immunological) level; ratio helps diagnosis.
• In bleeding patients a level below about 1.5 g/L often prompts cryoprecipitate or fibrinogen concentrate.

Normal values

• Typical adult range about 2.0–4.0 g/L (200–400 mg/dL); local ranges apply. Levels rise in pregnancy.

Quality control and pitfalls

• Very high heparin, direct thrombin inhibitors (dabigatran) and high FDPs can falsely lower results with some reagents.
• PT-derived fibrinogen can be falsely high in DIC and liver disease; Clauss is preferred.
• Run normal and low controls; recalibrate with new reagent lots.

Clinical use

• DIC scoring, bleeding and massive transfusion management, obstetric haemorrhage, liver disease, thrombolysis monitoring.