Standard Operating Procedure: Fibrinogen Testing (Clauss Method)

1. Purpose and Scope

To measure functional (clottable) fibrinogen by the Clauss method for the diagnosis and monitoring of disseminated intravascular coagulation (DIC), inherited hypofibrinogenemia and dysfibrinogenemia, and pre-procedure hemorrhagic risk.

2. Specimen Requirements

• Type: Citrated plasma (3.2% sodium citrate, 1:9 ratio), double-centrifuged to obtain platelet-poor plasma.
• Volume: Minimum 1.0 mL (2.7 mL tube filled to the line).
• Filling: Underfilling or overfilling alters the citrate:plasma ratio and is the most common preanalytical error; reject such tubes.
• Timing: The sample must not clot, and transport must avoid platelets; a clotted sample is rejected.
• Stability: Platelet-poor plasma stable 4 hours at room temperature, longer refrigerated, in a capped tube.

3. Safety and Precautions

• Standard PPE (gloves, lab coat). Blood is potentially infectious.
• Dispose of sharps and biohazard waste per laboratory policy.
• Do not mouth-pipette.

4. Equipment and Reagents

• Automated coagulation analyzer (optical or electromechanical end-point detection).
• Method: Clauss — high-concentration thrombin reagent added to diluted plasma (standard dilution such as 1:10 in Owren buffer); clotting time is inversely related to fibrinogen concentration, read against a calibration curve.
• Two levels of quality control material (low and normal fibrinogen).

5. Step-by-Step Procedure

1. Confirm the plasma is platelet-poor and the tube was filled correctly.
2. Load plasma and reagents on the analyzer per instrument instructions.
3. Run low and normal fibrinogen controls before patient samples.
4. Test the diluted plasma (Clauss) with high-concentration thrombin, reading the clot time.
5. Interpolate the fibrinogen concentration from the calibration curve; verify linearity across the standard range.
6. For results outside the linear range, re-test with adjusted dilution.
7. Release results after QC acceptance.

6. Quality Control

• Run low and normal controls daily, with every calibration, and each new reagent lot.
• Enforce Levey-Jennings rules; participate in an external quality assessment program.
• The Clauss method is sensitive to reagent thrombin strength and to fibrinogen degradation products; validate each lot.

7. Decision limits

The interpretive thresholds below are clinical decision limits; the canonical adult reference interval is displayed in the Reference Ranges panel.

• Low fibrinogen (below ~150 mg/dL): associated with bleeding risk; seen in DIC, severe liver disease, massive transfusion, and inherited deficiency.
• DIC supportive pattern: low fibrinogen plus low platelets, prolonged PT and APTT, and elevated D-dimer supports DIC according to the ISTH scoring system.
• Elevated fibrinogen: above the upper interval is a nonspecific acute-phase response (inflammation, pregnancy, malignancy) and reduces hemodilution risk interpretation.
• Method note: the Clauss assay measures functional fibrinogen; in dysfibrinogenemia the functional result may be low while the antigenic (immunologic) fibrinogen is normal. Suspect dysfibrinogenemia when the fibrinogen is low but the PT/APTT are disproportionately prolonged or the thrombin time fails to normalize.