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Fibrinogen Assays

Here is a fibrinogen assay question from Cara Calvo:

Hi George, I was the program director of the CLS program at Ohio Northern University in Ada, OH until this past November when I accepted a faculty position here at University of Washington.

Anyway, as I was reviewing some documents used in the UW coagulation course I will teach in the fall, I noted that one of the procedures was entitled Kinetic Fibrinogen Assay: Clauss Method. As I read the procedure it appeared to be the classic modified thrombin clotting time (TCT).

I find this confusing because I thought the kinetic fibrinogen assay and the Clauss procedure were two different methodologies. Can you clear this conundrum up for me – are these two different methodologies or not? Thanks!

Cara Calvo, MS, MT(ASCP), SH(ASCP)
Lecturer, Medical Technology Program
UW Department of Laboratory Medicine

Hi, Cara. Congratulations on your move to UW. That’s a big distance!

You are correct, the kinetic fibrinogen assay (KFA) is different from the Clauss (or von Clauss) method. There is a great reference describing both: Tan V, Doyle CJ, Budzynski AZ. Comparison of the kinetic fibrinogen assay with the von Clauss method and the clot recovery method in plasma of patients with conditions affecting fibrinogen coagulability. Am J Clin Pathol 1995;104:455-62.

I’m most familiar with the Clauss method, which as you write, is based on the TCT. In the TCT, thrombin at 1-2 U/mL is added to undiluted plasma and the mixture is timed to clot formation. The thrombin time is mostly used to screen for plasma heparin.

In the Clauss quantitative fibrinogen method, plasma is diluted 1:20 (or thereabouts) to eliminate the effects of potential thrombin inhibitors. Then thrombin is added at 50 or 100 U/mL and the mixture is timed to clot formation. The interval is compared to a standard curve and reported as mg/dL fibrinogen. The Clauss method is sensitive to both hypofibrinogenemia or dysfibrinogenemia. I routinely use the Clauss method as a student laboratory exercise.

The KFA measures fibrinogen by recording the velocity of thromboplastin-generated clot formation. Automated coagulometers that generate prothrombin time (PT) results through photometry or nephelometry are able to estimate fibrinogen activity by recording delta absorbance during clot formation and comparing it to a standard curve. I recall at least one optical-based automated PT/PTT instrument that reports a fibrinogen with every PT.

I hope this helps. By the way, the third method described in the article is a World Health Organization reference method in which an in vitro-induced clot is washed, dissolved in a urea solution, and measured spectrophotometrically. No one would choose this assay in a clinical lab, too much work. Geo

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