From Kim Kinney at Clarian (Indiana University) Laboratories in Indianapolis:
My pathologist is asking about prothrombin time (PT)-based fibrinogen levels. I would like to hear from sites that use this test about the advantages and disadvantages over using the Clauss fibrinogen level. Thanks, Kim.
Hello, Kim, thanks for your question. The Clauss assay is a clot-based modification of the thrombin time in which the clotting time is inversely proportional to fibrinogen concentration. The PT-derived fibrinogen assay computes fibrinogen concentration from the change in OD at 405 nm during clot formation, and is available from automated optical instruments like the TOPS using Siemens or Beckman-Coulter (IL) thromboplastins.
Based on the most recent CAP reference I could find, Bovill EG, McDonagh J, Triplett DA,et al. Performance characteristics of fibrinogen assays. Results of the College of American Pathologists Proficiency Testing Program 1988-1991. Arch Pathol Lab Med. 1993;117:58-66, the Clauss assay is not well-standardized or reproducible, although it is the method used most often in acute care clinical labs. Studies comparing the PT-derived assay to Clauss report that the PT-derived method gives higher results than the Clauss on specimens from patients taking warfarin and that it may miss dysfibrinogenemia in, for instance, liver disease.
From limited experience, I think it is helpful to have a fibrinogen result with every PT performed, but that the PT-derived fibrinogen assay should be taken as semiquantitative and that abnormal results should be confirmed using the Clauss assay. The down side of this is that you could end up tracking down a lot of clinically insignificant fibrinogen values.
Here are a couple of references that are available as free full-text articles that you and your pathologist may find to be helpful. I hope to hear from someone at Beckman-Coulter, Siemens, and CAP about the PT-derived method. Also, CAP should somewhere have published more recent fibrinogen assay survey data. Geo