Our November 2025 Quick Question asked, “Which is the numeral for the Stuart-Prower factor? Here are the results from our 31 participants:
- V: 0 (0%0
- VII: 3 (10%)
- X: 19 (61%, correct)
- XI: 5 (16%)
- XII: 4 (13%)
We scored well. Beginning with the International Committee on Nomenclature’s 1958 recommendations, we began using Roman numerals to identify coagulation factors. By the 1990s, we had essentially dropped the earlier nomenclature, with a few exceptions. In the mid-1950s, before being designated factor X (FX, ten), the enzyme, originally termed “thrombokinase,” earned the name Stuart-Prower factor when individuals from the Stuart and Prower families with bleeding disorders were shown through in vitro clotting techniques to lack the factor when a mixture of plasmas from both failed to correct the clotting time.
Navigate by this link to Dave McGlasson‘s article, “Do We Need To Standardize Coagulation Nomenclature In Today’s Laboratory Environment?” The article provides a discussion of coagulation factor terminology, including a list of the functional and proper noun names. For a history of the identification and naming of coagulation factors, go to your medical library to obtain: Milstone LM. Factor Xa: Thrombokinase from Paul Morawitz to J Haskell Milstone. J Thromb Thrombolysis. 2021;52:364–70. doi: 10.1007/s11239-021-02387-6. PMID: 33484373.
As always, we welcome your comments below.
Nomenclature in hemostasis is one of the leading perennial questions. Factor nomenclature is just one facet of this complexity. In our practice, the most common mix-ups are factor X for anti-Xa (drug) testing, factor V for factor V Leiden, and factor II for the prothrombin gene mutation. Probably, the latter two are the most common, and we do a lot of factor V and factor II clotting assays as part of the requested thrombophilia tests (e.g., protein C, protein S, and lupus anticoagulant). We have given up trying to get clarification from the test requestors. We just don’t have the manpower to spend 2 hours on the phone every day. The main issue we have with anti-Xa orders is that the intended drug is often missing from the test request. We have ‘fixed’ this for electronic orders since we force the test request to select a drug from a drop down menu, but this can be circumvented by a paper request. In 2025, we may do anti-Xa testing for half a dozen different drugs, including apixaban and rivaroxaban. I’m not convinced that changing roman numerals to standard numbers (e.g., 2, 5, 10) will fix these problems. We should also remember that italicized numbers are generally used to denote the genes for these clotting factors (e.g., italicized F2, F5, F10). Another big nomenclature issue is von Willebrand factor (VWF) testing. Here, there are published recommended nomenclatures from various ISTH committees, and still most people get it wrong. Whenever I review a paper where authors have listed ‘VWF activity’ without denoting what activity assay was actually used, it automatically gets a big red flag and suggests a clear lack of understanding from the authors. In regards to the post from Dave McGlasson, if the table from Hamilton is correct, FV and F5 is used for factor V Leiden, which could also cause order errors.