Factor VI does not exist as a distinct, independent coagulation factor. The substance originally designated factor VI, or accelerin, was later identified as activated factor V, now designated factor Va.
The designation factor VI was therefore withdrawn, and the Roman numeral was never reassigned. This explains why the coagulation factor list skips from factor V to factor VII.
How Were the Coagulation Factors Numbered?
In 1958, the International Committee for the Standardization of the Nomenclature of the Blood Clotting Factors standardized the naming of plasma procoagulants using Roman numerals, according to the order of their initial description or discovery.
These numerals are historical designations. They do not represent the sequence in which the factors become activated or function during coagulation.
Factor VI was initially included as a separate designation. Once it was identified as activated factor V, the designation was withdrawn, leaving a gap in the numbering system.
What Was Factor VI?
During early investigations of blood coagulation, researchers identified a substance that accelerated the conversion of prothrombin to thrombin. It was initially considered a separate procoagulant and designated factor VI, or accelerin.
Subsequent studies showed that this substance was not an independent coagulation factor. It was the activated form of factor V, also known as proaccelerin or the labile factor. Recognizing that relationship eliminated the need for a separate factor VI designation.
Factor V, Factor Va, and Factor VI
| Designation | Meaning |
|---|---|
| Factor V | Proaccelerin, also called the labile factor |
| Factor Va | The activated form of factor V |
| Factor VI | The retired designation for what was identified as factor Va |
The lowercase a in factor Va indicates activation, not a new factor number. Factor Va is an activated cofactor, not an enzyme.
REMEMBER
Factor VI was withdrawn because it was identified as activated factor V. The current designation is factor Va. The numeral VI was never reassigned.
Why This Matters in MedTech
This is a nomenclature distinction, not evidence that a separate coagulation protein is missing. When a question asks which factor designation was dropped because it represented activated factor V, the answer is factor VI, or accelerin. When it asks for the current designation of activated factor V, the answer is factor Va.
References
Rodak’s Hematology: Clinical Principles and Applications. 7th edition. Chapter 35.
Hematology by Harmening. Part 2. Chapter 25, “Nomenclature of Coagulation Factors,” Box 25-4.
