Factor IV – The ‘Coagulation Factor’ That Isn’t a Protein – ICHCC
Iranian Comprehensive Hemophilia Care Center (ICHCC) shared a post on LinkedIn:
“Factor IV: The ‘Coagulation Factor’ That Isn’t a Protein
Did you know that Factor IV is part of the historical coagulation factor numbering system, but unlike Factors VIII or IX, it is not a protein?
What is Factor IV?
Factor IV is Calcium (Ca²⁺)
Calcium is an essential cofactor in multiple steps of the coagulation cascade.
It helps coagulation factors assemble on negatively charged phospholipid surfaces and is particularly important for the formation and function of several coagulation complexes.
Where Does Calcium Act?
Ca²⁺ is essential for important coagulation complexes, including:
- Tenase complex
- Prothrombinase complex
For example:
Factor Xa, Factor Va, calcium ions, and phospholipids combine to generate thrombin.
Then:
Thrombin leads to Fibrin formation
Without adequate calcium availability, these reactions cannot proceed normally.
Why Is Calcium Important in Coagulation Testing?
Why do we collect coagulation samples in Sodium citrate tubes?
Because citrate chelates ionized calcium, temporarily preventing clot formation in the blood sample.
When performing:
- PT
- aPTT
the laboratory adds calcium back to the plasma to initiate the coagulation reaction.
So, conceptually:
- Citrate removes available Ca²⁺
- Laboratory adds Ca²⁺ back
- Coagulation can then be measured
Is There a ‘Factor IV Deficiency’?
Not in the same sense as Factor VIII or Factor IX deficiency.
There is no recognized inherited Factor IV deficiency disorder comparable to classic hemophilia.
Severe hypocalcemia can affect hemostasis, particularly in critically ill patients, but it does not typically present as a classic inherited bleeding disorder.
Clinical Pearl
Factor IV is calcium – not a coagulation protein.
It is a fascinating reminder that the historical coagulation factor numbering system does not represent a list of proteins alone.
Not every ‘coagulation factor’ is a protein.”
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