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Factor IV – The ‘Coagulation Factor’ That Isn’t a Protein – ICHCC
ICHCC/LinkedIn
Aug 18, 2026, 19:45

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:

  1. Citrate removes available Ca²⁺
  2. Laboratory adds Ca²⁺ back
  3. 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.”

ICHCC

Other posts about Iranian Comprehensive Hemophilia Care Center on Hemostasis Today.

aPTT Blood Calcium coagulation cascade Factor IV Factor IX Factor Va Factor VIII Factor Xa Fibrin Health Hematology Hemostasis Hemostasis News Hemostasis Today ICHCC Iranian Comprehensive Hemophilia Care Center Medicine 𝗣𝗹𝗮𝘀𝗺𝗮 Prothrombinase complex PT Tenase complex Thrombin
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