Working to Turn Off Factor Clotting

Protein C (PC) is a vitamin K-dependent protein. It is produced in the liver and through its activated form helps to stop the clotting activity of FVII. Thrombin activates protein C. The resulting activated protein C (APC) then works to shut down the clotting ability of activated FV and FVIII. In addition to its role in blood clotting, activated protein C is also involved in controlling inflammation and cell protection.


Protein C levels are dependent on the patient’s age and other medical conditions, with adult levels being reached at late adolescence. Several conditions can reduce protein C levels, including:

  • Liver disease

  • Complications during pregnancy or labor and delivery

  • Respiratory problems in the newborn

  • Blood clots

  • Gastrointestinal problems

  • Disseminated intravascular coagulation

  • Oral contraceptives or other medications

Protein C deficiency is an inherited condition and occurs in 1.4% to 8.6% of the population. In a study of healthy people, the frequency of the deficiency was found to be 1 in 200 to 1 in 300, while a study of almost 10,000 blood donors found a frequency of 1 in 500 to 1 in 700.


Protein C deficiency is divided into Type I or Type II deficiency.

  • Type 1: caused by low levels of normal protein C

  • Type 2: caused by an abnormally functioning protein

Homozygous protein C deficiency (where a person has two abnormal protein C genes, one from each parent) is usually evident in newborn infants. The first sign is often a rare and potentially catastrophic skin condition called purpura fulminans. Laboratory testing of babies with this condition reveals a severe deficiency (protein C levels of <1% of normal).

Some infants who do not have neonatal purpura fulminans but still have low levels of protein C (5% to 20%) often have a tendency to clot excessively at an early age. These patients require lifelong treatment with a blood-thinning medication to prevent recurrent blood clots.