Protein S and C


Protein C and protein S are glycoproteins, predominantly synthesized in the liver, that are important components of the natural anticoagulant system in the body.[1][2] They are Vitamin K dependent and serve as essential components in the maintenance of physiologic hemostasis.[1]

Deficiency of protein C and protein S results in the loss of these natural anticoagulant properties, thereby resulting in unchecked thrombin generation, leading to thromboembolism.

Etiology and Epidemiology

Protein C and S deficiencies can be secondary to inherited gene mutations or due to acquired causes.[3][1][4] The majority of the inherited forms are secondary to missense mutations (60% to 70%) followed by smaller percentages (1% to 15%) of nonsense mutations, splice site mutations, large deletions, small deletions/duplications/insertions, and point mutations.[5]

Protein C Deficiency


In the healthy general population, asymptomatic protein C deficiency incidence is reported to be 1 in 200 to 500 individuals, while clinically significant venous thromboembolism is estimated to occur in 1 in 20,000 individuals.[6] No clear racial or ethnic predispositions are known.[7]


1. Inherited form: This is typically an autosomal recessive disorder; however, de novo mutations have been reported as well. Around 160 mutations in the protein C gene (PROC) located on chromosome 2q14.3 have been described in the literature.[8] 

  • i) Type I: Low protein C antigen level and activity levels[9]
  • ii) Type II: Normal protein C antigen level, but low protein C activity levels[9]

2. Acquired form: Newborns have physiologically low levels of protein C at birth and have been reported to be as low as 35% in healthy full-term infants. This is an age-related acquired form of protein C deficiency, which increases to the lower level of adult reference range by 6 to 12 months of age.[10] Other causes of acquired protein C deficiency include inflammation/infection, liver disease, chemotherapy/malignancy, disseminated intravascular coagulopathy, and vitamin K deficiency or use of vitamin K antagonist medications.[7][11][12] Of note, warfarin results in a transient procoagulant state with a reduction of protein C levels, and results in a small risk of severe warfarin-induced skin necrosis in patients that have an underlying hereditary protein C deficiency.

Protein S Deficiency [13][14]

Epidemiology: The exact prevalence is not known. However, some studies have estimated the prevalence of 0.03% to 0.13% in healthy individuals.[14]


1. Inherited form: This is typically an autosomal dominant disorder. The PROS1 gene is located on chromosome 3q11.1, and about 200 mutations in this gene have been described in the literature.[15] The defect is quantitative in type I and III, but qualitative in type II.[14] 

  • i) Type I: Low total protein S level, low free protein S level, and protein S activity. This is the most common form. 
  • ii) Type II: Normal protein S level (both free and total), but low protein S activity levels. This is a rare form. 
  • iii) Type III: Normal total protein S level, but low free protein S level and low protein S activity.

2. Acquired form: Newborns have low levels of protein S at birth, which increases to the adult reference range by 6 to 10 months of age, typically sooner than protein C levels.[16] Other acquired causes include liver disease, infection, inflammation, nephrotic syndrome, disseminated intravascular coagulopathy, chemotherapy, malignancy, pregnancy, oral contraceptive use, hormone replacement therapy, vitamin K deficiency, and use of vitamin K antagonists.[11][7][17] Though warfarin-induced skin necrosis is typically described in protein C deficiency, rare cases in protein S deficiency have been reported in the literature as well.[18]


Protein C and protein S are primarily synthesized in the liver. Protein S is also synthesized by platelets, endothelial cells, osteoblasts, vascular smooth muscle cells, and circulates in plasma.[7] 

Protein C is activated by the thrombin-thrombomodulin complex, to form activated protein C (APC) on the surface of the vascular endothelial cells.[1] Once protein C is activated, free protein S in the plasma serves as a cofactor along with phospholipids and calcium, to inactivate factor V and factor VIIIa at specific polypeptide arginine cleavage sites.[1] This results in impaired prothrombin activation, thereby exerting their anti-coagulant action by reducing thrombin generation. About 60% to 70% of protein S is in a bound form, noncovalently attached to C4-binding protein (CBP).[19][20] This protein S-CBP complex enhances the cleavage of activated factor Va, but not as effectively as free protein S.[20][21] Protein S also enhances the effects of APC in fibrinolysis. Protein S also exerts APC independent effects as well by direct inhibition of the tenase & prothrombinase complex, acting as an important cofactor to tissue factor pathway inhibitor (TFPI) during the inactivation of activated factor X and further inhibiting thrombin generation.[14]  

In the setting of protein C or protein S deficiency, the coagulation cascade continues unchecked with the overactivity of factor V and factor VII, resulting in excessive thrombin production.[1][2][21]

Mutations to factor V(G1691A) in the activated protein C resistance disorder can prevent the deactivation from happening even in the presence of protein C and S, promoting blood clotting.[3][22] The resistance comes from a single nucleotide point mutation of adenine to guanine, further changing the polypeptide arginine to glutamine at the cleavage site of factor V and causing resistance to cleavage.[3]

Specimen Requirements and Procedure

Protein C  and S - antigen and activity levels are usually done by collecting a venous blood sample in citrate. They are centrifuged in the laboratory to separate plasma. The plasma is frozen in aliquots and stored at -80 °C until analysis. 

The typical volume of plasma required is 0.5 ml per 2.7 mL. The plasma needs to be frozen within 4 hours of collection. 

The patient should be off warfarin for at least 2 weeks before drawing the sample.

It is important to perform the testing several weeks after an acute thrombosis or inflammation to allow the levels to return to baseline.[1]

Diagnostic Tests

Protein C Deficiency [23][13]

  • Protein C functional assay - This is the preferred assay in the clinical setting, as this can help identify both type I and type II disorders. Available options include factor Xa based, activated partial thromboplastin time (aPTT) based, or a chromogenic assay
  • Total protein C - Measured by immunoassay. This helps distinguish type I and type II deficiencies. 
  • Mutational analysis - PROC1 mutation testing is done once the initial testing is suggestive of underlying protein C deficiency. This can help provide genetic counseling to patients and to understand the natural history of the disease. 

Protein S Deficiency [23][13]

  • Total protein S - Measured by immunoassay. Other detection methods include ligand-based or monoclonal antibody-based methods. 
  • Free protein S - Measured by immunoassay. Antibody-based methods are also used in some laboratories. 
  • Protein S functional assay - Measured by a clot-based assay. The amount of protein S activity is proportional to the time to a clot formation. 
  • Mutational analysis - PROS1 mutation testing is done once the initial testing is suggestive of underlying protein S deficiency.

Interfering Factors

Interfering factors include lupus anticoagulant, factor V Leiden mutation, APC resistance, elevated plasma factor VIII levels, and hyperlipidemia.[21]

Functional protein S assays should be employed alongside the free protein S immunoassays due to various interferences during testing.[19] These interferences in laboratory testing may disrupt analysis, resulting in false positive or false negative outcomes.[24]

Results, Reporting, Critical Findings

Normal reference ranges for protein C and S are as follows:[25] (These are the reference ranges listed in Nathan & Oski textbook of pediatric hematology, 8th edition)

Protein C [IU/dL, mean(range)]

  • 1-5 years: 66 (40-92)
  • 6-10 years: 69 (45-93)
  • 11-16 years: 83 (55-110)
  • Adult: 96 (64-128)

Total protein S [IU/dL, mean(range)]

  • 1-5 years: 86 (54-118)
  • 6-10 years: 78 (41-114)
  • 11-16 years: 72 (52-92)
  • Adult: 81 (60-113)

Free protein S [IU/dL, mean(range)]

  • 1-5 years: 45 (21-69)
  • 6-10 years: 42 (22-62)
  • 11-16 years: 38 (26-55)
  • Adult: 45 (27-61)

Clinical Significance

Patients with hereditary defects of the protein C and protein S pathway are prone to develop thromboembolic events such as deep venous thrombosis, pulmonary embolism, stroke, and organ ischemia.[23][7][26] Venous thromboembolism is more common than arterial. Patients who inherit heterozygous alleles for protein C or protein S deficiency will present with an onset later during adulthood when compared to individuals who inherit homozygous alleles, which will present critical blood clotting complexities at birth such as purpura fulminans.[7][4] Patients are also at risk for thromboembolism during high estrogenic states such as pregnancy and oral contraceptive use.[27] Treatment

The long-term treatment for protein C and S deficiencies is anticoagulation with heparin bridged to warfarin. The medications should overlap for five days until the therapeutic range of the international normalized ratio (INR) of 2.0 to 3.0 is reached for two consecutive days.[14][28][7] Protein C concentrate can be used as replacement therapy for protein C deficiency.[7] In homozygous newborns suffering from hemorrhagic and thrombotic complications of purpura fulminans, protein C concentrate in the form of fresh frozen plasma can be given.[7]

The warfarin dose should be carefully assessed and bridged with a therapeutic dose of heparin as it can impose warfarin-induced skin necrosis in protein C and S deficiency. Warfarin mechanism of action is carried out by inhibiting the Vitamin K dependent clotting factors and protein C, and S. Warfarin-induced skin necrosis transpires due to the relatively short half-life of protein C and S, which are inhibited first when warfarin is administered. This further promotes the procoagulant effects of other vitamin K-dependent clotting factors and forms microthrombi.[29][30]


Patients who have been prescribed warfarin for long-term use warrant recurrent monitoring to confirm the medication is in optimal range and benefits exceed the risk of harm.[28] The medication should be carefully assessed regularly, so the INR is in the therapeutic range.[28]

Enhancing Healthcare Team Outcomes

Protein C and S are glycoproteins synthesized in the liver, which function to maintain the physiologic function of coagulation within the body. When mutated or dysfunctional, they can cause symptoms of blood clotting in all ages, with onset from birth to late adulthood. These thrombophilias prompt care from interprofessional healthcare teams, which includes primary care physicians, hematologists, nurses, and pharmacists. This team-based approach provides an integrated, evidence-based strategy to treat patients with symptomatic thrombophilias and monitor asymptomatic thrombophilias. The interprofessional team should be up-to-date with the latest management guideline for anticoagulation use and should monitor the INR regularly to maintain therapeutic ranges. Patients should be educated on their disease, the importance of medication compliance, and factors that may interfere with medication to cause sub-therapeutic or toxic levels. Genetic counseling should be offered to at-risk patients with a history of thrombophilia or a family history of the disease. The interprofessional team should be able to inform their patients about the risk and probability of the disease to the offsprings. The care of protein C and protein S deficiency is most beneficial when managed in an interprofessional team strategy to form a therapeutic alliance and enhance patient-centered care to achieve the desired outcome.[27][13][23]

Article Details

Article Author

Inderbir S. Padda

Article Author

Poras Patel

Article Editor:

Divyaswathi Citla Sridhar


5/8/2022 1:43:43 AM

PubMed Link:

Protein S and C



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