Protein C: A Natural Anticoagulant and the Risk of Thrombosis

Protein C is one of the main natural blood anticoagulants. It is synthesized in the liver, circulates in an inactive form, and, once activated, begins to limit excessive clotting, helping the body maintain a balance between bleeding and thrombosis. Its clinical interest is primarily related not to bleeding, but to the increased susceptibility to pathological thrombus formation associated with its deficiency. [1]

The most important practical idea is that low protein C alone does not indicate a hereditary disorder. A decrease can be congenital, but it is much more often acquired and associated with liver disease, vitamin K deficiency, severe infection, disseminated intravascular coagulation, certain medications, and other conditions. Therefore, the test is always evaluated in a clinical context, not as a standalone number. [2]

Protein C is typically tested as part of the evaluation of thrombophilia, that is, a congenital or acquired predisposition to venous thrombosis. The most common inherited thrombophilias include antithrombin, protein C, and protein S deficiencies, as well as the factor V Leiden mutation and the prothrombin variant G20210A. However, even if thrombophilia is suspected, testing should be meaningful, because the results do not always change treatment. [3]

It's especially important for the reader to distinguish between two objectives. The first is to determine whether the patient has a persistent protein C deficiency. The second is to avoid confusing a true hereditary deficiency with a temporary decrease due to illness, pregnancy, medication, or acute thrombosis. Most diagnostic errors occur at this stage. [4]

The modern approach to protein C testing has become more cautious. Guidelines do not recommend routine testing of everyone after any venous thrombosis. The test is useful when the results can truly impact the duration of prophylaxis, family risk assessment, pregnancy planning, the choice of treatment strategy for newborns, and rare decisions regarding replacement therapy in severe congenital deficiency. [5]

What is protein C and how does it work?

Protein C is a vitamin K-dependent plasma serine protease. Under normal conditions, it circulates in an inactive form. When thrombin binds to thrombomodulin on the endothelial surface, protein C is activated, forming activated protein C. It is this active form that performs the primary anticoagulant function. [6]

The primary function of activated protein C is to cleave and inactivate factors Va and VIIIa. This reduces thrombin generation and weakens the coagulation cascade. Clinically, this means that protein C serves as a natural brake on the hemostatic system and protects the body from excessive thrombus formation. [7]

Protein C does not work alone. For full anticoagulant activity, it requires protein S as a cofactor. This is why protein C and protein S tests are often ordered together: these proteins are functionally related, and their deficiency can lead to similar clinical consequences. [8]

Protein C levels vary with age. In newborns and young children, values are physiologically lower than in adults, gradually reaching adult levels later. This is important because pediatric results cannot be automatically interpreted using adult reference values. Results in full-term and premature infants are evaluated with particular caution. [9]

Clinically, protein C deficiency is primarily associated with venous thrombosis. The association with arterial thrombosis in adults is considered much less certain, and routine testing for hereditary thrombophilia in patients with arterial thrombosis is not recommended. Therefore, protein C testing is primarily needed in the context of venous thromboembolism, rather than as a general test for any thrombosis. [10]

Table 1. The role of protein C in the hemostasis system
Where is it synthesized?	Mainly in the liver
What does synthesis depend on?	From sufficient supply of vitamin K
In what form does it circulate?	Inactive
Where is it activated?	On the surface of the endothelium through the complex of thrombin and thrombomodulin
The main action of the active form	Inactivation of factors Va and VIIIa
Clinical outcome of normal functioning	Limiting excess clotting
Clinical outcome of deficiency	Increased susceptibility to venous thrombosis

Sources for the table. [11]

Congenital and acquired protein C deficiency

Protein C deficiency can be congenital or acquired. The congenital form is associated with mutations in the PROC gene. Mild or moderate inherited deficiency typically affects one copy of the gene, while severe forms involve two abnormal variants and become apparent in the neonatal period. [12]

Hereditary deficiencies are divided into two main types. In type 1 deficiency, both functional activity and antigen quantity are reduced, meaning the protein is actually low. In type 2 deficiency, the antigen may be normal, but activity is reduced because the protein is present but malfunctioning. This distinction is important for laboratory interpretation but does not always significantly alter the clinical risk. [13]

Hereditary deficiency is rare, and severe clinical forms are even rarer. According to reviews, milder forms may occur in approximately 1 in 200-500 people, while severe congenital deficiency is extremely rare. However, the presence of a laboratory deficiency does not necessarily mean that thrombosis will develop in every carrier: some people remain asymptomatic for years. [14]

Acquired deficiency is much more common. It can be caused by liver disease, vitamin K deficiency, severe infections, malignant tumors, disseminated intravascular coagulation, certain chemotherapy drugs, kidney disease, and especially treatment with vitamin K antagonists, primarily warfarin. Therefore, the first decrease in protein C in an adult patient is almost never immediately considered hereditary. [15]

It is the distinction between congenital and acquired deficiencies that determines the diagnostic logic. First, temporary and secondary causes of decline are excluded, then, if necessary, the test is repeated in a stable state, activity and antigen are compared, and genetic testing is used selectively when the result will actually influence the clinical decision. [16]

Table 2. Congenital and acquired protein C deficiency
Congenital deficiency	Associated with a variant of the PROC gene
Acquired deficit	Occurs due to illness or medication
Type 1	Both activity and antigen are reduced
Type 2	Reduced activity with normal antigen
Frequent clinical context of the congenital form	Young age, family history, recurrent venous thrombosis
Frequent clinical context of the acquired form	Liver disease, vitamin K deficiency, severe infection, warfarin
Particularly severe form	Neonatal purpura fulminans with severe congenital deficiency

Sources for the table. [17]

When is the test prescribed and who really needs it?

Protein C testing is not a routine test for all patients with any thrombosis. It is ordered when there is reason to suspect thrombophilia and when the results may change the patient's management. Classic indications include venous thrombosis in young adults without an obvious cause, recurrent episodes, thrombosis in unusual locations, and a strong family history. [18]

Unusual locations include, for example, thromboses of the veins of the liver, kidneys, or brain. Such situations do not automatically indicate protein C deficiency, but require a more thorough evaluation. However, current guidelines emphasize that even in unusual locations, hereditary thrombophilia is not automatically investigated, but rather in selected patients when there is no more obvious cause. [19]

A separate group are relatives of patients with severe hereditary deficiencies. They are not tested en masse on a "just in case" basis. Selective testing of first-degree relatives is recommended in situations where the results could impact life decisions, such as pregnancy management, choice of hormonal therapy, or prophylaxis during surgery and prolonged immobilization. [20]

In newborns and children, testing is especially important in cases of severe thrombotic manifestations. If a newborn has purpura fulminans or multiple unexplained thromboses, testing for protein C deficiency should be performed urgently, as the result may require immediate replacement therapy with protein C concentrate and anticoagulation. [21]

In contrast, after a first venous thromboembolism in an adult patient, routine testing for inherited thrombophilia is not recommended as a universal strategy. Guidelines assume that in many patients the result does not change treatment, and inappropriate timing of blood sampling and transient acquired changes create more confusion than benefit. [22]

Table 3. When is protein C testing warranted and when is it not?
Acquitted	Venous thrombosis in young people without obvious cause
Acquitted	Recurrent venous thrombosis
Acquitted	Thrombosis of unusual localization with an unclear cause
Acquitted	Severe family history of venous thrombosis
Acquitted	Newborn with purpura fulminans or multiple thromboses
Selectively acquitted	First-degree relatives in families with confirmed severe deficiency
Usually not needed routinely	Any first episode of venous thrombosis without clinical benefit from the outcome
Not suitable	Mass screening of healthy people

Sources for the table. [23]

When the analysis may be false and how to choose the right time for the study

The most common cause of erroneous conclusions is incorrect testing timing. If the test is performed too early after acute thrombosis, the results may be inaccurate. MedlinePlus explicitly states that after an episode of thrombosis, testing should be performed after recovery, and British guidelines recommend assessing physiological anticoagulant deficiency only after 3 months of anticoagulant therapy for acute thrombosis. [24]

Warfarin is another classic cause of diagnostic confusion. It reduces protein C activity, as protein C itself is a vitamin K-dependent protein. Therefore, while taking warfarin and for a period after stopping it, the test may appear to indicate a deficiency, although in fact it is a drug effect. Some laboratories explicitly recommend against performing the test if the patient has taken warfarin in the previous 2-4 weeks. [25]

Other anticoagulants can also interfere with the results. Clot-based functional tests are sensitive to direct oral anticoagulants and heparin, while chromogenic methods are generally less susceptible to interference. Therefore, when it is impossible to discontinue the anticoagulant, the choice of method becomes particularly important, and for this reason, chromogenic tests are often preferred as initial tests. [26]

Coexisting conditions are equally important. Liver disease, vitamin K deficiency, severe infections, malignant tumors, disseminated intravascular coagulation, and some acquired conditions can reduce protein C. In such situations, the test reflects the overall severity of the condition or secondary deficiency, and not necessarily hereditary thrombophilia. [27]

In children and newborns, age-specific interpretation is important. In healthy full-term and premature newborns, protein C antigen levels can be significantly lower than adult values, and this does not always indicate pathology. Therefore, pediatric testing requires knowledge of age-specific reference values and particularly careful interpretation. [28]

Table 4. What most often distorts the analysis results
Acute thrombosis	Makes the result less reliable
Warfarin	Decreases protein C and may mimic deficiency
Direct oral anticoagulants	Particularly affect some clotting methods
Heparin	May distort functional tests
Liver diseases	Cause secondary reduction
Vitamin K deficiency	Causes secondary reduction
Severe infection	May be accompanied by a decrease
Disseminated intravascular coagulation	It can sharply reduce the indicator
Newborn age	Requires separate age references

Sources for the table. [29]

How the study is conducted and how the activity and antigen are interpreted

Laboratory assessment of protein C involves two main approaches: activity testing and antigen testing. The functional test determines how well the protein functions, while the antigen test determines how much protein is present in the plasma. The combination of these two indicators helps determine whether there is a quantitative deficiency or a qualitative defect in the molecule. [30]

For the initial evaluation of suspected congenital deficiency, many laboratory sources recommend a functional activity test, not just an antigen test. Mayo Clinic Laboratories specifically states that a functional activity test is preferred as the initial test for suspected congenital deficiency. This is because in type 2, the protein may be present in normal amounts but functionally defective. [31]

If activity is decreased, the next step is an antigen assessment. When both activity and antigen are decreased, this corresponds to type 1. When activity is decreased but antigen is normal, type 2 is more likely. However, even this approach does not eliminate the need to repeat the analysis outside of acute and drug-induced conditions, lest a transient decrease be mistaken for a hereditary condition. [32]

Laboratory thresholds vary by method. Mayo Clinic Laboratories lists the reference interval for activity in adults as 70-150%, and values below approximately 60-70% may suggest a congenital deficiency if secondary causes are excluded. However, this is not a universal global norm, but rather a benchmark for a specific laboratory, so in practice, specific reference values for a specific method are always used. [33]

Elevated protein C levels generally have no proven clinical significance. MedlinePlus reports that elevated levels of protein C or protein S are not known to cause medical problems. This is important for everyday practice: clinical attention is almost always focused on the decrease or dysfunction, not the increase. [34]

Table 5. How to read the combination of protein C activity and antigen
Activity is normal, antigen is normal	A shortage is unlikely
Activity is reduced, antigen is reduced	Looks more like type 1 deficiency
Activity is reduced, antigen is normal	Looks more like type 2 deficiency
Activity is extremely low or undetectable	Severe congenital deficiency should be considered, especially in newborns.
A single decrease due to illness or warfarin	Does not confirm hereditary deficiency
Normal result in the stable phase	Makes severe, persistent deficiency less likely
Increased results	Generally not considered clinically significant

Sources for the table. [35]

What are the clinical manifestations associated with protein C deficiency?

In adults, the most common manifestation is venous thromboembolism, primarily deep vein thrombosis and pulmonary embolism. The risk is particularly increased when a hereditary predisposition is combined with precipitating factors: surgery, trauma, immobilization, pregnancy, use of estrogen-containing drugs, and active inflammation. Deficiency itself does not always lead to thrombosis, but it lowers the threshold for its development. [36]

Some carriers of the inherited deficiency may remain asymptomatic. This is a fundamental point: laboratory detection of a variant does not necessarily mean that the event is inevitable. Therefore, the clinical value of the test is determined not only by the number of results but also by family history, the patient's age, predisposing factors, and whether the result will influence preventive measures. [37]

In severe congenital deficiency, the picture is different. Newborns may develop purpura fulminans, skin necrosis, and widespread venous and arterial thrombosis, sometimes as early as the first day of life. This is not just a laboratory abnormality, but a life-threatening condition requiring emergency protein C replacement. [38]

Protein C deficiency also has important pharmacological implications. Patients with congenital deficiency have a higher risk of warfarin-induced skin necrosis, especially if warfarin is initiated without adequate parenteral anticoagulant coverage. This is because protein C declines faster than procoagulant factors, paradoxically creating a more thrombogenic state for a short time. [39]

The association with arterial thrombosis in adults remains less well-established than with venous thrombosis. Therefore, protein C is not considered a universal explanation for stroke or heart attack, and routine testing for hereditary thrombophilia in arterial thrombosis in adults is not supported by current guidelines. [40]

Table 6. Main clinical scenarios for protein C deficiency
Mild to moderate hereditary deficiency	May not give symptoms for a long time
Typical manifestation in adults	Venous thrombosis
Possible complication	Pulmonary embolism
Increased risk	Surgery, trauma, immobilization, pregnancy, estrogens
Particularly severe form in newborns	Purpura fulminans
Drug risk	Warfarin-induced skin necrosis
Role in arterial thrombosis in adults	Routinely not considered a proven basis for testing

Sources for the table. [41]

Treatment, prevention and patient management

If a patient has already experienced venous thrombosis, treatment is aimed primarily at the thrombosis itself, not just the protein C level. The Merck Manual indicates that symptomatic thrombosis in patients with protein C deficiency is treated with direct oral anticoagulants or warfarin. The choice of drug depends on the clinical situation, renal function, bleeding risk, comorbidities, and pregnancy plans. [42]

Particular caution is required when using warfarin. Due to the rapid decline in protein C at the beginning of therapy, a transient increase in thrombotic readiness and the development of skin necrosis are possible if the parenteral anticoagulant is discontinued prematurely. Therefore, initiation of treatment requires proper overlap and close monitoring, and in many cases, direct oral anticoagulants can circumvent this problem. [43]

In severe congenital deficiency, particularly in neonates with purpura fulminans, standard anticoagulation may be insufficient without replacement therapy. The U.S. Food and Drug Administration (FDA) lists human protein C concentrate as approved for the prevention and treatment of venous thromboembolism and purpura fulminans in patients with severe congenital protein C deficiency. This is one of the rare cases where a specific protein is truly replaced as a life-saving drug. [44]

Prevention in asymptomatic carriers is discussed on an individual basis. Continuous anticoagulation is not necessary for everyone. More often, situational prophylaxis is considered during surgery, prolonged immobility, pregnancy, and the postpartum period, as well as avoiding additional risk factors, including smoking and uncontrolled use of estrogen-containing drugs. Therefore, meaningful identification of deficiency can influence lifestyle and preventive measures even before the patient has experienced thrombosis. [45]

Genetic testing is not required for diagnosis in most patients. It is considered when a phenotypically confirmed deficiency exists and when the genetic response is likely to influence family counseling or clinical decisions. In routine practice, the basis for diagnosis remains repeated laboratory measurements at the appropriate time and sound clinical interpretation. [46]

Frequently Asked Questions

Can a single low result immediately diagnose congenital protein C deficiency?
No. A single low result does not prove a hereditary deficiency, as the result is affected by acute thrombosis, warfarin, liver disease, vitamin K deficiency, severe infection, and other acquired conditions. A repeat test during the stable phase is usually required. [47]

When is the best time to get tested after a thrombosis?
Usually not during the acute phase. British guidelines recommend testing for physiological anticoagulant deficiency only after 3 months of anticoagulant therapy following acute thrombosis, while MedlinePlus specifically notes that testing after an episode of thrombosis should be performed after recovery. [48]

Can I have the test while taking warfarin?
This is highly undesirable because warfarin reduces protein C and can create a false impression of deficiency. Some laboratories recommend not performing the test if warfarin has been taken in the previous 2-4 weeks. [49]

Which is more important: activity or antigen?
For initial evaluation, activity is often more important because it shows how the protein functions. Antigen helps clarify the type of deficiency: quantitative or qualitative. In practice, these two tests complement each other. [50]

Will a person with this congenital deficiency always develop thrombosis?
No. Some carriers remain asymptomatic and never experience clinical thrombosis. However, their risk is higher, especially with additional triggering factors, so prevention and awareness are important. [51]

Is elevated protein C dangerous?
Elevated protein C levels are not generally considered a clinically significant problem. Current literature does not link them to proven medical complications. [52]

When should severe protein C deficiency in a newborn be urgently considered?
In cases of purpura fulminans, widespread skin necrosis, multiple thromboses, and severe condition in the first hours or days of life, urgent examination and early replacement therapy with protein C concentrate are required. [53]

Should all relatives of a patient with protein C deficiency be tested?
No. Current guidelines do not recommend routine testing of all first-degree relatives. The approach should be selective and depend on whether the results will impact pregnancy, hormonal therapy, surgery, or other important preventive measures. [54]

Conclusion

Protein C is a vital natural anticoagulant, and its deficiency is primarily associated with venous thromboembolism. However, the key practical implication is that a low level does not automatically indicate inherited thrombophilia. A correct diagnosis requires the correct timing of testing, exclusion of secondary causes, and matching of functional activity with the antigen. [55]

The current clinical value of protein C testing lies not in mass screening, but in targeted use: in patients with suspected significant thrombophilia, in certain relatives from selected families, and in newborns with severe thrombotic manifestations. In cases of severe congenital deficiency, life-saving replacement therapy with protein C concentrate is possible, while for typical venous thromboses, the basis remains properly administered anticoagulation and prevention of recurrent events. [56]