The normal protein C level in a healthy adult is approximately 65-135 IU/dL. For a healthy term infant, the average value is 40 IU/dL, which will increase as the child ages, to approximately 60 IU/dL; the adult range typically is not reached until after puberty. Activated protein C is found at levels approximately 2000 times lower than this.
Protein C deficiency is considered mild at plasma levels greater than 20 IU/dL but below the reference range. Moderate-to-severe deficiency is blood concentrations ranging from 1-20 IU/dL. Severe protein C deficiency is a value of less than 1 IU/dL or if protein C is not detectable. 
Elevated levels of protein C and/or protein S are not clinically significant and usually are not associated with medical problems. If the activity and concentrations of protein C and protein S antigens are normal, this usually indicates clotting regulation is adequate.
A low level of protein C or protein S activity can cause excessive or inappropriate blood clotting. If the protein is not functioning properly (ie, normal protein levels but improper function), insufficient regulation of the coagulation process ensues, which can result in an increased risk of clot development and vein blockage. The severity of the risk is dependent on the magnitude of the deficiency and/or the degree of dysfunction of the protein.
The tests used for protein C testing can include enzyme-linked immunosorbent assay (ELISA), chromogenic assays, and the activated partial thromboplastin time (aPTT)–based functional protein C assay. (A thrombin-generation–based test has also been shown to detect protein C deficiency.) However, no single test for protein C is 100% sensitive or specific for detecting abnormalities.
An ELISA measures protein C immunological levels with very high sensitivity but cannot detect functional defects. ELISAs show differences in specificity, especially in clinical samples (eg, from patients on vitamin K antagonists). Some ELISAs may recognize protein C complexed to its inhibitor, but others may not.
Chromogenic assays can detect low levels of protein C with high sensitivity and can detect most functional defects. Those not detected include impaired phospholipid binding due to a mutation in the Gla domain, because the chromogenic assay is phospholipid independent. Similarly, the chromogenic assay is not dependent on the presence of protein S.
The aPTT-based functional protein C assay may yield misleadingly low protein C levels in the presence of (1) a factor V Leiden mutation and some other causes of activated protein C resistance, (2) elevated plasma factor VIII levels, or (3) hyperlipidemia. In addition, patients with lupus anticoagulants or those on direct thrombin inhibitors may have falsely normal results from an aPTT-based functional assay.
The following are causes of apparently low protein C levels:
Factor V Leiden
Elevated plasma factor VIII levels
Other causes of activated protein C resistance
Other factor deficiencies - Decreased antithrombin and inherited conditions such as factor V Leiden or a prothrombin 20210 mutation; if present, the effects of a protein C or S deficiency can be exacerbated
The following are causes of genuinely low protein C levels:
Heterozygous protein C deficiency - Occurs in 0.2% of the population and in 3% of unselected patients with venous thromboembolism
Homozygous protein C deficiency - Rare condition
Acquired deficiency (much more common than hereditary deficiency) from conditions such as acute phase reaction, disseminated intravascular coagulation (DIC), liver disease, vitamin K antagonist use, sickle cell disease
If test results show decreased activity or a decreased quantity of protein C, the test should be repeated before a diagnosis is made. If an acquired deficiency is identified, protein C or protein S concentrations may be monitored occasionally as the underlying condition progresses or resolves. If an inherited deficiency is identified, monitoring is not usually necessary. However, the patient must be made aware of the risk of clotting when exposed to situations such as surgery, chemotherapy for cancer, or oral contraceptive use.
Collection and Panels
A blood sample is drawn from a vein in the patient’s arm.
The procedure involves only minor risk. Potential problems are minimal but may include infection (the skin barrier is broken), excessive bleeding, hematoma (blood accumulation under the skin), and possible fainting or light-headedness.
The patient’s venous thromboembolism should be treated, and the patient should be put on a limited course of anticoagulant therapy (often approximately 3-6 mo). Additionally, investigate for underlying diseases or conditions such as liver disease, vitamin K deficiency, or cancer that may cause inappropriate blood clotting (bleeding or thrombosis).
Before the test can be performed, at least 10 days should have elapsed since the thrombotic episode.
The patient should be off oral anticoagulant therapy for 2 weeks before the test is performed.
What is being tested?
The protein C test is usually performed as part of the evaluation to detect a possible clotting disorder. The protein S test, while usually performed for the same reason, is a separate test. Either test measures the amount of each protein and helps assess whether the protein is performing its proper function in the body.
The following tests are used to measure protein C:
Immunological by means of an ELISA - Measures only the amount of protein C present, not its functional activity
A clot-based functional aPTT assay – Determines the time to clot formation after the addition of a protein C activator; from this, the amount of protein C present can be determined
Chromogenic assay - Protein C is activated, commonly with the use of a protein C activator (Protac) derived from the venom of Agkistrodon contortrix contortrix (Southern copperhead), and the concentration of protein C is determined from the rate of color change in the test sample, owing to cleavage of a chromogenic substrate
A thrombin-generation–based test has also been shown to help detect protein C deficiency.
Commercial kits should use a reference standard calibrated against the current international standard for protein C. These kits are readily available commercially.
Most ELISAs use either monoclonal or polyclonal antibodies against protein C.
Functional, clotting-based protein C assays are based on either the prothrombin time (PT) or the aPTT, but the aPTT is used more commonly. The patient’s platelet-poor plasma is incubated at 37°C with phospholipid, a contact activator (eg, Kaolin), and a protein C activator (eg, Protac). After an incubation period of typically 1-4 minutes, calcium is added to initiate clotting. The time elapsed to clot formation is recorded. From this, the protein C level is determined from a reference curve. The clotting time of the aPTT (or PT) is influenced by the amount of factor Va or factor VIIIa present in the reaction mixture; this, in turn, is influenced by the activity of the activated protein C. Activated protein C is generated from the conversion of protein C to activated protein C by the protein C activator (Protac). Therefore, if circulating levels of protein C are reduced, less activated protein C is generated, less factor Va and factor VIIIa is inactivated, and clotting times areshorter.
For the chromogenic assay, platelet-poor plasma is incubated at 37°C with the protein C activator (eg, Protac). After the typical incubation period of 5 minutes, a chromogenic substrate for activated protein C is added. The change in optical density is measured and is compared against a standard reference curve. From this, the protein C level is determined. Calcium, phospholipid, and coagulation activator are not needed for the chromogenic assay because the plasma used in the test serves only as a source of protein C; clot formation is unnecessary for this test. 
Protein C is an inactive (zymogenic) protein. When activated, it plays a significant part in blood clot, inflammation, and cell death regulation, as well as in maintenance of blood vessel cell wall permeability. Protein C is also termed autoprothrombin IIA and blood coagulation factor XIV.  These operations are performed by activated protein C primarily by proteolytic inactivation of the proteins factor Va and factor VIIIa. Activated protein C contains a residue of serine in its active site; therefore, it is classified as a serine protease. Human protein C is found on chromosome 2 (band 2q13-q14) and is encoded by the PROC gene. 
The inactive form of protein C circulates in blood plasma and is a vitamin K–dependent glycoprotein. It is structurally similar to other vitamin K-dependent proteins that affect blood clotting, including factors VII, IX, and X and prothrombin. A light chain (21 kd) and a heavy chain (41 kd) connected by a disulfide bond (between Cys183 and Cys319) comprise this 2-chain polypeptide.  When inactive protein C binds to thrombin, it becomes activated. The activation of protein C is strongly promoted by the presence of thrombomodulin and endothelial protein C receptors (EPCRs). Activated protein C is found primarily near cells that make up the walls of blood vessels (the endothelium), likely because of the role of EPCRs. Activated protein C affects these endothelial cells, along with white blood cells.  Activated protein C has a half-life of approximately 15 minutes. 
Protein C has a crucial role as an anticoagulant, and individuals with a protein C deficiency or with some type of protein C activation dysfunction are at much greater risk of thrombosis. 
The 2 main classes of functions of protein C are anticoagulation and cytoprotection. Whether protein C is involved with anticoagulation or cytoprotection depends on if activated protein C stays bound to EPCRs after it is activated. Anticoagulative effects occur if it does not remain bound to EPCRs. If activated protein C remains bound to EPCRs after activation, its function is cytoprotection. It has been suggested that because the expression of one pathway is not necessarily affected by the existence of the other pathway, the cytoprotective properties are independent of the anticoagulant properties. 
Protein C plays a significant role in anticoagulation. It acts as a serine protease zymogen. Normally, when body tissue or a blood vessel wall is injured, the process termed hemostasis initiates plug formation at the injury site to help stop the bleeding. Small cell fragments called platelets adhere to and aggregate at the site, and a coagulation cascade ensues, with clotting factors being activated one after the other. Towards completion of the cascade, thrombin converts fibrinogen into insoluble fibrin threads, which then cross-link together to form a stabilized fibrin net at the site of injury. The fibrin net, along with the platelets, adheres to the site of injury to form a stable blood clot. The clot prevents additional blood loss and remains in place until the injured area has healed. Adequate platelets and adequate amounts of each coagulation factor must be present, and each must function normally to achieve a stable clot.
Proteins C and S are instrumental in the regulation of blood clot formation. Further, thrombin, which can accelerate or decelerate blood clot development, works together with protein C and protein S in a “feedback system.” Initially, thrombin combines with the protein thrombomodulin, and it subsequently activates protein C. The activated protein C combines with the cofactor protein S and, both work in concert to degrade coagulation factors VIIIa and Va (activated factors VIIIa and Va are required to produce thrombin). The net effect of this process is a slowing of new thrombin generation and the inhibition of further clotting. If the amount of protein C or protein S is inadequate or if either one is not functioning properly, thrombin generation essentially remains undeterred, which may promote inappropriate or excessive clotting, with resultant blockage of blood flow in the veins and, rarely, the arteries (thrombosis). [8, 9]
The usual reason a protein C test is ordered is to evaluate a thrombotic episode, such as unexplained venous thromboembolism, especially in the following situations:
Relatively young patient (< 50 y)
Unusual location (eg, veins leading to the liver or kidney, blood vessels of the brain [cerebral])
Patient has a family history of blood clots
Newborn with a possible severe clotting disorder (eg, DIC, purpura fulminans)
Protein C testing is also used to screen relatives of patients with a known protein C deficiency.
Additionally, protein C testing may also be performed to investigate the cause of multiple miscarriages.
What abnormal results mean
Protein C and protein S help regulate blood clotting. Protein C and/or protein S deficiency may result in blood clots forming in veins. These clots tend to form in veins, not arteries.
Protein C deficiency can develop secondary to conditions such as chemotherapy use, DIC, liver disease, long-term antibiotic use, and warfarin use; or it can be the result of an inherited (familial) trait.
Protein C levels typically increase as an individual ages, but this generally is not a cause of any health problems.
Protein C test results can be affected by the presence of certain drugs, such as anticoagulants (eg, warfarin); they decrease protein C and protein S levels. These types of medications and related supplements should be avoided before testing is performed.