From: Elizabeth M. Van Cott, M.D., and Michael Laposata, M.D., Ph.D., "Coagulation." In: Jacobs DS et al, ed. The Laboratory Test Handbook, 5th Edition. Lexi-Comp, Cleveland, 2001; 327-358.

Index of Tests

Protein C [CO004200]

Related Information

Abstract Protein C, with protein S as a cofactor, is a natural anticoagulant protein. A hereditary deficiency of protein C leads to a hypercoagulable state with an increased risk for venous thrombosis. Type I deficiency is a quantitative deficiency of protein C. Type II deficiencies result from qualitatively abnormal (but often quantitatively normal) protein C.

Patient Preparation Determine if patient is on oral anticoagulants. Protein C levels are decreased by warfarin (Coumadin®).

Specimen Plasma

Container One blue top (sodium citrate) tube

Sampling Time Testing should be deferred until patients have not received Coumadin® for at least 10 days, because Coumadin® decreases protein C levels.

Collection Routine venipuncture. If multiple tests are being drawn, draw blue top tubes after any red top tubes but before any lavender top (EDTA), green top (heparin), or gray top (oxalate/fluoride) tubes. Immediately invert tube gently at least 4 times to mix. Tubes must be appropriately filled. Deliver tubes immediately to the laboratory

Storage Instructions Separate plasma from cells as soon as possible. Plasma may be stored on ice for up to 4 hours; otherwise, store frozen.

Causes for Rejection Specimen received more than 4 hours after collection, tubes not filled, clotted specimens

Turnaround Time Several hours to several days, depending on how often test batches are performed

Reference Interval Results are reported as a percent of the amount expected in normal plasma. By definition, the mean value in normal plasma is 100%. The reference range is approximately 70% to 140%.1 At birth, protein C levels are only 35% (range 17% to 53%) of adult normal values.2 Mean protein C levels rise to above 50% of adult normal values by age 6 months, but may remain below adult normal range until the age of 10-16 years.3

Use A functional assay should be performed first, because both type I and type II protein C deficiencies will be detected. The antigen assay is needed only if the functional assay is decreased, in order to determine if the deficiency is type I or type II. If the antigen assay is performed without the functional assay, type II deficiencies will not be detected (see Additional Information).

Limitations Acquired protein C deficiencies are more common than hereditary deficiencies (see Additional Information).

Chromogenic (functional) assays: Certain type II protein C deficiencies may not be detected in the chromogenic assay but will be detected by clot-based assays.4,5 Assays are usually designed to tolerate up to 1 unit/mL heparin. The advantage of this assay is that it is not affected by lupus anticoagulants, factor VIII levels, factor V Leiden, or other coagulation abnormalities that can interfere with clot-based protein C assays.

Clot-based (functional) assays: Commonly encountered coagulation conditions can interfere. For example, lupus anticoagulants can artifactually increase the protein C test result. Elevations in factor VIII (>200%) can artifactually decrease the result; factor VIII elevations occur in patients with an acute phase reaction. Falsely low values have been reported in patients with the factor V Leiden mutation.6,7 The advantage of this assay is that all known type I and type II variants should be detected.4,5 Assays that tolerate up to 1 unit/mL heparin are available. Cannot be performed in patients on hirudin or argatroban anticoagulation.

Antigen (immunologic) assays: If not used in conjunction with a functional assay, type II deficiencies will not be detected (see Additional Information).

Methodology Assays are functional (chromogenic or clot-based) or antigenic.

Chromogenic assays: Protein C in the patient plasma sample is activated, usually by a specific snake venom. The activated protein C cleaves a synthetic substrate that resembles the natural substrate of protein C, liberating a chromogenic substance that can be measured spectrophotometrically.8

Clot-based assays: Protein C in the patient plasma sample is activated, usually with a specific snake venom. The activated protein C then degrades factors Va and VIIIa, thereby prolonging a PTT-based clotting time.

Antigenic (immunoassay): Enzyme-linked immunosorbent assay (ELISA)9

Additional Information Protein C, a vitamin K dependent zymogen of a serine protease (activated protein C), has a molecular weight of 62,000 daltons. Protein C functions as an anticoagulant by using protein S as a cofactor to degrade activated factors V and VIII. Protein C must first be converted into activated protein C by interacting with a thrombin-thrombomodulin complex on the surface of endothelial cells. Protein C also indirectly promotes fibrinolysis.10

Hereditary protein C deficiency is present in 0.14% to 0.50% of the general population.11,12 It accounts for 3% of unselected patients with venous thrombosis and up to 9% of patients younger than 70 years of age with thrombosis.13,14 Over 160 mutations in the protein C gene are known to cause hereditary protein C deficiency.15,16 Individuals heterozygous for protein C deficiency have a sevenfold increased risk for venous thrombosis.13 Heterozygotes generally have protein C levels between 35% to 65%, although levels as high as 68% have been reported.17 The risk for thrombosis is further increased in the presence of a second risk factor.18 The age at onset of thrombosis is usually between 10-50 years in heterozygous individuals. Coumadin®-induced skin necrosis may occur if protein C deficient patients are treated with Coumadin® without the addition of an immediate-acting anticoagulant (eg, heparin) until the Coumadin® levels are therapeutic. Homozygous deficiencies are rare, and are fatal if untreated. They present in the newborn period with severely decreased protein C, purpura fulminans, and disseminated intravascular coagulation (DIC).

Decreased protein C can also arise from acquired conditions, such as:

* decreased hepatic synthesis from liver disease or L-asparaginase treatment

* synthesis of a dysfunctional protein due to vitamin K deficiency or warfarin (Coumadin®) use

* consumption from thrombosis, DIC, or surgery

A case of an acquired inhibitor (autoantibody) to protein C has been reported.19 If a patient with low protein C has any of the conditions listed above, the test should be repeated once the condition is no longer present. Confirmation of a hereditary protein C deficiency may require documenting protein C deficiency in a relative. In nephrotic syndrome, protein C may increase, decrease, or remain unchanged. Malm et al. reported that protein C can increase with oral contraceptives and pregnancy,20 whereas Kjellberg et al reported no significant increase in protein C during pregnancy.21 Women may have slightly decreased protein C levels in comparison to men, and premenopausal women may have slightly lower levels than postmenopausal women.22

Protein C has a relatively short half-life of 6-8 hours; therefore, it is one of the first hepatic coagulation proteins to decrease with liver dysfunction as well as with Coumadin® initiation.

Protein C deficiencies are quantitative (type I) or qualitative (type II). In type I deficiencies, normal protein C molecules are made, but in reduced quantity. In type II deficiencies, normal amounts of protein C are made, but the protein C is defective. Functional assays measure protein C function (activity). Antigenic assays are immunoassays that measure the quantity of protein C, regardless of the quality of its function. Accordingly, type I deficiencies have decreased protein C in both functional and antigenic assays. Type II deficiencies have normal antigenic protein C levels, with decreased functional protein C. Thus, if only antigenic assays are performed, type II deficiencies will not be detected. Therefore, a functional assay should be used as the initial screening assay. If the result is decreased, an antigenic assay may be performed to determine if the deficiency is type I or type II.


1. Allaart CF, Poort SR, Rosendaal FR, et al, "Increased Risk of Venous Thrombosis in Carriers of Hereditary Protein C Deficiency Defect,"Lancet, 1993, 341:134-8.

2. Andrew M, Paes B, Milner R, et al, "Development of the Human Coagulation System in the Full-Term Infant,"Blood, 1987, 70(1):165-72.

3. Andrew M, Vegh P, Johnston M, et al, "Maturation of the Hemostatic System During Childhood,"Blood, 1992, 80(8):1998-2005.

4. Vasse M, Borg JY, and Monconduit M, "Protein C: Rouen, a New Hereditary Protein C Abnormality With Low Anticoagulant but Normal Amidolytic Activities,"Thromb Res, 1989, 56:387-98.

5. Wojcik EGC, Simioni P, Berg MVD, et al, "Mutations Which Introduce Free Cysteine Residues in the Gla-domain of Vitamin K Dependent Proteins Result in the Formation of Complexes With alpha1-microglobulin,"Thromb Haemost, 1996, 75(1):70-5.

6. Ireland H, Bayston T, Thompson E, et al, "Apparent Heterozygous Type II Protein C Deficiency Caused by the Factor V 506 Arg to Gln Mutation,"Thromb Haemost, 1995, 73(4):731-2.

7. Jennings I, Kitchen S, Cooper PC et al, "Further Evidence That Activated Protein C Resistance Affects Protein C Coagulant Activity Assays,"Thromb Haemost, 2000, 83(1):171-2.

8. Francis RB Jr and Seyfert U, "Rapid Amidolytic Assay of Protein C in Whole Plasma Using an Activator From the Venom of Agkistrodon Contortrix,"Am J Clin Pathol, 1987, 87(5):619-25.

9. Boyer C, Rothschild C, Wolf M, et al, "A New Method for the Estimation of Protein C by ELISA,"Thromb Res, 1984, 36:579-89.

10. Nesheim M, Wang W, Boffa M, et al, "Thrombin, Thrombomodulin and TAFI in the Molecular Link Between Coagulation and Fibrinolysis,"Thromb Haemost, 1997, 78(1):386-91.

11. Miletich J, Sherman L, and Broze G Jr, "Absence of Thrombosis in Subjects With Heterozygous Protein C Deficiency,"N Engl J Med, 1987, 317:991-6.

12. Tait RC, Walker ID, Reitsma PH, et al, "Prevalence of Protein C Deficiency in the Healthy Population,"Thromb Haemost, 1995, 73(1):87-93.

13. van der Meer FJ, Koster T, Vandenbroucke JP, et al, "The Leiden Thrombophilia Study (LETS),"Thromb Haemost, 1997, 78(1):631-5.

14. Melissari E, Monte G, Lindo VS et al, "Congenital Thrombophilia Among Patients With Venous Thromboembolism,"Blood Coagul Fibrinolysis, 1992, 3(6):749-58.

15. Reitsma PH, Bernardi F, Doig RG, et al, "Protein C Deficiency: A Database of Mutations, 1995 Update,"Thromb Haemost, 1995, 73:876-89.

16. Reitsma PH, "Protein C Deficiency: From Gene Defects to Disease,"Thromb Haemost, 1997, 78(1):344-50.

17. Finazzi G and Barbui T, "Different Incidence of Venous Thrombosis in Patients With Inherited Deficiencies of Antithrombin III, Protein C and Protein S,"Thromb Haemost, 1994, 71:15-8.

18. Simioni P, Sanson BJ, Prandoni P, et al, "Incidence of Venous Thromboembolism in Families With Inherited Thrombophilia,"Thromb Haemost, 1999, 81(2):198-202.

19. Mitchell CA, Rowell JA, Hau L, et al, "A Fatal Thrombotic Disorder Associated With an Acquired Inhibitor of Protein C,"N Engl J Med, 1987, 317:1638-42.

20. Malm J, Laurell M, and Dahlbäck B, "Changes in the Plasma Levels of Vitamin K-Dependent Proteins C and S and of C4b-Binding Protein During Pregnancy and Oral Contraception,"Br J Haematol, 1988, 68(4):437-43.

21. Kjellberg U, Andersson NE, Rosen S, et al, "APC Resistance and Other Haemostatic Variables During Pregnancy and Puerperium,"Thromb Haemost, 1999, 81(4):527-31.

22. Henkens CM, Bom VJ, van der Schaaf W et, al, "Plasma Levels of Protein S, Protein C and Factor X: Effects of Sex, Hormonal State and Age,"Thromb Haemost, 1995, 74(5):1271-5.


Alhenc-Gelas M, Gandrille S, Aubry ML, et al, "Thirty-three Novel Mutations in the Protein C Gene,"Thromb Haemost, 2000, 83:86-92.

De Stefano V, Finazzi G, and Mannucci PM, "Inherited Thrombophilia: Pathogenesis, Clinical Syndromes, and Management,"Blood, 1996, 87(9):3531-44.

Gandrille S and Aiach M, "Identification of Mutations in 90 of 121 Consecutive Symptomatic French Patients With a Type I Protein C Deficiency. The French INSERM Network on Molecular Abnormalities Responsible for Protein C and Protein S Deficiencies,"Blood, 1995, 86(7):2598-605.

Sanson BJ, Simioni P, Tormene D et al, "The Incidence of Venous Thromboembolism in Asymptomatic Carriers of a Deficiency of Antithrombin, Protein C, or Protein S: A Prospective Cohort Study,"Blood, 1999, 94(11):3702-6.

Schofield KP, Thomson JM, and Poller L, "Protein C Response to Induction and Withdrawal of Oral Anticoagulant Treatment,"Clin Lab Haematol, 1987, 9(3):255-62.


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