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]
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®).
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,
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)
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
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
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
* decreased hepatic synthesis from liver disease or L-asparaginase
* 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
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,
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,
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,
15. Reitsma PH, Bernardi F, Doig RG, et al, "Protein C Deficiency:
A Database of Mutations, 1995 Update,"Thromb Haemost, 1995,
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,
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,
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,
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,
Schofield KP, Thomson JM, and Poller L, "Protein C Response to
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