CFX - Clinical: Protein C Activity, Plasma

Test Catalog

Test Name

Test ID: CFX    
Protein C Activity, Plasma

Useful For Suggests clinical disorders or settings where the test may be helpful

As an initial test for evaluating patients suspected of having congenital protein C deficiency, including those with personal or family histories of thrombotic events


Because coagulation testing and its interpretation is complex, Mayo Medical Laboratories suggests ordering THRMP / Thrombophilia Profile.


Detecting and confirming congenital Type I and Type II protein C deficiencies, detecting and confirming congenital homozygous protein C deficiency, and identifying decreased functional protein C of acquired origin (eg, due to oral anticoagulant effect, vitamin K deficiency, liver disease, intravascular coagulation and fibrinolysis/disseminated intravascular coagulation.)

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test


Protein C is a vitamin K-dependent anticoagulant proenzyme. It is synthesized in the liver and circulates in the plasma. The biological half-life of plasma protein C is approximately 6 to 10 hours, similar to the relatively short half-life of coagulation factor VII.


Protein C is activated by thrombin, in the presence of an endothelial cell cofactor (thrombomodulin), to form the active enzyme activated protein C (APC). APC functions as an anticoagulant by proteolytically inactivating the activated forms of coagulation factors V and VIII (factors Va and VIIIa). APC also enhances fibrinolysis by inactivating plasminogen activator inhibitor (PAI-1).


Expression of the anticoagulant activity of APC is enhanced by a cofactor, protein S, another vitamin K-dependent plasma protein.



Congenital homozygous protein C deficiency results in a severe thrombotic diathesis, evident in the neonatal period and resembling purpura fulminans.


Congenital heterozygous protein C deficiency may predispose to thrombotic events, primarily venous thromboembolism; arterial thrombosis (stroke, myocardial infarction, etc.) may occur. Some individuals with hereditary heterozygous protein C deficiency may have no personal or family history of thrombosis and may or may not be at increased risk. Congenital heterozygous protein C may predispose to development of coumarin-associated skin necrosis. Skin necrosis has occurred during the initiation of oral anticoagulant therapy.


Two types of hereditary heterozygous protein C deficiency are recognized:

-Type I (concordantly decreased protein C function and antigen)

-Type II (decreased protein C function with normal antigen level)


Acquired deficiencies of protein C may occur in association with:

-Vitamin K deficiency

-Oral anticoagulation with coumarin compounds

-Liver disease

-Intravascular coagulation and fibrinolysis/disseminated intravascular coagulation (ICF/DIC)


The clinical hemostatic significance of acquired protein C deficiency is uncertain.


Assay of protein C functional activity is recommended for the initial laboratory evaluation of patients suspected of having congenital protein C deficiency (personal or family history of thrombotic diathesis), rather than assay of protein C antigen (PCAG / Protein C Antigen, Plasma).

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

Adults: 70%-150%

Normal, full-term newborn infants or healthy premature infants may have decreased levels of protein C activity (15%-50%), which may not reach adult levels until later in childhood or early adolescence.*

*See Pediatric Hemostasis References in Coagulation Studies in Special Instructions.

Interpretation Provides information to assist in interpretation of the test results

Values <60% to 70% may represent a congenital deficiency state, if acquired deficiencies can be excluded.


Protein C activity (and antigen) is generally undetectable in individuals with severe, homozygous protein C deficiency.


Oral anticoagulant therapy (warfarin, Coumadin) decreases protein C activity, compromising the ability to distinguish between congenital and acquired protein C deficiency. Concomitant measurement of the activity of coagulation factor VII (or factor X) may aid in differentiating congenital deficiency state from acquired protein C deficiency due to oral anticoagulant effect, but the ratio of the activities of protein C:factor VII (or factor X) has not been demonstrated to provide certainty about  this distinction.


The clinical significance of acquired protein C deficiency and of increased protein C is unknown.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Protein C Activity result may be affected by:

-Heparin (Unfractionated) >4 U/mL

-Heparin (low-molecular-weight) >2 U/mL

-Hemoglobin >500 mg/dL

-Bilirubin >21 mg/dL

-Triglycerides >890 mg/dL


Heparin therapy may temporarily decrease plasma protein C activity into the abnormal range.


Lipemia may interfere with functional protein C assay. Blood specimens for protein C functional assay should be drawn in the fasting state, if possible.


Protein C functional assay using a venom activator and a chromogenic peptide substrate has the potential of not detecting certain congenital protein C variants that might be detectable using clot-based assay of protein C function.

Clinical Reference Provides recommendations for further in-depth reading of a clinical nature

1. Mannucci PM, Owen WG: Basic and clinical aspects of proteins C and S. In Haemostasis and Thrombosis. Second edition. Edited by AL Bloom, DP Thomas. Edinburgh, Churchill Livingstone, 1987, pp 452-464

2. Marlar RA, Mastovich S: Hereditary protein C deficiency: a review of the genetics, clinical presentation, diagnosis and treatment. Blood Coagul Fibrinolysis 1990;1:319-330

3. Marlar RA, Montgomery RR, Broekmans AW: Diagnosis and treatment of homozygous protein C deficiency. Report of the Working Party on Homozygous Protein C Deficiency of the Subcommittee on Protein C and Protein S, International Committee on Thrombosis and Haemostasis. J Pediatr 1989;114:528-534

4. Miletich J, Sherman L, Broze G Jr: Absence of thrombosis in subjects with heterozygous protein C deficiency. N Engl J Med 1987;317:991-996

5. Pabinger I, Allaart CF, Hermans J, et al: Hereditary protein C-deficiency: laboratory values in transmitters and guidelines for the diagnostic procedure. Report on a study of the SSC Subcommittee on Protein C and Protein S. Protein C Transmitter Study Group. Thromb Haemost 1992;68:470-474

Special Instructions and Forms Describes specimen collection and preparation information, test algorithms, and other information pertinent to test. Also includes pertinent information and consent forms to be used when requesting a particular test