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Interpretive Handbook

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Test 9127:
Protein C Antigen, Plasma

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

Physiology:

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.

 

Pathophysiology:

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.

 

The 2 types of hereditary heterozygous protein C deficiencies that are recognized are:

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

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

 

Acquired deficiency 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)

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

Differentiating congenital Type I protein C deficiency from Type II deficiency

 

Evaluating the significance of decreased functional protein C, especially when decreased protein C activity might be congenital rather than acquired (eg, due to oral anticoagulant effect, vitamin K deficiency, liver disease, or intravascular coagulation and fibrinolysis/disseminated intravascular coagulation)

Interpretation Provides information to assist in interpretation of the test results

Values <70% to 75% may represent a congenital deficiency state, if acquired deficiencies can be excluded.

 

Protein C antigen and activities generally are undetectable in individuals with severe, homozygous protein C deficiency.

 

Acquired protein C deficiency is of uncertain clinical hemostatic significance.

 

Clinical significance 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

Assay of protein C functional activity (CFX/9339 Protein C Activity, Plasma) is recommended for initial laboratory evaluation of patients suspected of having congenital protein C deficiency (personal or family history of thrombotic diathesis).

 

Not useful for predicting a thrombotic event.

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 antigen (15%-50%), which may not reach adult levels until later in childhood or early adolescence.*

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

Clinical References 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. 2nd 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 Haemeostasis. J Pediatr 1989;114:528-534

4. Miletrich 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