Cite this page: Parsons JC. Protein C assays. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/coagulationproteinCassays.html. Accessed March 9th, 2021.
Definition / general
- Deficiencies are either quantitative (type I - reduced amount of normal protein) or qualitative (type II - normal amount of defective protein, Thromb Haemost 1984;51:1)
- Assays are either functional (measure protein activity) or antigenic (immunoassays that measure quantity, not function)
- Perform functional assay first - if decreased, perform antigenic assay; must exclude acquired causes (Arch Pathol Lab Med 2002;126:1337)
- Low values should be confirmed on a new specimen
- Assays should be performed with platelet poor plasma, using sodium citrate collection tubes
- Functional assays are clot-based or chromogenic
- Clot based functional assays
- Detect all known type I and II variants; patients protein C is activated by Southern Copperhead venom (Agkistrodon contortrix contrortrix), which degrades synthetic substrate, factor Va or factor VIIIa with clot based PTT assay; the prolongation of clotting time is proportional to the amount of factor activity
- PT based assay or amidolytic assays are affected by lupus anticoagulants (raises protein C result), elevations of factor VIII > 200% (decreases the result), acute phase reactions or factor V Leiden mutation (decrease the result); cannot perform on patients taking hirudin or argatroban
- Chromogenic functional assays
- Not affected by lupus anticoagulants, factor VIII levels, factor V Leiden or other coagulation abnormalities that interfere with clot-based functional assays; may not detect qualitative deficiencies detected by clot-based assays; patients protein C is activated by snake venom, which cleaves a synthetic substrate, which releases a chromogenic that is measured spectrophotometrically
- Other assays
- Antigenic assays: either ELISA, electroimmunoassay (Laurell rocket method) or radioimmunoassay; variable levels, so use 3 standard deviations as cutoff
- ELISA: uses antibody to protein C immobilized to microtiter place; add plasma; add secondary anti-protein C antibody coupled to an enzyme for colorimetric detection; use standard curve to determine plasma protein C
- Laurell rocket antigenic assay: agarose gel has antibody to protein C; plasma samples are put into wells and electrophoresed; antigen-antibody complexes precipitate during electrophoresis, and height of precipitin arc is proportional to plasma protein C, which is compared to standard curve using pooled normal plasma; may be unable to detect protein C levels < 5%
- Radioimmunoassay: similar to ELISA but uses single, radiolabeled antibody
Etiology
- Acquired causes of low Protein C levels: more common than hereditary deficiencies - clot formation, surgery, liver disease, warfarin (should be discontinued at least 10 - 30 days prior to testing), DIC, vitamin K deficiency, vitamin K antagonist therapy and L-asparaginase therapy; repeat protein C test once these conditions are no longer present
- Acquired causes of increased Protein C (may mask protein C deficiency): ischemic heart disease, pregnancy, postmenopausal women, hormone replacement therapy and oral contraceptives
Diagnosis
- Necrotic skin in newborns days 1 - 3 of life (purpura fulminas neonatorum, can also test parents also for heterozygosity); evaluation of cause of venous thromboembolism (recommended to use chromogenic protein C assays initially)
- Non-indications: screening before oral contraceptives or oral anticoagulants (discontinue for 10 days, or test family members)
Interpretation
- Values falsely increased by bivalirudin, lepirudin, argatroban and fondaparinux (Arch Pathol Lab Med 2004;128:1142), lowered by warfarin (must discontinue for 10 days prior to testing)
- Reference range: 70 - 140% of normal; newborns levels are 20 - 30% of adult values; usually rise to near adult levels by age 6 months, but may remain below adult normal levels until age 10 years
