What blood tests are used to measure Protein S and Protein C?

Blood Tests for Protein S and Protein C

Protein S and Protein C are measured through specialized coagulation assays using citrated blood samples that require specific collection and processing techniques to ensure accurate results.

Collection and Processing Requirements

• Venous blood must be collected into tubes containing 3.2% (0.105-0.109 mol/L) trisodium citrate 1
• Samples require double-centrifugation to ensure platelet count <10.0 × 10^9/L 1
• Testing should be performed within 4 hours of collection or properly frozen:
  • At -20°C if stored ≤2 weeks
  • At -70°C or below if stored >2 weeks 1
• Samples should be rapidly thawed at 37°C and thoroughly mixed before testing 1

Types of Protein C Assays

Protein C can be measured using two primary methods:

1. Functional (Activity) Assays:

   • Amidolytic (Chromogenic) Assay: Recommended as the initial test for protein C deficiency 2
   • Measures the ability of activated protein C to cleave a chromogenic substrate
   • More reliable than clotting-based assays

2. Antigenic (Immunological) Assays:

   • Used for subclassification of protein C deficiency type 2
   • Measures the total amount of protein C present regardless of function

Types of Protein S Assays

Protein S exists in two forms: free (active) and bound to C4b-binding protein. Testing includes:

1. Functional Assays:

   • Measures protein S's ability to act as a cofactor for activated protein C 3
   • Based on factor Va inactivation by activated protein C 3
   • Can detect functional abnormalities not identified by antigenic assays

2. Antigenic Assays:

   • Total Protein S: Measured by immunoelectrophoresis 3
   • Free Protein S: Can be measured by:
     • PEG precipitation followed by immunoassay (traditional method) 4
     • Direct one-step ELISA using monoclonal antibodies specific for free protein S (newer method) 4

Testing Considerations

• Timing: Tests should be performed at least 3 months after acute thrombotic events and at least 2 weeks after discontinuing oral anticoagulants 5
• Confirmation: Low protein C or S levels should be confirmed with a subsequent assay on a new specimen 2
• Interferences:
  • Icterus, hemolysis, and lipemia can interfere with results 1
  • Direct oral anticoagulants (DOACs) can interfere with both protein C and S assays 5
  • Vitamin K antagonists significantly affect results 5

Clinical Application

• These tests are part of a comprehensive thrombophilia panel for patients with:
  • Venous thrombosis under age 50
  • Recurrent venous thrombosis
  • Thrombosis in unusual sites
  • Strong family history of thrombotic disease 5

Interpretation Pitfalls

• Protein C and S levels can be decreased during:

  • Acute thrombotic events
  • Pregnancy
  • Liver disease
  • DIC
  • Vitamin K deficiency
  • Anticoagulant therapy 5

• False results may occur if testing is performed too soon after a thrombotic event or while on anticoagulation therapy 5

For accurate diagnosis of deficiencies, testing should be repeated when the patient is not on anticoagulant therapy and not experiencing an acute thrombotic event 5, 2.