Can Heparin Drip Affect Protein C and Protein S Assay Results?
Yes, heparin can affect functional protein C and protein S assay results, but the degree of interference depends on the specific assay method used, the heparin concentration, and the timing of sample collection.
Impact on Protein C Assays
Heparin's effect on protein C testing is assay-dependent and generally manageable with proper technique:
- Chromogenic protein C assays (the recommended standard) are largely unaffected by therapeutic heparin levels 1
- Clotting-based protein C assays can show falsely elevated results in the presence of heparin 1
- Heparin accelerates the inhibition of activated protein C (APC) by plasma inhibitors in a dose-dependent manner, with maximal stimulation occurring at heparin concentrations ≥10 U/mL 2, 3
Clinical studies demonstrate that during heparin treatment for acute deep vein thrombosis, protein C antigen and activity levels remained unchanged over 5 days of therapy 4. This suggests that while heparin may affect the kinetics of APC inhibition in vitro, it does not substantially alter measured protein C levels when using appropriate assays.
Impact on Protein S Assays
The effect on protein S is more complex and depends on which component is being measured:
- Total protein S remained unchanged during heparin treatment 4
- Free protein S (the functionally active form) was initially decreased before heparin treatment but increased to normal values after 5 days of therapy in thrombosis patients 4
- Functional protein S assays based on activated protein C cofactor activity can be affected by heparin's enhancement of APC inhibition 5
Practical Recommendations
When Testing is Necessary During Heparin Therapy:
For Protein C:
- Use chromogenic assays rather than clotting-based methods 1
- Chromogenic assays have good specificity and are not significantly affected by therapeutic heparin levels
- Avoid clotting-based assays as they produce falsely elevated results
For Protein S:
- Measure both total and free protein S antigen levels immunologically 6
- If using functional assays, be aware that heparin may enhance the rate of APC inhibition, potentially affecting results
- Consider that free protein S levels may actually normalize during heparin treatment in acute thrombosis patients
Optimal Testing Strategy:
The best approach is to delay testing until after anticoagulation is discontinued whenever clinically feasible 7. However, if testing cannot be delayed:
- Draw samples at trough heparin levels (just before next dose for intermittent dosing)
- Perform simultaneous anti-Xa or aPTT testing to document heparin levels
- Ensure baseline clotting times are within expected ranges 7
- Use immunologic (antigen) assays when possible, as these are less affected by heparin than functional assays
Critical Caveats:
- High heparin levels (>1.0 U/mL) can overcome the heparin neutralizers present in many modified assays, leading to invalid results 7
- The ionic strength of the assay system significantly affects heparin's stimulation of enzyme inhibition by protein C inhibitor—higher ionic strength (>0.15) drastically reduces heparin interference 3
- Partially clotted samples will give falsely raised protein C levels by chromogenic assay but reduced levels by clotting-based assay 1
Bottom Line
If hereditary protein C or S deficiency testing is needed urgently in a patient on heparin, use chromogenic protein C assays and immunologic protein S antigen assays, as these are minimally affected by therapeutic heparin levels. For functional protein S testing or when using clotting-based protein C assays, postpone testing until heparin is discontinued to avoid misinterpretation of results 7, 6, 1.