Interpretation of Activated Protein C Resistance Ratio of 2.6
An activated protein C resistance (APC-R) ratio of 2.6 with a reference range >2.3 indicates a NORMAL result, meaning Factor V Leiden mutation is unlikely and the patient does not have resistance to activated protein C. 1
Understanding the Test Result
Normal vs. Abnormal Values
- The APC-R ratio measures how well activated protein C can inhibit coagulation in the patient's plasma. 1
- A ratio >2.3 is considered normal, indicating adequate response to activated protein C. 1
- Your result of 2.6 falls ABOVE the cutoff threshold, placing it in the normal range. 1
- Lower ratios (typically <2.0-2.3, depending on the assay) indicate APC resistance and suggest Factor V Leiden mutation. 1, 2
What This Means Clinically
Factor V Leiden is unlikely:
- The most common cause of inherited APC resistance is the Factor V Leiden mutation (Arg506Gln), which accounts for the vast majority of APC-R cases. 1, 3
- Your normal ratio effectively excludes this mutation with high sensitivity and specificity when using modern predilution assays. 1, 4
Thrombotic risk implications:
- Factor V Leiden heterozygotes have approximately 4-7 fold increased risk of venous thromboembolism (VTE), while homozygotes have 9-80 fold increased risk. 5
- With a normal APC-R ratio, you do not have this inherited thrombophilic risk factor. 2
- The link between APC resistance and arterial stroke is tenuous in adults, though it may play a larger role in cerebral venous thrombosis (CVT) with an odds ratio of 3.38. 1
Important Caveats About the Test
Assay-Specific Considerations
- Modern predilution assays (where patient plasma is diluted in Factor V-deficient plasma) have near 100% sensitivity and specificity for Factor V Leiden mutations. 1, 4
- First-generation undiluted APTT-based assays had only 50-86% sensitivity and were prone to interference from lupus anticoagulants, elevated Factor VIII, pregnancy, oral contraceptives, and low protein S levels. 1, 6
- If your test was performed using a predilution method (most modern laboratories use this), the result is highly reliable. 1
When Further Testing May Be Needed
Molecular DNA testing for Factor V Leiden is NOT indicated with your normal result unless:
- You have a strong personal history of unprovoked VTE at young age (<50 years). 2
- You have recurrent thrombotic events despite a normal APC-R ratio. 2
- There is a documented family history of Factor V Leiden with high-risk clinical situations. 5
Acquired APC Resistance
- Acquired APC resistance can occur without Factor V mutations and represents an independent risk factor for venous thrombosis. 1
- However, your normal ratio of 2.6 indicates you do not have clinically significant acquired APC resistance either. 1
Clinical Bottom Line
Your result of 2.6 (above the >2.3 threshold) is reassuring and indicates normal activated protein C function, effectively ruling out Factor V Leiden thrombophilia as a cause of any thrombotic concerns. 1, 2 No further testing for this specific thrombophilic disorder is warranted based on this result alone. 1