Transferring Blood for INR Testing: Not Recommended
Blood or serum should not be transferred into a blue-top (sodium citrate) tube for INR testing as this will lead to inaccurate results that could negatively impact patient care and anticoagulation management.
Why Transferring Blood Is Problematic
Citrate Concentration Issues
- The concentration of citrate used to anticoagulate plasma significantly affects the INR, especially for high INR values 1
- Blue-top tubes contain sodium citrate (3.2% or 0.109M is recommended) in a specific ratio to blood (1:9) 1
- Transferring blood or serum would disrupt this precise ratio, leading to unreliable results
Sample Integrity Concerns
- Transferring blood that has already begun to clot (as in serum) into a citrated tube cannot reverse the clotting process 2
- The presence of microclots in transferred samples can interfere with accurate INR measurement 2
- Cross-contamination between tube additives occurs during transfer, which can significantly alter test results 2
Impact on INR Accuracy
- Even small variations in collection methods can cause clinically significant differences in INR results 3
- Studies show that variations in tube types can lead to INR differences of up to 10-14%, which could affect clinical dosage of oral anticoagulants 4, 5
- Inaccurate INR results directly impact warfarin dosing decisions, potentially leading to under- or over-anticoagulation 6
Proper INR Collection Protocol
Correct Collection Method
- Blood for INR testing must be drawn directly into blue-top (sodium citrate) tubes 1
- The recommended citrate concentration is 0.109M (3.2%), with concentrations in the range of 0.105-0.11M being acceptable 1
- Higher citrate concentrations (3.8% or 0.129M) should not be used for PT tests 1
Tube Filling Requirements
- Blue-top tubes must be filled completely to maintain the correct blood-to-citrate ratio 1
- Underfilling the tube results in excess citrate relative to blood volume, which spuriously prolongs the PT 1
- The tube should be gently inverted 3-4 times immediately after collection to ensure proper mixing with the anticoagulant
Alternative Approaches
- If proper collection in a blue-top tube is not possible, a new sample should be collected directly into the appropriate tube 2
- Small-volume citrate tubes (e.g., MiniCollect 3.2% citrate) can be used when only small blood volumes can be obtained 7
- Point-of-care INR testing may be considered as an alternative when venipuncture is difficult 6
Clinical Implications of Inaccurate INR Results
- Inaccurate INR results can lead to inappropriate warfarin dosing decisions 6
- Underestimated INR values may result in excessive anticoagulation and increased bleeding risk
- Overestimated INR values may lead to inadequate anticoagulation and increased thrombotic risk
- The therapeutic range for most indications is narrow (INR 2-3), making accurate measurement critical 1
Remember that proper specimen collection is fundamental to accurate INR testing and appropriate anticoagulation management. Always collect blood directly into the appropriate blue-top tube rather than attempting to transfer from another tube type.