Acceptable Sample for Coagulation Testing
The correct sample for coagulation testing is plasma collected using a 9:1 ratio of whole blood to 3.2% sodium citrate (option C). This is the standardized and recommended sample collection method for accurate coagulation testing according to international guidelines.
Proper Sample Collection for Coagulation Testing
Anticoagulant Type and Concentration
- 3.2% sodium citrate is the preferred anticoagulant concentration for coagulation testing 1
- The ratio must be precisely 9:1 (whole blood to sodium citrate) to ensure accurate test results 1, 2
- 3.8% sodium citrate (option D) is also acceptable but more sensitive to underfilling effects, making 3.2% the preferred concentration 2, 3
Why Other Options Are Incorrect
- Heparin (option A) is unsuitable for coagulation testing as it directly interferes with clotting mechanisms
- EDTA (option B) inhibits calcium-dependent coagulation factors and alters platelet morphology, making it inappropriate for coagulation studies 2
Critical Collection Parameters
- Tubes must be filled to at least 90% capacity to maintain the correct blood-to-anticoagulant ratio 2, 4
- Underfilled tubes can cause falsely prolonged coagulation times due to excess anticoagulant 3, 4
- Research shows that underfilling affects activated partial thromboplastin time (aPTT) results when tubes are less than 89% filled, and prothrombin time (PT) when less than 80% filled 4
Sample Processing Requirements
- Double centrifugation is recommended to ensure a residual platelet count of <10.0 × 10^9/L 1
- For standard processing: centrifuge at 2500 ×g for 15 minutes at room temperature 1
- Samples should be processed within 1 hour of collection or properly stored if testing is delayed 1, 5
Common Pitfalls to Avoid
- Hemolysis can significantly interfere with coagulation test results, even at low levels (0.9% lysis) 6
- Pneumatic tube transport systems can alter test parameters and should be avoided 1
- Improper mixing after collection can lead to inaccurate results - tubes should be gently inverted 3-4 times immediately after collection 2
Storage Considerations
- If testing is delayed, plasma should be frozen at –20°C (for ≤2 weeks) or –70°C (for >2 weeks) 1
- Frozen plasma should be rapidly thawed at 37°C and thoroughly mixed before testing 1
By adhering to these standardized collection procedures using the 9:1 ratio of whole blood to 3.2% sodium citrate, laboratories can ensure accurate and reliable coagulation test results that directly impact patient care decisions, particularly for those on anticoagulation therapy.