Acceptable Sample for Coagulation Testing
The correct answer is C: plasma collected using a 9:1 ratio of whole blood to 3.2% sodium citrate, which is the standardized and recommended sample collection method for accurate coagulation testing according to the International Society on Thrombosis and Haemostasis. 1
Why 3.2% Sodium Citrate is Preferred
The 3.2% sodium citrate concentration is specifically recommended over 3.8% sodium citrate for several important reasons:
- It is less sensitive to underfilling effects, which can significantly alter test results 1, 2
- Studies have shown that 3.2% sodium citrate allows for more reliable results even when tubes are not completely filled 2
- The 9:1 ratio of whole blood to anticoagulant is critical for standardized testing 1, 3
Why Other Options Are Incorrect
Option A (10:1 ratio of whole blood to heparin): Heparin is an anticoagulant that interferes with coagulation testing by inhibiting thrombin and other coagulation factors, making it unsuitable for these tests 1
Option B (4:1 ratio of whole blood to EDTA): EDTA completely removes calcium from the sample, which prevents proper assessment of the coagulation cascade. Additionally, the ratio is incorrect for any coagulation testing 1
Option D (9:1 ratio of whole blood to 3.8% sodium citrate): While historically used, 3.8% sodium citrate is more sensitive to underfilling effects compared to 3.2% sodium citrate. Research has shown that with 3.8% citrate, statistically significant differences in PT results occur when tubes are less than 80% filled, whereas with 3.2% citrate, reliable results can be obtained with tubes filled to as low as 60% capacity for PT and 70% for aPTT 1, 2
Critical Sample Collection Parameters
For accurate coagulation testing, the following parameters must be observed:
- Light-blue top tubes containing 3.2% sodium citrate must be used 1
- The tube must maintain a precise 9:1 blood-to-anticoagulant ratio 1, 3
- Tubes should be filled completely and gently inverted 3-4 times immediately after collection 1
- Tubes should be filled to at least 90% of capacity to avoid falsely elevated results 1, 4
Processing Requirements
After collection, proper processing is essential:
- Samples should be processed within 1 hour of collection 1, 3
- Double centrifugation is recommended to ensure a residual platelet count of <10.0 × 10^9/L 1
- Standard processing involves centrifuging samples at 2500 ×g for 15 minutes at room temperature 1
- Alternative processing can include centrifugation at 3000g for 5 minutes, which has been shown to produce reliable results while reducing turnaround time 5
Common Pitfalls to Avoid
- Underfilling tubes: This causes excess anticoagulant relative to blood volume, which spuriously prolongs coagulation times, especially aPTT 1, 4
- Hemolysis: Even moderate blood cell lysis (as low as 0.9%) can significantly influence the reliability of coagulation test results 6
- Improper mixing: Inadequate mixing can lead to partial clotting and inaccurate results 1
- Using butterfly needles without discarding the first tube: The air volume in butterfly tubing can affect the blood-to-anticoagulant ratio 1