What effect can under-filled blue top (blood collection) tubes have on coagulation test results?

Effect of Underfilled Blue Top Tubes on Coagulation Tests

Underfilled blue top tubes cause prolonged clotting times from excess anticoagulant (option A), which can lead to falsely elevated coagulation test results. 1

Mechanism of Interference

When blue top tubes (containing sodium citrate) are underfilled, the ratio of anticoagulant to blood volume becomes disproportionate, resulting in:

• Excess citrate relative to blood volume 2, 1
• Increased calcium binding by the anticoagulant 1
• Reduced availability of calcium ions necessary for coagulation 1
• Artificially prolonged clotting times in laboratory tests 3, 4

Impact on Specific Coagulation Tests

Prothrombin Time (PT/INR)

• Underfilled tubes cause falsely prolonged PT/INR results 1, 3
• The effect is more pronounced with higher citrate concentrations (3.8% vs 3.2%) 4
• The impact is greater when the true PT is already elevated (e.g., in patients on warfarin) 3, 5

Activated Partial Thromboplastin Time (aPTT)

• aPTT is even more sensitive to underfilling than PT 3, 5
• Significant prolongation occurs when tubes are filled to less than 90% capacity 4

Filling Requirements

• Blue top tubes must be filled completely to maintain the correct blood-to-citrate ratio 1

• For tubes with 3.2% sodium citrate:

  • PT results remain accurate down to 60-65% fill volume in normal samples 3, 4
  • PT results for patients on anticoagulants require at least 80-90% fill volume 3
  • aPTT requires at least 70-90% fill volume for accurate results 4

• For tubes with 3.8% sodium citrate (less commonly used now):

  • PT results require at least 80% fill volume 4
  • aPTT results require at least 90% fill volume 4

Clinical Implications

• Falsely prolonged coagulation tests can lead to:
  • Inappropriate reduction in anticoagulant dosing 1
  • Increased thrombotic risk due to subtherapeutic anticoagulation 1
  • Unnecessary diagnostic workups for suspected coagulopathies
  • Delayed procedures or inappropriate blood product administration

Best Practices

• Use 3.2% sodium citrate tubes (more tolerant to underfilling than 3.8%) 2, 4
• Fill tubes to at least 90% of capacity to avoid falsely elevated results 3
• Ensure proper mixing by gently inverting tubes 3-4 times immediately after collection 1
• If underfilling is suspected, recollect the sample properly 1
• Never transfer blood from other collection tubes to blue top tubes due to risk of contamination with other additives 2, 1

Important Caveat

While underfilling causes falsely prolonged clotting times, a normal PT or aPTT result from an underfilled tube is likely to be truly normal, as underfilling does not produce false normal results 5.