Sequence of Filling of Tubes for Hematological Investigations

The correct sequence of filling blood collection tubes during hematological investigations is critical for accurate lab results. Incorrect order can lead to cross-contamination between additives, skewed results, and even the need for repeat testing. This guide explains the importance of tube filling sequence and provides a step-by-step breakdown to ensure accurate hematology results.

Why is the Sequence Important?

During blood collection, different tubes contain various additives such as anticoagulants, clot activators, and preservatives. Filling the tubes in the wrong sequence can cause additive carryover, potentially contaminating samples and leading to inaccurate test results. Therefore, adhering to the correct order of draw is essential for both patient safety and reliable results.

What is the Correct Sequence of Tube Filling?

According to the Clinical and Laboratory Standards Institute (CLSI) guidelines, the order of draw is as follows:

1. Blood Culture Tubes

• Purpose: Blood cultures are used to detect infections in the bloodstream.
• Additive: No additive or sodium polyanethol sulfonate (SPS).
• Why First: Blood culture tubes are collected first to minimize the risk of contamination by other tube additives.

2. Coagulation Tubes (Light Blue)

• Purpose: These tubes are used for coagulation studies, such as Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT).
• Additive: Sodium citrate (anticoagulant).
• Why Second: Sodium citrate binds calcium in the blood to prevent clotting, so this tube is drawn next to ensure an uncontaminated sample for accurate coagulation tests.

3. Serum Tubes (Red, Gold, or Tiger Top)

• Purpose: These tubes are used for serum chemistry tests, serology, and immunology.
• Additive: May contain a clot activator and gel for serum separation.
• Why Third: Drawing serum tubes after coagulation tubes prevents any potential carryover of anticoagulants that could interfere with the serum’s ability to clot.

4. Heparin Tubes (Green)

• Purpose: Heparin tubes are used for plasma chemistry tests.
• Additive: Lithium heparin or sodium heparin (anticoagulant).
• Why Fourth: Heparin prevents clotting and does not interfere with most plasma chemistry tests. Drawing it after the serum tubes prevents interference with the clotting process needed for serum collection.

5. EDTA Tubes (Lavender or Pink)

• Purpose: EDTA tubes are commonly used for hematology tests such as Complete Blood Count (CBC) and blood typing.
• Additive: EDTA (ethylenediaminetetraacetic acid) is an anticoagulant that binds calcium to prevent clotting.
• Why Fifth: EDTA is a strong anticoagulant that can interfere with calcium-dependent tests, so it is drawn after coagulation and serum tubes to avoid contamination.

6. Glucose Tubes (Gray)

• Purpose: Glucose tubes are used for glucose testing, including fasting blood sugar and glucose tolerance tests.
• Additive: Sodium fluoride (a glycolysis inhibitor) and potassium oxalate (anticoagulant).
• Why Sixth: Sodium fluoride prevents glycolysis (the breakdown of glucose), making these tubes crucial for accurate glucose measurement. They are drawn last to avoid contamination of other tubes by fluoride or oxalate.

Special Considerations in Tube Filling Sequence

Pediatric Patients

In pediatric patients, the sequence of tube filling may be adjusted due to the limited blood volume. Blood culture tubes are still drawn first, followed by EDTA tubes to ensure enough blood is collected for hematological testing.

Venipuncture vs. Capillary Collection

For venipuncture, the order of draw is critical, as described above. However, in capillary blood collection, the sequence differs due to the small volume of blood. Typically, EDTA tubes are filled first, followed by other additive tubes.

Potential Risks of Incorrect Tube Sequence

1. Additive Carryover

Additives like EDTA or sodium fluoride can interfere with other tests if carried over into subsequent tubes. For example, EDTA can bind calcium, leading to falsely low calcium levels in serum samples.

2. Incorrect Coagulation Results

Drawing a coagulation tube after an EDTA tube can result in prolonged clotting times, leading to inaccurate diagnoses or improper treatment.

3. Repeated Testing

Failure to follow the correct sequence can lead to the need for repeated blood draws, which is uncomfortable for the patient and adds unnecessary costs and delays to diagnosis.

Conclusion

Following the correct sequence of tube filling for hematological investigations is essential for ensuring accurate lab results. Healthcare professionals, especially phlebotomists, must be well-trained in this sequence to prevent cross-contamination, erroneous results, and the need for retesting.

By adhering to these guidelines, you ensure both the safety of the patient and the reliability of the diagnostic process, ultimately improving patient care and outcomes.

For further information on best practices in hematological investigations, consult the latest CLSI guidelines or speak with a certified laboratory professional.