University of Alabama at Birmingham clinical pathologist Chad Siniard MD, shared the following streamlined approach to adjusting the sodium citrate volume to accommodate specimens from patients whose hematocrit exceeds 55%. The formula is adapted from CLSI Collection, Transport, and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays, 6th ed. Guideline H21, Clinical Laboratory and Standards Institute, 2024.
Standard blue-top tubes are calibrated to collect 2.7 mL of whole blood to mix with the 0.3 mL of 3.2% sodium citrate in the tube. When the hematocrit (HCT) level exceeds 55%, the reduced specimen plasma volume necessitates decreasing the volume of sodium citrate using these steps:
- Calculate the proper amount of anticoagulant (AC) required:
Blood volume (mL) X (100 – HCT %) | = | AC volume (mL) required |
595 – HCT % |
Example for HCT = 60%:
2.7 mL X (100 – 60)
|
= | 0.202 mL |
595 – 60 |
- Subtract the corrected AC volume from the original AC volume in the tube, The remainder is the amount of AC to be removed.
Example: AC in tube = 0.3 mL, AC volume required = 0.202 mL, 0.3 – 0.202 = remove 0.098 mL
- Shortcut: because most polycythemic HCTs range between 55–65%, withdraw by pipette 0.1 mL (100 uL) from several standard tubes, label, and make available when necessary.
- As the adjusted tubes have no vacuum, the specimen is collected by a syringe, and the correct volume is transferred safely to the adjusted tube. The tube is capped, mixed gently, and labeled.
- This formula may be used to prepare larger-volume tubes with sterile reagent-grade sodium citrate.
- Although discouraged, the formula may be applied to tubes smaller than 3.0 mL.
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