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Adjusting AC Volume in Polycythemia

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:

  1. 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


  1. 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

  1. 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.
  2. 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.
  3. This formula may be used to prepare larger-volume tubes with sterile reagent-grade sodium citrate.
  4. Although discouraged, the formula may be applied to tubes smaller than 3.0 mL.
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