Formula for citrate volume adjustment In high hematocrit

When collecting blood into a citrate bulb (commonly used for coagulation tests like PT, APTT, etc.), the standard anticoagulant-to-blood ratio is 1 part citrate to 9 parts blood. However, if the patient has a significantly elevated hematocrit (usually above 55%), this ratio needs to be adjusted because the plasma volume (the actual liquid portion of blood) is reduced in these cases. If you don’t correct for this, the excess citrate can interfere with clotting test results.

Formula for citrate volume adjustment:

C=(1.85×10−3)×(100−HCT)×VC = (1.85 \times 10^{-3}) \times (100 – HCT) \times VC=(1.85×10−3)×(100−HCT)×V

Where:

• C = volume of citrate to be used (mL)
• HCT = patient hematocrit (%)
• V = volume of whole blood sample required (mL)

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Explanation:

• High hematocrit means less plasma, so the citrate anticoagulant is too concentrated.
• The formula reduces the citrate volume accordingly.

Example Calculation:

• Target blood volume (V) = 5 mL
• Patient hematocrit = 65%

C=(1.85×10−3)×(100−65)×5=0.323875 mL citrateC = (1.85 \times 10^{-3}) \times (100 – 65) \times 5 = 0.323875 \text{ mL citrate}C=(1.85×10−3)×(100−65)×5=0.323875 mL citrate

This is less than the usual 0.5 mL citrate for a 5 mL tube (since the default assumes normal hematocrit).

In practice:

• Labs may use pre-filled citrate tubes that aren’t adjustable. In these cases, you might need to either:
  • Draw a smaller volume of blood.
  • Use specialized low-volume citrate tubes for high HCT patients.
• Most analyzers also allow you to input a corrected citrate ratio if needed.

Quick Reference Chart (for common volumes)

HCT (%)	Required Blood Volume (mL)	Corrected Citrate Volume (mL)
45	5ml	0.5 mL (Standard tube)
55	5ml	0.416 mL
60	5ml	0.370 mL
65	5ml	0.324 mL
70	5ml	0.277 mL
75	5ml	0.231 mL