On September 12, 2016, Michelle Fahs MT (ASCP) asked about how to prepare tubes with the proper volume of anticoagulant for people whose hematocrit exceeds 55%. Here is a question from Sandy Odegaard asking about the correction formula’s derivation:
Hi George, BD could not explain where the conversion factor for calculating the appropriate sodium citrate volume came from: X = (100 – PCV) vol / (595 – PCV), where PCV = packed cell volume or hematocrit, X = volume of citrate anticoagulant and vol = volume of whole blood added to the tube. Do you have some knowledge of this? I would be very grateful. Thank you.
From George, I once had an article from the 1970s that provided the derivation of this formula as stated in the CLSI H21 format: C = (1.85 X 10 –e3) (100 – HCT) V, where C = volume of citrate anticoagulant, HCT is hematocrit in percent, and V is the volume of whole blood added. The article made it clear how the constant, 1.85 X 10 –e3 (.00185) was developed. Regrettably, I’ve lost the article, and am not enough of a mathematician to derive the formula on my own. I’ve been in the habit, like many, of simply applying the CLSI H21 nomogram.
The article, Marlar RA, Potts RM, Marlar AA, Effect on routine and special coagulation testing valused of citrate anticoagulant adjustment in patients with high hematocrit values. Am J Clin Pathol 2006:128; 400–5 provides evidence for the need for citrate sdjustment. They compared 28 high hematocrit patient results produced from standard and citrate-adjusted whole blood specimens and found significant differences for PTT, PT, fibrinogen, factor VIII, and protein C activity. In this study the authors withdrew the computed citrate volume to discard using a tuberculin syringe, thus maintaining the tube’s vacuum.
Participants, please respond with the derivation of the constant in the formula. Thank you.
Based on literature, 45%
Based on literature, a 45% hematocrit (HCT%) is considered normal and an increased HCT up to 55% (polycythemia) or decreased down to 20% (anemia) has no significant impact on PT or PTT results. Personally I believe the above assumption is reagent-dependent and we should be cautious about the application of a generalized rule like this.
Anyway, an application of the formula for adjustment of citrate volume in commercial tubes is only recommended for patients with a HCT of above 55%. There are many rearrangement of the same formula in literature but I like this one better than the others because of its simplicity. Moreover it works for all sizes of citrate tubes (Bennett ST, Lehmann CM, Rodgers GM. Laboratory Hemostasis, A Practical Guide for Pathologists, 2nd Edition, Springer, 2015):
This is from page 27 of the first edition, 2007:
R = V [0.1 – (100-HCT)/(595-HCT)]
R is volume of citrate solution that needs to be removed from tube.
V is total filled volume in a tube, which is typically a value of 5, 3, 2 or 1 mL. It is the sum of patient’s blood volume and volume of citrate solution in the tube.
HCT is patient’s hematocrit and it must be more than 55%. The logic behind this formula is based on providing an identical citrate concentration in liquid phase of collected blood sample.
Here we use a simple formula
Here we use a simple formula that gives the amount of anticoagulant to remove from a NaCitrate tube based on the amount of citrate in the tube, and the patient’s Hct (if over 55% according to our policy). The idea being that the amount of anticoagulant that is left remains proportional to the (relatively decreased) amount of plasma in a tube with a high Hct.
The amount of liquid anticoagulant to remove from a tube equals:
OCV – [OCV x (100-pHct)/65], where OCV is the original citrate volume in the tube, pHcT is the patient’s Hct, and 65 is what we consider a normal “plasma crit”.
However, I cannot find a reference for this formula either! But I will say that other than having to break the vacuum on a tube and use a adjustable micro-pipettor to remove the proper amount of anticoagulant, this is a pretty easy method to understand. Just hope it is valid!
A comment from Tom Exner,
A comment from Tom Exner, Haematex Research; Hi George, Thanks for asking. I have no idea at present but will think on it. The derivation must be quite logical but I have forgotten most of my algebra..
Another alternative to reducing the citrate level appropriately is to increase the calcium chloride concentration. This has been looked at and published by M.Kanahara et al but they only used 35mM CaCl2 and the correlations were not good. Tom Exner