The International Council for Standardization in Haematology and NCCLS have recommended K2EDTA as the anticoagulant of choice for blood cell counting and sizing for the following reasons1,2:
• K3EDTA results in greater RBC shrinkage with increasing EDTA concentrations
(11% shrinkage with 7.5 mg/ml blood).
• K3EDTA produces a larger increase in cell volume on standing (1.6% increase after 4 hours).
• K3EDTA leads to lower MCV values (typically a -0.1 to -1.3% difference is observed compared with K2EDTA).
• K3EDTA is a liquid additive, and therefore, will result in the dilution of the specimen. All directly measured values (Hgb, RBC, WBC, and platelet counts) have been reported to be 1-2% lower than results obtained with K2EDTA2,3.
• With some instrument systems, K3EDTA gives lower WBC counts when used at high concentrations. Brunson, et al., reported that plastic tubes containing K2EDTA gave complete blood count and differential results in excellent agreement with glass tubes containing K3EDTA, though they confirmed the earlier results of 1-2% higher WBC, RBC, hemoglobin, and platelet count results with the former tube, owing to dilution observed with K3EDTA4.
• Our internal studies showed no clinically significant differences when comparing K3EDTA glass tubes to K2EDTA plastic tubes.