CAP TODAY Pathology/Laboratory Medicine/Laboratory Management
Editor: Frederick L. Kiechle, MD, PhD
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Which CBC parameters require correction?
How many NRBCs before WBC count correction?
Q. When performing a platelet count from a blood sample collected in a sodium citrate tube, the result is multiplied by 1.1 to correct for the volumetric difference in anticoagulant compared to EDTA. Which other CBC parameters, if any, should be similarly corrected?
A. Blood specimens for complete blood count testing are usually collected in a lavender-top tube containing ethylenediaminetetraacetic acid (EDTA) anticoagulant. However, EDTA occasionally induces platelet clumping as an in vitro phenomenon. If platelet clumps are flagged by the hematology analyzer and/or identified on the blood smear, steps should be taken to eliminate the clumps because they can artificially decrease the platelet count, referred to as pseudothrombocytopenia.1 In addition, platelet clumps may be counted as leukocytes, leading to pseudoleukocytosis. The measured mean platelet volume (MPV) may also be affected.
The first step to resolve platelet clumping is to vortex the specimen for one to two minutes at or near the highest setting, which will be successful in approximately 50 percent of cases.2 If this does not resolve the clumps, the sample should be recollected in a tube containing a non-EDTA anticoagulant, such as a blue-top sodium citrate tube. The platelet count obtained from the sodium citrate tube must be multiplied by 1.1 to account for the different blood-to-anticoagulant ratio in the citrate tube.
In our laboratory, if vortexing does not resolve platelet clumping, we request the specimen be recollected in both EDTA and citrate tubes. Both tubes are then run on the hematology analyzer and slides are made. If platelet clumps are still present in the EDTA tube, the platelet count and white blood cell count from the citrate tube are multiplied by 1.1 and reported. The MPV from the citrate tube is also reported, but no correction factor is applied because the MPV is not affected by dilution. All other CBC parameters are reported from the EDTA tube.
To report CBC data from a citrate tube only, apply the 1.1 correction factor to all parameters reported per unit volume, which includes the red blood cell count, WBC count, platelet count, hemoglobin, and absolute counts for all cell types in the WBC differential.3 The hematocrit should also be multiplied by 1.1 or, equivalently, should be calculated using the corrected RBC count (Hct = RBC × MCV/10). The MCH (Hgb/RBC × 10) and MCHC (Hgb/Hct × 100) do not require correction because the 1.1 correction factor cancels out in the numerator and denominator of these calculations. The MCV, RDW, and MPV are not affected by dilution and do not require correction.