Minimum blood volume for CBC tubes.
A The amount of anticoagulant that manufacturers recommend placing in their tubes is calculated to provide the proper blood:anticoagulant ratio when the tube is completely filled. Therefore, all tubes should be filled to their stated volume. Short draws disrupt this ratio and can affect results. When an EDTA tube is underfilled, the specimen is excessively anticoagulated. When the ratio of EDTA to blood is too high, the red cells shrink. As a result, hematocrit, mean cell volume, and the mean corpuscular hemoglobin concentration will be affected. Sedimentation rates conducted on excessively anticoagulated specimens can also be erroneous.
If filling a standard-sized tube is not possible because of a difficult draw or because a limited volume of blood has been obtained by syringe, smaller-volume tubes should be used so that a completely filled tube can be submitted for testing.
If a facility seeks to establish a policy to accept underfilled tubes, it should carefully consider the consequences of filling collection tubes against the manufacturer's recommendations. Only after consulting with the tube manufacturer and conducting internal evaluations that demonstrate no significant difference in results should facilities establish specimen-rejection criteria for underfilled specimens. Such a study should evaluate the effects that underfilling has on all components of a CBC, including indices, on tubes underfilled to various degrees and at the typical temperatures and storage duration practiced at the facility.
Without properly assessing the effect of underfilled specimens on test results and establishing strict specimen-rejection criteria, facilities put the patient at risk of being diagnosed, medicated, and/or treated according to inaccurate results.
--Dennis Ernst, MT(ASCP), Director
The Center for Phlebotomy Education Inc.