If a lavender-top blood collection tube is only partially filled there will be an excess of EDTA anticoagulant in the tube. This causes increased red blood cell shrinkage. While this may not cause significant changes in automated measurements of MCV and hematocrit because of the isotonic diluents used, the excess EDTA can cause morphologic distortion of the red cells when examined on a blood smear. The most common morphologic distortion seen in samples with excess EDTA is the appearance of crenated red cells, as shown in the top image on the right. Although crenated red cells are an artifact, they can easily be confused with burr cells.
Normal erythrocytes on smears in which the stain is too alkaline may appear to have a bluish tinge. This could lead to erroneously reporting polychromatophilia.
Macrocytes and spherocytes may incorrectly be reported when examining erythrocyte morphology in a thin area of the smear where cells are flattened and lack central pallor. Examining in a too thick area may lead to erroneous reporting of rouleaux or agglutination, as shown in the bottom image on the right.