Final answer:
A medical assistant should use the lavender top tube during phlebotomy when a complete blood count (CBC) is ordered by the physician. The lavender top tube contains an anticoagulant that helps preserve the blood sample and allows accurate testing of the complete blood count.
Step-by-step explanation:
During phlebotomy, if a complete blood count (CBC) is ordered by the physician, the medical assistant (MA) should use the lavender top tube. The lavender top tube, also known as the EDTA tube, contains ethylenediaminetetraacetic acid (EDTA), an anticoagulant that helps preserve the blood sample. The EDTA in the lavender top tube prevents clotting and allows accurate testing of the complete blood count, which includes various blood cell counts and indices.