Final answer:
The buffy coat, a layer containing leukocytes and platelets, should be used to perform the differential blood cell count on a blood smear.
Step-by-step explanation:
The portion of a blood smear that should be used to perform the differential blood cell count is the buffy coat. The buffy coat is a thin, pale layer that contains leukocytes (white blood cells) and platelets, which are key components in evaluating the body's immune response and clotting status.
During a hematocrit test, the blood sample is centrifuged, and the buffy coat forms between the plasma on top and the erythrocytes (red blood cells) at the bottom of the tube. The differential cell count focuses primarily on the white blood cells and their different subtypes, and the buffy coat provides a concentration of these important cells for analysis.