What the heck is a Buffy coat?   We spin a tube of anticoagulated blood. The RBCs settle first, they're the densest. The WBCs make a layer or "Coat" on top of the RBCs. The more WBCs, the greater the thickness of the Buffy layer. Note in the case of the leukemia in this picture, there is not only a thickened buffy layer, but the there are fewer RBCs, owing to the anemia associated with the leukemic process.