Why do I need to use Annexin V Binding Buffer?

Annexin V binding alone cannot differentiate between necrotic and apoptotic cells. It requires the presence of calcium in the solution. The annexin binding buffer is an isotonic buffer designed to promote the labeling of apoptotic cells with annexin V conjugates. the binding buffer contains an optimal concentration of calcium that facilitates the binding of annexin V to exposed phosphatidylserine residues on the surface of apoptotic cells.