Disclosed herein are apparatus, systems, and methods for optically identifying and enumerating cells present in a blood sample. A light scatter detector array may be used having no more than three light scatter detectors. The array may include a side scatter detector, an intermediate angle light scatter detector, and one of an axial light loss detector and a forward light scatter detector. A lytic reagent system is disclosed that allows for the identification and enumeration of five major leukocyte populations in normal whole blood on an instrument using no more than three light scatter detectors.