Breast cancer diagnosis is a rapidly expanding research field, whose early-stage detection may reduce the burden of the disease and permit more effective treatment. It is well documented that tumor growth requires specific conditions for its development from the occurrence of a mutation to a clinically detectable cancer. It has been shown that living peripheral blood mononuclear cells (PBMC) have different ability of cytosolic enzymes (esterases) to convert hydrophobic substrate fluorescein diacetate (FDA) into hydrophilic fluorescent fluorescein. It depends on PBMC cycle stage, size, surface state, physiological states, such as activation and apoptosis.