Cytotoxicity assay chemistries are formulated to detect loss of membrane integrity associated with cell death. The current most useful cytotoxicity biomarkers are constitutive, conserved and relatively stable, high-abundance enzymes "released" into the extra-cellular environment (culture medium) following loss of membrane integrity. Unlike viability assays whereby a decline in cell response is strongly inferred to be caused by cell-cycle arrest or death, detection of cytotoxic biomarker activities outside the cellular compartment after treatment is proof-positive for a compound's cytotoxic effect. Unfortunately, activity-based surrogates of cell death have a finite enzymatic half-life, which can limit their utility during long compound/cell contact incubations (48-72 hr) [3, 27] . Enzymatic decay as a function of time in culture medium can lead to underestimation of the absolute level of cytotoxicity [27] . The quantitative HTS (qHTS) approach eliminates a majority of this error because the kinetics of cytotoxicity is intrinsically linked to dose [42] .