A host of new assays have been described and utilized which measure biomarkers of cellular stress or specific signaling events more proximal to initial cytotoxic insult (i.e. glutathione, caspases) [4] . These methods offer early indication of potential cytotoxicity, but are typically relegated to secondary screening because they are more difficult to employ as endpoint assays due to the transient nature of the biomarker and kinetic differences associated with cell death progression [5, 6] . Therefore assay chemistries predicated upon the detection of changes in membrane integrity remain the gold standard for in vitro cytotoxicity testing.