Morphological methods (including ultrastructural studies) require highly skilled personnel,
expensive equipment and results are not usually quantifiable; cells must also be destroyed so
continuous monitoring or kinetic studies are ruled out. Various cell viability and cell proliferation assays are used to determine the effect of a test compound on cells propagated in vitro. Indirect techniques that assess cell viability by monitoring cell membrane integrity after drug exposure, i.e., dye exclusion and preferential dye uptake, also destroy or interfere with the cell’s functioning and hence are terminal assays. Other assays that measure cell viability, indirectly, by quantitation of reduction of the intracellular environment using indicator metabolic markers are useful and offer fewer limitations than other methods. Assays that quantify intracellular ATP (the ATP status of cells reflects the cells’ energy capacity and viability) have also been developed where the concentration of ATP in indicative of the number of viable cells in that culture. Other biomarkers that can be quantified include NADH, capases, LDH, in addition to live- and dead-cell proteases (www.promega.com). One potential drawback of some of these metabolic assays is there is no differentiation between cells that are actively dividing and those that are quiescent which may result in an over-estimation of cell number. Also, an increase in cell growth may indicate cell viability but, a decrease in viability can be interpreted as the result of either cytotoxic effects of a particular test compound or induction of sub-optimal
assay conditions.