Variability in viability


A viability assay is often viewed as one of the simplest assays performed in cell therapy. Yet, the interpretation of the results is quite confusing. Common viability assays make use of the fact that cell death is accompanied by deterioration of the cell membrane and hence, allows intracellular access to dyes that otherwise are not able to pass the membrane. Likewise, deterioration of the cell membrane results in a decrease of the electrical impedance of the cell.

Generally, viability of a cell product is simply expressed as ‘percentage (%) viable cells’ (e.g. the viability of cell product X is 92%).

Although it is assumed that the % viable cells is derived from the total number of cells (including, live, dead and dying cells), its meaning and implications are not clear without knowing the actual number of live cells. In fact, a cell count of live cells is essential for proper interpretation of viability, while determined over several consecutive timepoints it provides even more relevant information about the quality of a cell product.

It should be noted that dye exclusion viability assays are intended to specifically identify live cells, rather than to identify dead cells. There are significant challenges in measuring dead and dying cells. For example, in contrast to programmed cell death (apoptosis), cell processing- and sample preparation-induced uncontrolled cell death (necrosis) results in the complete disintegration of cells very rapidly. Measuring dead and dying cells therefore, is sensitive to the rate at which cells disintegrate and thus provide an unreliable denominator for calculating % viable cells.

Particularly in scaled-up cell processing, viability assessment can cause a seemingly high degree of variability in the expected quality of a cell product. Therefore, when the percentages of viable cells in a cell product appears to increase after scale up, it is likely due to variability in measurements on dead and dying cells rather than variability in the quality of the product itself. By including the absolute number of viable cells, the quality of a cell product is better described and could proof to be more reliable than percentage viable cells alone.