Cell viability is often defined as the number of healthy cells in a
sample. Often the same assays used to determine viability are
used repeatedly over a period of time to investigate cell proliferation
within a population. Cell viability methods loosely can be
categorised into those which analyse whole populations and those
which involve analysis of individual cells. Generally speaking, the
population analysis is more rapid, but gives a less detailed result
than those which involve viability measurements on the single cell
level. One of the earliest methods for assessing cell viability was
trypan blue dye exclusion assay, which is still widely used today. It
is based on the principle that viable cells have an intact cell membrane
which can therefore exclude the trypan blue dye. Dead cells
take up trypan blue, and appear blue as a consequence, as their
membrane is no longer able to control the passage of macromolecules.
The assay requires the cells to be in a single cell suspension
and they are then visualised and counted under a microscope
using a haemocytometer of a defined volume or using automated
counting devices which have recently become available. From
these counts, it is relatively simple to calculate the total number
of cells and the percent of viable cells within a population.