How to determine cell viability?

Cell viability is a calculation of the number of viable or living cells within the total number of cells.  Although there are now commercially available alternatives to the historical method, trypan blue exclusion is the tried and true method commonly used in the lab.  Briefly, trypan blue dye is added to a cell suspension and visualized on a hemocytometer.   Any cell in the suspension that is not viable will have a weakened cell membrane and allow trypan blue to pass through the membrane.  When visualizing, the total cell population and trypan blue stained cells are counted.  Healthy cells are considered to be a suspension composed of greater than 90% viable cells as calculated by the following equation:

% viable cells = [ 1 – (number of blue cells/number of total cells) ] * 100