Patent ID: 8216778

Claim:
A method of measuring the fraction of viable cells in a cell population maintained in a cell culture medium comprising: (a) detecting a cell death-stable enzyme activity in a portion of a cell culture conditioned medium not containing cells of the cell population; (b) detecting a cell death-stable enzyme activity in the cells and conditioned medium of a portion of the cell culture medium containing cells of the cell population; and (c) comparing the level of cell death-stable enzyme activity in the cells and conditioned medium portion of the cell culture medium containing cells to the level of cell death-stable enzyme activity in the portion of the cell culture conditioned medium not containing cells, wherein the cell death-stable enzyme activity is measured by the steps comprising: (i) contacting a sample with a substrate of the cell death-stable enzyme activity wherein the substrate is conjugated to a detectable leaving group; and (ii) detecting the leaving group, wherein the amount of leaving group detected is proportional to the level of cell death-stable enzyme activity, wherein the method additionally comprises adding an agent which attenuates the signal of the detectable leaving group, and wherein the fraction of viable cells in the cell population is directly proportional to the difference between the level of cell death-stable enzyme activity in the portion of the cell culture medium containing cells and conditioned medium and the level of cell death-stable enzyme activity in the portion of the cell culture conditioned medium not containing cells, wherein the agent which attenuates the signal of the leaving group is phenol red.