An improved technique for calculating relative response in cellular proliferation experiments

Investigating the response of cells to specific agonists may involve the use of cell tracking dyes to assess the extent of stimulated proliferation, frequently reported as the proliferation index (PI). Calculation of PI uses a model for cell division that expects the cell number to double as cells proliferate through each successive generation. It is often useful to compare the PI of a stimulated control population with that of a population in the presence of some agent, whether chemical, pharmacologic, or cellular. For such comparison studies, the nature of the metric being used must be taken into account to accurately assess the extent of inhibition. Specifically, the metric used in ModFit LT (Verity Software House, Topsham, ME) and in FCS Express (De Novo Software, Los Angeles, CA) uses a metric with a lower limit of unity, whereas the metric used in FlowJo (Treestar, Ashland, OR) has a lower limit of zero. For studies involving cell proliferation comparisons using tracking dye dilution, a new equation is proposed as the appropriate calculation to use when determining the percent of relative response based on proliferation index values for a metric whose lower limit is unity.