FAQ - Passage vs PDL
Passage refers to the transfer or subculture of cells from one culture vessel to another. Usually, but not necessarily, this implies subdivisions of a proliferating cell population enabling propagation of a cell line. Thus "passage number" is the number of times a culture has been subcultured. By choosing an appropriate size culture vessel and seeding density, the "passage" of a cell culture can be a matter of convenience for the laboratory--once a week or twice a week. Passage numbers are incremented by one with each subculture in order to keep track of the number of manipulations a particular cell line has undergone. In incrementing passage numbers, the specific number of cells present in the population is rarely considered.
Population doubling level is an intrinsic measure of the "age" of the particular culture of a cell line. In culture, an untransformed cell line has a finite life span expressed in the number of cumulative population doublings that can be achieved. Population doubling levels refer to the total number of times the cells in the population have doubled since their primary isolation in vitro. The formula for calculating PDL is
PDL = 3.32(log (total viable cells at harvest/total viable cells at seed))
The "life span" of a cell line is plotted as the cumulative PDLs versus time in culture. Subcultures are carried out until the cell line reaches senescence: that is, there is no change in PDL from one subculture to the next.