The passage number simply refers to the number of times the cells in the culture have been subcultured, often without consideration of the inoculation densities or recoveries involved. The population doubling level (PDL) refers to the total number of times the cells in the population have doubled since their primary isolation in vitro. This is usually a very crude estimate rounded off to the nearest whole number. A formula to use for the calculation of population doublings is as follows: n = 3.32 (log UCY - log l) + X, where n = the final PDL number at end of a given subculture, UCY = the cell yield at that point, l = the cell number used as inoculum to begin that subculture, and X = the doubling level of the inoculum used to initiate the subculture being quantitated.

A more complete discussion of this subject was provided by L. Hayflick (1973) Tissue Culture Methods and Applications, P.F. Kruse, Jr. and M.K. Patterson, Jr. eds., p. 220 (Academic Press, New York).