What is the difference between passage number and population doublings (PD)?

Passage number and population doublings (PDs) describe different aspects of cell culture expansion.

• Passage number indicates how many times cells have been subcultured (detached and re-seeded).
• Population doublings (PDs) indicate how many times the cell population has doubled in number.

Unlike passage number, PDs reflect the actual proliferation and biological age of a cell culture. For example, a 1:2 split and a 1:6 split are both counted as one passage, but they result in different levels of cell expansion and therefore different numbers of PDs.
For this reason, PromoCell specifies and guarantees population doublings (PDs) rather than passage numbers. Under typical culture conditions, 15 PDs correspond to approximately 6-8 passages, depending on the split ratio, seeding density, and cell yield.
 
Summary
Passage number counts subculturing events, whereas PDs measure actual cell proliferation. Therefore, PDs provide a more meaningful indication of culture age and expansion history.