Cell culture
Passage and population doublings
Track how much a culture has actually grown, not just how many times you split it. From seed and harvest numbers it gives population doublings, doubling time and a cumulative doubling level; from a split ratio it estimates doublings when you did not count.
Population doublings are log₂ of the fold increase from seed to harvest. Doubling time is the culture time divided by the doublings. Cumulative PDL is the running total across passages and is the honest record of a line’s age — a split ratio alone is not, because how far the cells grew back before harvest varies.
From split ratio
When cells are split at a fixed ratio and grown back to the same density each time, the doublings per passage are log₂ of the ratio: a 1:8 split is three doublings. This estimates cumulative doublings from the ratio and passage count when exact seed and harvest numbers were not recorded.
Formula
population doublings = log₂(harvest ÷ seed); doubling time = culture time ÷ doublings
- seed
- cells put down at the start of the passage
- harvest
- cells recovered at the end
- cumulative PDL
- the running sum of doublings over the life of the line
- from a ratio
- doublings per passage = log₂(split ratio), assuming a full grow-back each time
Worked example
Seeding 5 × 10⁵ and harvesting 4 × 10⁶ after 72 hours
- seed 500,000
- harvest 4,000,000
- time 72 hours
- Fold increase: 4,000,000 over 500,000 is 8.
- Doublings: log₂ of 8 is 3.
- Doubling time: 72 hours over 3 is 24 hours.
Answer 3 doublings, a 24-hour doubling time
Common mistakes
- Recording passage number as if it measured age. Two labs at passage 20 can be tens of doublings apart depending on split ratio and grow-back; cumulative PDL is the figure that travels.
- Reading a doubling time off two harvests days apart. Growth is not steady through a passage; the exponential phase gives the honest rate, the plateau does not.
- Mixing hours and days. Enter the culture time in one unit and keep the doubling time in the same one.
Frequently asked
Split ratio or exact counts?
Counts are better because grow-back varies. The ratio route is a reasonable estimate for the stretch of history where you only recorded how you split.
Why does cumulative PDL matter?
Many lines drift or senesce past a certain doubling level. Logging PDL lets you retire a line at a defined age and makes work reproducible.
What if harvest is below seed?
Then there was no net growth and no doubling time exists; check for a plating or viability problem rather than forcing a number.