I am producing lentiviruses and as a preliminary experiment, I produced the virus using HEK293T cells and checked the functional titre using primary T cells.

I had 3 conditions for titre: virus+cells left for 24 hours. Post 24 hours, I took half of the volume from these wells and observed for GFP expression (FYI: success). The remaining half, I spun down, removed the virus, and cultured in fresh medium for another 24 hours.

The final condition was that the cells were cultures with the virus for 48 hours.

I used undiluted, 1:10, 1:100 and 1:1000 virus.

I started with 500,000 cells in each well. However, since the 24 hour conditions were split into two, the titre will now be calculated with 250,000 cells per condition. while the 48 h sample has 500,000 cells.

My question is, since the titre calculation formula is cell number dependent, how do I compare the titres of 24 and 48 h? Can these values be scaled up and down?