American Society for Clinical Pathology (ASCP) Technologist in Cytogenetics certification (ASCP CG) Practice Test

The best time to freeze cells for long-term storage is during the 80% confluent log phase. At this stage, cells are actively dividing and are in their optimal growth condition. This phase typically features healthy, metabolically active cells that are well-nourished and capable of recovering effectively after thawing. The 80% confluence indicates that the cells have enough space for further growth while also being dense enough to ensure adequate cell-to-cell communication and interaction, which is vital for maintaining cell health.

Freezing cells during this phase helps to ensure a high viability rate after thawing. Cells that are too confluent may start to exhibit signs of contact inhibition, potentially affecting their growth dynamics and recovery post-thaw. In contrast, freezing during the lag phase might lead to cells that are not actively dividing and could have lower viability, while freezing at the end of the exponential phase may stress the cells due to overcrowding, also impacting their ability to recover once thawed. Hence, the log phase with about 80% confluence strikes the right balance for optimal preservation.