Cell Counting Process
Cell counting success is highly influenced by cell sample preparation, which needs to be performed consistently and carefully in order to ensure cell counting measurements are of the highest quality. The procedures for preparing the cell sample for cell counting are critical, as improper cell sample handling can increase counting error and standard deviation, especially between operators, which has been published previously. Numerous bioprocessing steps are required when processing from primary patient cell samples to final cell and gene therapy products, where the sampling and dilution steps are critical when preparing cell samples for cell counting.
Important general considerations for sample processing include how the sampling and diluting occurs, which sample treatments are performed, and what staining procedures are implemented for the assay probes. These considerations can be further detailed when designing cell counting protocols, to ensure that all operators are performing the assay consistently. Cell sampling factors include sample size, frequency, and sample mixing, as well as calibration and correct use of sampling tools such as pipettes.
One major example demonstrating the importance of cell counting process is sample mixing and waiting time, which can cause cell settling thus affecting the cell counting results. To measure the effect of cell settling time on cell counting measurements, a 2 mL sample of Jurkat cells was aliquoted into a 15 mL tube. After uniform mixing by inverting the tube 10X and pipetting up and down 10X, approximately 200 µL of cells were removed from the middle of the suspension at 0, 1, 5, and 20 mins and analyzed. The results demonstrate that only 5 min of settling time can cause a 26% reduction in cell concentration. (Figure 1). This serves to emphasize how even slight variability in sample processing can have significant effects on cell counting accuracy.
Figure 1. Effect of cell settling time on measured cell concentrations
Protocols that include dilution should consider dilution buffer type (i.e. pH, temperature, etc.), as well as rounds of dilution. Previous publications have found that higher dilution factors generated lower CV; however, too many dilution steps can also increase CV. Sample treatments may include lysing, de-clumping, and any form of purification.
Finally, when assessing staining procedures, careful attention should be paid to the amount of stain, as well as the staining conditions such as temperature, light exposure, and time. As previously mentioned, many probes are sensitive to time, especially when staining dead cells, therefore, researchers must carefully consider their probes and signals through the context of operator processing. All of these steps for sample collection and processing must be performed consistently between all operators, as even minor variations can negatively affect cell counting accuracy and data.