How can I tell if my cell sample is dying?

Cell death in a sample can be characterized by cell crenation, blebbing, and debris or membranous remain, all of which are invisible to the naked eye. When cell death is not evident, assays can be used to assess cell death. The most common way to identify dead cells is to assess cell membrane health using cell-impermeant dyes, such as trypan blue (vital stain used in light microscopy) or propidium iodide (fluorescent DNA-binding dye).

The trypan blue dye exclusion test is used to determine the number of viable cells present in a cell suspension. It is based on the principle that viable cells with intact cell membranes are impermeable to polar dyes, such as trypan blue, whereas in dead cells with porous membranes, trypan blue can readily penetrate and stain the cytoplasm blue. Upon analysis by light microscope, the number of stained cells can be examined against the total cell population. The number of stained cells will represent the percentage of dead cells in the entire population.