Since the goal of any cell separation technique is to isolate a particular cell type of interest so that it may be analyzed independently, one of the most important factors to consider when selecting a separation method is purity.
Purity is a measure of how many isolated cells are of the cell type of interest vs. how many belong to undesired cell types. Most often, purity is conveyed as a percentage of desired cells out of the total number of isolated cells. For example, a cell separation having a purity of 90% indicates that 90 out of every 100 cells are the cell type of interest while only 10 out of the 100 are contaminating cell types.
What is considered sufficient cell purity can vary depending on the goals of the experiment, the overall complexity of the sample, and the specificity of the characteristic being used to isolate the target cells. However, greater purity means that the results obtained from a cell separation are more likely attributable to the cell type of interest.