It depends on what this is in reference to, but often you might enrich the population your cells of interest through a procedure such as MACS (magnetic associated cell sorting) isolation before running the sample on the cytometer. For example, if you were interested in a specific population of CD4 T cells, you might isolate your CD4+ population first, then stain these cells and analyse them by flow. This helps reduce background noise from other cells that you have no interest in, and is particularly helpful if you want to sort a rare population of cells.
The principle reason behind enrichment is to increase the signal to noise ratio, particularly if you are trying to identify a rare population. For instance, if you record 50,000 events per sample and the ratio of cells of interest to other particles is less than 1 in 50,000, then you might not be able to detect it. Even if you do, you can't be confident that the observed particle within the specified gate is your cell of interest (unless you use a carefully validated multi-colour multi-gating strategy). Therefore, the best option is generally to increase the number of cells of interest through enrichment using an appropriate enrichment broth, for a few hours or overnight depending on the type and number of cells, before analysing the sample.
Using negative selection method for magnetic associated cell sorting (MACS) or any way of concentrating a particular population of cell is called as enrichment. In this all except your cell of interest will be collected at the bottom of the tube and all other cells will bing to the column.
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