FACS data analysis – Gating strategies

Importance of Gating Strategies in Data Interpretation

Accurate gating is essential for transforming raw flow cytometry data into meaningful biological insights. Since each gate determines which events are included or excluded in the analysis, gating directly impacts:

• Population frequency estimates (e.g., % of T cells in a lymphocyte gate)
• Quantification of marker expression levels
• Identification of rare or transitional subpopulations

A poorly defined gate can introduce false positives or negatives, inflate background, or overlook subtle biological differences—especially in multicolor experiments or clinical samples. On the other hand, a well-constructed gating strategy enables:

• Reproducible and standardized data interpretation across experiments or users
• Improved resolution of closely related populations (e.g., naïve vs. memory T cells)
• Confident discrimination between specific signal and autofluorescence or nonspecific staining

Furthermore, robust gating is crucial in diagnostic flow cytometry, where clinical decisions may hinge on the presence or absence of specific cell subsets (e.g., leukemic blasts, activated immune cells). Therefore, developing and validating gating strategies—often with the use of FMO controls, isotype controls, and compensation settings—is a best practice in both research and clinical applications.

Key Gating Strategies in Flow Cytometry

Various gating strategies are used to identify and analyze specific cell populations depending on experimental goals.

Forward and Side Scatter (FSC & SSC) Gating

Cells are first gated on the basis of their scatter properties. Forward (FSC) and side scatter (SSC) give an idea of the size and granularity of the cells respectively. As an example, in the analysis of blood cell populations, scatter based gating is highly useful since blood is made of cells of distinct sizes and granularity (see dot plot below). The granulocytes in the red square have a high SSC and can be easily demarcated from the monocytes and lymphocytes. The monocytes in the yellow square form a distinct population from the lymphocytes (pink square) which are smaller and less granular. Lastly, the violet square contains the debris and RBCs with the lowest FSC and SSC values. Scatter gating is a useful tool for the bulk sorting of cells when the purity is not an issue. For more refined sorting and analysis, marker expression is an obvious criterion.

In the dot plots shown above, blood cells are first gated on the basis of CD3 and CD14 expression to separate T cells and non-lymphocytes. The populations in the green, red and blue ovals represent the T cells, granulocytes and monocytes respectively –. To confirm the identity of these populations vis-à-vis this gating strategy, they can be back-gated along FSC and SSC parameters. The dot plot on the right confirms these populations on the basis of their scatter properties.

*Note: Different graph types and statistics differ between different analysis software programs and are best studied using specific manuals for your flow cytometry instrument.