Question 2 of 6 Intermediate Scenario
How would you design a defensible gating strategy?
Short answer
Set gates in a documented hierarchy: acquisition QC, cells, singlets, viable events, lineage and the target population. Use appropriate biological negatives and FMO controls for ambiguous positivity, apply the same rules across comparable samples and report the parent denominator.
Why
Start with the biological outcome, not an attractive plot. Confirm instrument stability and time-dependent anomalies. Exclude debris, distinguish singlets from aggregates, and restrict to live cells when appropriate.
Then identify the lineage with robust markers before gating rare subsets. In a multicolor T-cell experiment, for example, you might define live singlet CD45-positive cells, then CD3-positive T cells, then CD4 or CD8 subsets and finally CD69 or cytokine-positive events. FMO controls can inform a dim positive threshold where spreading matters, but the gate should also be biologically plausible and evaluated using appropriate negative samples.
Record the exact sequence of gates and counts at every stage. If treatment alters forward scatter or marker intensity, hard-coded gates may introduce differential exclusion, so investigate sensitivity and use prespecified guidance. Verify compensation, staining specificity and sufficient events.
Report both the numerator and the biologically meaningful parent population, and ideally absolute counts where supported by the assay.
Common mistakes
- Reporting a rare-cell percentage without the parent population or selecting gates after seeing the treatment effect
The interviewer might ask next
What would you check if your target frequency doubles when a viability gate changes?