SCYM Data Analysis 2 — Questions and Answers
Question 1: In flow cytometry, what does the term 'compensation' correct for?
- Detector quantum efficiency differences
- Spectral spillover of fluorochromes into adjacent channels (Correct answer)
- Laser power fluctuations between samples
- Cell size variation affecting scatter signals
Correct answer: Spectral spillover of fluorochromes into adjacent channels
Compensation mathematically removes spectral spillover, where emission from one fluorochrome bleeds into a detector channel assigned to another fluorochrome.
Question 2: Which gating strategy is most appropriate for identifying rare cell populations comprising less than 0.01% of total events?
- Broad scatter gates followed by single fluorochrome threshold gates
- Sequential hierarchical gating with back-gating validation (Correct answer)
- Single-parameter histogram analysis only
- Ratio gating using FSC/SSC parameters exclusively
Correct answer: Sequential hierarchical gating with back-gating validation
Sequential hierarchical gating progressively enriches the population of interest, and back-gating confirms the identified population is correctly placed in earlier scatter plots.
Question 3: What is the purpose of using a 'fluorescence minus one' (FMO) control?
- To set voltage on each detector independently
- To determine the boundary of positivity in the presence of spectral spillover (Correct answer)
- To measure autofluorescence of unstained cells
- To calibrate the laser power for each experiment
Correct answer: To determine the boundary of positivity in the presence of spectral spillover
FMO controls contain all fluorochromes except the one being measured, revealing where the negative population boundary falls after compensation is applied.
Question 4: When analyzing a bivariate dot plot, a 'cloud' of events forming a diagonal line from low-low to high-high typically indicates:
- Two completely independent markers being measured
- High spectral spillover requiring additional compensation
- A positive correlation between the two measured parameters (Correct answer)
- Instrument malfunction causing cross-channel interference
Correct answer: A positive correlation between the two measured parameters
A diagonal cluster from low-low to high-high in a bivariate plot indicates that the two parameters co-vary positively, suggesting co-expression or biological co-regulation.
Question 5: In mass cytometry (CyTOF), which approach replaces traditional spectral compensation?
- Electronic gating to exclude cross-reactive signals
- Bead-based normalization without compensation matrices
- Minimal signal spillover due to narrow isotope mass windows (Correct answer)
- Automated software deconvolution of overlapping isotope peaks
Correct answer: Minimal signal spillover due to narrow isotope mass windows
Mass cytometry uses heavy metal isotope tags with minimal mass overlap, so the need for spectral compensation is largely eliminated compared to fluorescence-based cytometry.
Question 6: Which statistical method is most commonly used to assess whether two cell populations in a cytometry experiment are significantly different?
- Linear regression analysis
- Chi-square goodness-of-fit test
- Student's t-test or non-parametric equivalent (e.g., Mann-Whitney U) (Correct answer)
- Pearson correlation coefficient
Correct answer: Student's t-test or non-parametric equivalent (e.g., Mann-Whitney U)
The t-test (or Mann-Whitney U for non-normal distributions) compares means of two groups, making it appropriate for evaluating differences between cell population frequencies or median fluorescence intensities.
Question 7: What does a high coefficient of variation (CV) in a single-peak bead population indicate during quality control?
- Excellent instrument alignment and sensitivity
- Broad peak width suggesting instrument performance issues (Correct answer)
- Successful compensation of spectral spillover
- Optimal photomultiplier tube voltage settings
Correct answer: Broad peak width suggesting instrument performance issues
A high CV in a bead peak indicates broad distribution of events, which may reflect poor instrument alignment, suboptimal PMT voltage, or debris contamination.
SCYM Data Analysis 2
In flow cytometry, what does the term 'compensation' correct for?