ISAC Data Analysis and Interpretation 3 — Questions and Answers
Question 1: When analyzing cell cycle data stained with propidium iodide (PI), which cell cycle phase corresponds to the G2/M peak?
- The sub-G1 peak to the left of G1
- The peak with exactly twice the DNA content of G1 (Correct answer)
- The broad S-phase region between G1 and G2/M
- The population with fragmented DNA
Correct answer: The peak with exactly twice the DNA content of G1
G2/M cells have undergone DNA replication and contain twice (4N) the DNA content of G1 (2N) cells, appearing as a distinct peak at double the G1 channel.
Question 2: In spectral flow cytometry, what is the key mathematical process that separates overlapping emission spectra into individual fluorochrome contributions?
- Logarithmic transformation of raw photon counts
- Spectral unmixing using a reference library of pure spectra (Correct answer)
- Autofluorescence subtraction using UV laser excitation
- Principal component analysis of scatter parameters
Correct answer: Spectral unmixing using a reference library of pure spectra
Spectral unmixing uses least-squares fitting against a library of single-fluorochrome reference spectra to deconvolve overlapping signals across all detectors.
Question 3: A dot plot showing FSC-A on the X-axis and FSC-H on the Y-axis is PRIMARILY used to:
- Separate live and dead cells based on membrane integrity
- Identify cell doublets by discriminating cell aggregates from singlets (Correct answer)
- Set voltage on the forward scatter PMT
- Exclude debris based on side scatter characteristics
Correct answer: Identify cell doublets by discriminating cell aggregates from singlets
Doublet discrimination plots use FSC-A vs. FSC-H because doublets have proportionally larger area than height, falling off the diagonal of singlets.
Question 4: Which statistical measure is MOST appropriate for describing the central tendency of a highly skewed fluorescence distribution?
- Mean fluorescence intensity (MFI)
- Median fluorescence intensity (Correct answer)
- Mode of the histogram
- Standard deviation of the peak
Correct answer: Median fluorescence intensity
Median is resistant to outliers and skewed distributions, making it more representative of the central tendency than mean for non-normal fluorescence data.
Question 5: In the context of SPADE (Spanning-tree Progression Analysis of Density-normalized Events), nodes in the tree represent:
- Individual cells color-coded by marker expression
- Clusters of phenotypically similar cells (Correct answer)
- Time points in a developmental trajectory
- Compensation matrix entries for each fluorochrome
Correct answer: Clusters of phenotypically similar cells
SPADE clusters cells into nodes of similar phenotype and connects them in a spanning tree that reflects developmental or phenotypic relationships.
Question 6: When should a researcher use log-scale transformation rather than linear scale for displaying cytometry data?
- When analyzing scatter parameters like FSC and SSC only
- When displaying fluorescence parameters that span several decades of intensity (Correct answer)
- When the population of interest has a very narrow range of expression
- When compensation has not yet been applied to the dataset
Correct answer: When displaying fluorescence parameters that span several decades of intensity
Fluorescence data typically spans 3–5 orders of magnitude, requiring logarithmic (or biexponential) scaling to simultaneously visualize dim and bright populations.
Question 7: What artifact can occur if the stop condition in a flow cytometry acquisition is set to 'time' rather than 'cell count' in a sample with variable flow rate?
- Increased doublet frequency due to faster flow
- Inconsistent cell numbers across samples, compromising quantitative comparisons (Correct answer)
- Automatic triggering of the compensation matrix recalculation
- Loss of the smallest cells through threshold exclusion
Correct answer: Inconsistent cell numbers across samples, compromising quantitative comparisons
Time-based stops collect variable cell counts if flow rate fluctuates, making cross-sample quantitative comparisons unreliable.
ISAC Data Analysis and Interpretation 3
When analyzing cell cycle data stained with propidium iodide (PI), which cell cycle phase corresponds to the G2/M peak?