ISAC Quality Control and Assurance 3 — Questions and Answers
Question 1: Which statistical tool is most appropriate for comparing flow cytometry results between two instruments to assess method equivalence?
- Levey-Jennings chart
- Bland-Altman plot (Correct answer)
- Receiver operating characteristic (ROC) curve
- Frequency histogram
Correct answer: Bland-Altman plot
Bland-Altman plots display the agreement and systematic bias between two measurement methods across the range of values.
Question 2: For a multi-site flow cytometry network, which standardization strategy best ensures inter-site comparability of T-cell subset results?
- Using identical patient populations at each site
- Harmonizing instrument settings using target MFI values derived from common reference beads (Correct answer)
- Running all samples at a single central laboratory
- Using the same brand of antibody regardless of clone
Correct answer: Harmonizing instrument settings using target MFI values derived from common reference beads
Target MFI-based harmonization using shared reference beads aligns photomultiplier tube voltages across instruments for comparable results.
Question 3: A QC bead lot change results in a shift in the mean channel number on a Levey-Jennings chart. The appropriate corrective action is to:
- Recalibrate the instrument to the old bead values
- Establish a new baseline mean and SD using the new lot before resuming patient testing (Correct answer)
- Continue using the old mean and SD for comparison
- Report results with a disclaimer
Correct answer: Establish a new baseline mean and SD using the new lot before resuming patient testing
A new lot requires establishing its own baseline statistics; using old lot parameters would produce false QC failures or false passes.
Question 4: In clinical immunophenotyping, 'gating strategy validation' means:
- Confirming that all gates are drawn identically to published literature
- Demonstrating that the gating approach correctly identifies cell populations across a variety of clinical samples (Correct answer)
- Ensuring that scatter gates exclude all dead cells automatically
- Verifying that software gates are password-protected
Correct answer: Demonstrating that the gating approach correctly identifies cell populations across a variety of clinical samples
Gating validation requires demonstrating clinical accuracy and reproducibility across diverse sample types, not just matching a template.
Question 5: Which document provides the primary regulatory framework for quality systems in US clinical laboratories performing flow cytometry?
- ISO 9001:2015
- CLIA '88 (42 CFR Part 493) (Correct answer)
- CAP Checklist only
- ISAC Data File Standard
Correct answer: CLIA '88 (42 CFR Part 493)
CLIA '88 (42 CFR Part 493) establishes federal quality standards for all US clinical laboratory testing including flow cytometry.
Question 6: A 10-log decade detector is displaying saturation (off-scale events) for a highly expressed antigen. The best corrective action is:
- Increase laser power
- Decrease the PMT voltage or use a lower antibody concentration (Correct answer)
- Switch to a scatter parameter for that antigen
- Extend acquisition time
Correct answer: Decrease the PMT voltage or use a lower antibody concentration
Saturated detectors clip signal data; reducing PMT voltage or titrating antibody concentration brings the population into the linear detection range.
Question 7: In a flow cytometry proficiency testing program, a laboratory's result is flagged as an outlier. The first step in investigation should be:
- Immediately change the antibody clone used
- Review the raw FCS data files and gating strategy to determine if a technical or interpretive error occurred (Correct answer)
- Report the discrepancy to regulators
- Repeat the survey sample in triplicate
Correct answer: Review the raw FCS data files and gating strategy to determine if a technical or interpretive error occurred
Root cause analysis begins with reviewing the original data and gating to distinguish technical errors from interpretive ones.
ISAC Quality Control and Assurance 3
Which statistical tool is most appropriate for comparing flow cytometry results between two instruments to assess method equivalence?