CLT Quality Control & Assurance in Testing 2 — Questions and Answers
Question 1: A laboratory's Levey-Jennings chart shows 6 consecutive QC values all falling on the same side of the mean. Which Westgard rule is violated?
- 1-2s rule
- R-4s rule
- 10x rule (Correct answer)
- 2-2s rule
Correct answer: 10x rule
The 10x rule is triggered when 10 consecutive QC values fall on the same side of the mean, indicating a systematic shift; 6 consecutive points triggers the 6x warning rule.
Question 2: Which of the following BEST describes 'accuracy' in laboratory quality control?
- The ability to reproduce the same result repeatedly
- How close a measured value is to the true value (Correct answer)
- The range within which 95% of values fall
- The coefficient of variation of repeated measurements
Correct answer: How close a measured value is to the true value
Accuracy refers to how closely a measured result agrees with the true or accepted reference value.
Question 3: A new reagent lot is being validated. QC results show acceptable precision but a consistent positive bias. What is the MOST appropriate corrective action?
- Accept the lot and adjust patient results manually
- Recalibrate the instrument using the new lot (Correct answer)
- Reject the lot and return to the manufacturer
- Run additional QC levels to confirm precision
Correct answer: Recalibrate the instrument using the new lot
A consistent positive bias indicates a systematic error that can often be corrected by recalibrating the instrument with the new reagent lot.
Question 4: The total allowable error (TEa) for a glucose assay is ±10 mg/dL. If bias is 3 mg/dL and imprecision (2 SD) is 5 mg/dL, the method's total error is:
- 8 mg/dL — method is acceptable (Correct answer)
- 8 mg/dL — method is unacceptable
- 15 mg/dL — method is acceptable
- 15 mg/dL — method is unacceptable
Correct answer: 8 mg/dL — method is acceptable
Total error = bias + 2SD = 3 + 5 = 8 mg/dL, which is within the TEa of 10 mg/dL, so the method is acceptable.
Question 5: Which statistical measure is MOST useful for comparing the variability of two analytes with different units or concentration ranges?
- Standard deviation (SD)
- Coefficient of variation (CV%) (Correct answer)
- Variance
- Range
Correct answer: Coefficient of variation (CV%)
The coefficient of variation expresses SD as a percentage of the mean, allowing comparison of variability across different analytes and concentration ranges.
Question 6: A laboratory uses a peer-group comparison program where mean QC values are compared to other laboratories using the same method. This is an example of:
- Internal quality control
- External quality assessment
- Proficiency testing
- Interlaboratory comparison (Correct answer)
Correct answer: Interlaboratory comparison
Comparing QC data among laboratories using the same method and reagents is interlaboratory comparison, a form of external quality assessment.
Question 7: When a QC result violates a warning rule but not a rejection rule, the BEST course of action is to:
- Reject the run and repeat all patient samples
- Document the warning and proceed with caution, running additional QC (Correct answer)
- Immediately recalibrate the instrument
- Report patient results with a disclaimer
Correct answer: Document the warning and proceed with caution, running additional QC
Warning rules signal increased probability of error; the run should not be automatically rejected but additional QC should be run and the situation documented.
CLT Quality Control & Assurance in Testing 2
A laboratory's Levey-Jennings chart shows 6 consecutive QC values all falling on the same side of the mean.
Which Westgard rule is violated?