CLT Quality Control & Assurance in Testing 3 — Questions and Answers
Question 1: A laboratory implements a delta check for sodium results. A patient's sodium has changed from 138 mEq/L to 155 mEq/L in 4 hours. The FIRST step is to:
- Report the result immediately as critical
- Verify patient identity and repeat the test (Correct answer)
- Notify the physician of the critical value
- Check for sample hemolysis or lipemia
Correct answer: Verify patient identity and repeat the test
When a delta check flag is triggered, verifying patient identity and repeating the test are the first steps to rule out mislabeling or analytical error.
Question 2: The reference interval for a test was established using 120 healthy individuals. Approximately how many values are expected to fall outside the reference interval?
- 0 values
- 6 values (Correct answer)
- 12 values
- 24 values
Correct answer: 6 values
Reference intervals are typically set at the central 95%, so approximately 5% of healthy individuals fall outside; 5% of 120 = 6 values.
Question 3: A laboratory notices that QC precision has increased (wider SD) specifically after the instrument's daily maintenance cycle. This pattern MOST likely indicates:
- Reagent deterioration during storage
- An instrument problem introduced by maintenance (Correct answer)
- Operator technique variation
- A shift in the QC material itself
Correct answer: An instrument problem introduced by maintenance
If precision worsens specifically after maintenance, the maintenance procedure itself may be introducing variability, such as improper component reinstallation.
Question 4: Which document specifies the criteria for acceptable performance in external proficiency testing in the United States?
- ISO 15189
- CLIA '88 regulations (Correct answer)
- CAP checklist requirements
- Joint Commission standards
Correct answer: CLIA '88 regulations
CLIA '88 regulations establish the performance criteria that laboratories must meet on proficiency testing surveys for regulated analytes.
Question 5: A laboratory method has a sensitivity of 95% and specificity of 90% for detecting a disease with 1% prevalence. What is the APPROXIMATE positive predictive value (PPV)?
- 8.7% (Correct answer)
- 50%
- 90%
- 95%
Correct answer: 8.7%
At 1% prevalence with 95% sensitivity and 90% specificity, the PPV is approximately 8.7% due to the large number of false positives in a low-prevalence population.
Question 6: In a method comparison study, the slope of the regression line is 1.08 and the y-intercept is -2.0. What type of error does this PRIMARILY indicate?
- Random error only
- Constant systematic error only
- Proportional systematic error only
- Both constant and proportional systematic errors (Correct answer)
Correct answer: Both constant and proportional systematic errors
A slope other than 1.0 indicates proportional error and a y-intercept other than 0 indicates constant error; both are present here.
Question 7: A laboratory is validating a new hematology analyzer. Carryover testing is performed by running three replicates of a high sample followed by three replicates of a low sample. This test evaluates:
- Analytical precision at two concentration levels
- Contamination of a low sample by a preceding high sample (Correct answer)
- Linearity across the measuring range
- Between-run reproducibility
Correct answer: Contamination of a low sample by a preceding high sample
Carryover testing assesses whether residual analyte from a high-concentration sample contaminates subsequent low-concentration samples.
CLT Quality Control & Assurance in Testing 3
A laboratory implements a delta check for sodium results.
A patient's sodium has changed from 138 mEq/L to 155 mEq/L in 4 hours.
The FIRST step is to: