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Questions and Answers
What does the Negative Predictive Value indicate?
Which measure of central tendency is indicated by the peak of a Gaussian curve?
What is the definition of variance in a dataset?
What type of error is characterized by a consistent change in one direction?
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In quality control charts, what is the primary purpose of the Levey-Jennings chart?
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If two test methods show that the difference is constant regardless of the concentration, what type of systematic error is present?
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Which statistical test would be applicable for comparing the variances of two datasets?
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What does a lower coefficient of variation (CV) imply about measurement precision?
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Which phase of quality assurance involves patient preparation and specimen collection?
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What does a control in laboratory testing monitor?
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What is established by testing a minimum of 120 healthy subjects?
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Which term describes the ability of a method to detect only the analyte it is designed to determine?
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Which of the following best represents the definition of reliability in a laboratory setting?
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What is the formula for calculating clinical sensitivity?
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What is the purpose of external quality control?
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Which measure indicates the probability of an individual having the disease when the test result is abnormal?
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What is the primary purpose of a Youden plot in laboratory analysis?
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Which control rule is triggered as a warning to inspect control data?
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In what context would you use a CUSUM graph?
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What does the 13s rule indicate when applied in quality control?
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Which Westgard control rule would cause rejection if 10 consecutive readings fall on one side of the mean?
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Which rule requires rejection of results when 2 out of 3 measurements exceed the mean plus or minus 2s?
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What does the R4s control rule involve?
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What does the term 'consecutive control measurements' refer to in the context of control rules?
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Study Notes
Quality Assurance
- Ensures accurate, precise, and timely results for patients
- Laboratory's system for identifying and minimizing analytical errors
- Composed of three phases:
- Pre-Analytical: Patient preparation, specimen collection, transport, and storage
- Analytical: Condition of reagents/equipment, pipetting, QC, personnel competency
- Post-Analytical: Reporting of results, timely release of results, record keeping
Materials for Quality Control
- Control: Resembles the test specimen; contains known analyte concentrations; used to monitor precision of the test system; CLIA requires at least two levels of controls (HIGH and LOW)
- Calibrator: Formerly known as "standard"; usually contains one analyte; tests and adjusts the instrument program to measure the concentration of an unknown
Types of Quality Control
- Internal QC (Intralab): Daily monitoring of the accuracy and precision of analytical methods through analysis of control samples within the laboratory.
- External QC (Interlab): Long-term monitoring of the accuracy of analytical methods through participation in proficiency testing programs.
Important Terms
- Accuracy: Closeness of a result to the true value.
- Precision: Closeness of replicates; reflects the degree of reproducibility.
- Reliability: Ability to maintain accuracy and precision over an extended period; consistent.
- Delta Check: Comparison of patient data to their previous results.
- Reference Range: Formerly called "normal value"; established by testing at least 120 healthy subjects and determining the range in which 95% of them fall.
Method Evaluation
- Analytical Sensitivity: Ability of a method to detect small quantities of an analyte.
- Analytical Specificity: Ability of a method to detect only the analyte it is designed to determine.
Measures of Diagnostic Efficiency
- Clinical Sensitivity: Percentage of people with the disease who have a positive test result. Formula: TP/(TP + FN) x 100.
- Clinical Specificity: Percentage of people without the disease who have a negative test result. Formula: TN/(TN + FP) x 100.
- Positive Predictive Value: Probability of an individual having the disease if the result is abnormal or outside the reference range. Formula: TP/(TP + FP) x 100.
- Negative Predictive Value: Probability that a patient does not have a disease if the result is normal or within the reference range. Formula: TN/(TN + FN) x 100.
Gaussian Distribution
- A symmetrical bell-shaped curve generated by plotting the assay of a reliable method used to measure an analyte. (Johann Karl F. Gauss)
- Assay values obtained are plotted on the x-axis, and their relative frequencies on the y-axis.
Measures of Central Tendency
- Mean: The peak of a Gaussian curve indicates the mean value for a data set. It's the average of all the values in a data set.
- Median: The midpoint observation. If there is an even number of values, the median is the average of the two innermost values.
- Mode: The most frequent observation.
- Mean = Median = Mode
Measures of Dispersion
- Range: Difference between the highest and lowest values in a data set.
- Variance: Average distance from the center (mean) and every value in a data set.
- Standard Deviation (SD): Square root of the variance; most frequently used measure of variation.
- Coefficient of Variation (CV): Also known as relative standard deviation; expresses standard deviation as a percentage. SD/CV = index of analytical precision (↓ CV = results in HIGH PRECISION)
Measurement Error
- Random Error (indeterminate error): Due to chance; affects precision. (Ex: mislabeling a sample, variations in technique, pipetting error, voltage fluctuations).
- Systematic Error (determinate error): Consistent changes in one direction. Usually seen as a trend (gradual increase or decrease). (Ex: Deterioration of reagents, poorly prepared calibrators, sample instability, dirty photometer).
Types of Systematic Error
- Constant Error: Difference between two test methods is constant regardless of concentration.
- Proportional Error: Difference between two test methods is proportional to concentration.
Inferential Statistics
- T-test: Used to compare two related means.
- F-test: Used to compare two standard deviations (variances).
Quality Control Charts
- Levey-Jennings/Shewhart: Most widely used chart in the laboratory; involves analysis of a control sample over a period of 20 consecutive days.
- Youden Plot: Graphical method to analyze interlaboratory data where laboratories have analyzed two control specimens (low and high controls) for the same analyte. Can help differentiate random and systematic error.
- CUSUM Graph: Used in the laboratory requiring computer implementation; very responsive to systematic error and sensitive to small, persistent shifts that commonly occur in modern, low-calibration-frequency analyzers. Monitors deviation from the target value (CUSUM chart is an alternative to Shewhart control charts).
Westgard Control Rules
- Rules utilized to indicate criteria for judging out-of-control situations.
Westgard Control Rules - Specific Rules
- 12s & 13s: Used with a Levey-Jennings chart when control limits are set as the mean plus/minus 2s or 3s respectively.
- 22s: Reject when two consecutive control measurements exceed the same mean plus 2s or the same mean minus 2s control limit.
- R4s: Reject when one control measurement in a group exceeds the mean plus 2s and another exceeds the mean minus 2s. Only interpreted within-run, not between-run.
- 41s: Reject when four consecutive control measurements exceed the same mean plus 1s or the same mean minus 1s control limit.
- 10x, 8x, 12x: Reject when ten, eight, or twelve consecutive control measurements fall on one side of the mean.
- 2of32s: Reject when two out of three control measurements exceed the same mean plus 2s or mean minus 2s control limit.
- 31s: Reject when three consecutive control measurements exceed the same mean plus 1s or mean minus 1s control limit.
- 6x & 9x: Reject when six or nine consecutive control measurements fall on one side of the mean.
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Description
This quiz focuses on the key aspects of quality assurance in laboratory testing, including pre-analytical, analytical, and post-analytical phases. Understand the importance of quality control materials like controls and calibrators, and the types of quality control used in laboratories. Test your knowledge about ensuring accurate results for patients.
