Chapter 10: Quality Control (Handout) Flashcards
a process to periodically examine a measurement procedure to verify that it is performing according to pre-established specifications
Quality Control
Dispersion of results for repeated measurements of aliquots of the same QC material
Imprecision
measure of expected imprecision in a measurement procedure when it is performing correctly
Standard Deviation
Interval of ±1SD
68% of measured values
Interval of ±2SD
95% of measured values
Eliminates systematic bias
Correct Calibration
Difference between the observed mean and the expected value for a QC material
- Brought about by changes in calibration
Systematic Bias
- the combination of systematic bias and imprecision that occurred for that specific measurement
Accuracy
refer to an average systematic bias that may be present in a given method
Trueness
most often performed by the laboratory using calibrator materials provided by the method or instrument manufacturer
Method calibration
methods are calibrated during the manufacturing process, and the laboratory performs a verification of that calibration
(E.g. Point-of-care devices)
frequently specify calibration intervals
In vitro device (IVD) manufacturers
Calibration of routine methods should be traceable to a higher-order reference measurement procedure or international reference material
Calibration Traceability to a Reference System and Commutability Considerations
- evaluates a measurement procedure by periodically assaying a QC sample for which the expected result is known in advance.
- part of the process management component of the quality system that integrates good laboratory practices to ensure correct patient results.
Statistical QC
Most common presentation for evaluating QC results
Levey- Jennings or Shewhart Plot
±1SD
68.3% of observations
±2SD
95.4% of observations
±3SD
99.7% of observations
Selection of QC Materials
Two different concentrations are necessary for adequate statistical QC
- Analyte concentrations that monitor the analytic measurement range of the method, should be selected for Quantitative Methods
Frequency to Assay Quality Control Samples
Analytic stability of the measurement procedure
- Risk of harm to a patient from clinical action being taken before a significant error is detected
- Number of patient results produced in a period of time when an error condition existed but was not yet detected
- Events such as recalibration or maintenance that may alter the current performance condition of the measurement system
- Training and competency of the test operator, particularly for manual or semi-automated methods
- Risk of failure of the measuring device
Establishing Quality Control Target Value and Standard
Deviation That Represent a Stable Measurement Operating Condition
- a minimum of 20 observations is recommended for the initial SD estimate
- determine the SD for stable measurement performance from the cumulative SD over a 6- to 12-month period for a single lot of QC material.
➔ commonly used to assess how well a method performs relative to the medical requirement.
➔ Compares the variability in a measurement process (in SD) vs. the acceptable variability because it will not cause an error diagnosis or treatment of a patient
Sigma Metric
➔ conventional way to express QC interpretive rules is by using an abbreviation nomenclature popularized among clinical laboratories
Westgard Rules
Corrective Action When a Quality Control Result
Indicates a Measurement Problem
- A QC alert occurs when a QC result fails an
evaluation rule, which indicates that an analytic
problem may exist. - There is a high probability that the assay is
producing results unreliable for patient care
Reviewing Quality Control Data
Immediate impact of QC data is to determine if patient results can be reported and used for clinical decision making
Weekly by Senior Technologist/and or Supervisors
- Ensure that correct follow-up of any QC alerts was conducted
- Patient samples that may have had erroneous results
were repeated - Ensure that process was properly documented in QC
records
Monthly by Laboratory Director
- Address/Include any issues identified by the weekly review process
- Examination of the Levey-Jennings chart* or other tool to identify trends or changes in assay performance that
may need to be addressed - Review consistency of reagent lot changes, calibrator
lot changes, and any patient data–based QC procedures
Major Function of the QC Review Process
o It verifies that test procedures are stable and meet performance specifications,
o Identifies test procedures that may need intervention to address trends in performance deterioration
recommends 3 or more samples
CLSI document EP26
Alternate
10 or more patient samples that span the analytic measurement range and use Deming regression analysis
Results from patient samples support QC processes in a laboratory by verification of
o Consistency of patient results when changing lots of reagent or calibrators for a method
o Identified inconsistent results using a delta check with a previous result for a patient
o Consistency of patient results when an analyte is measured using more than one instrument or method in a health care system
o Method performance using results from patient samples in a statistical QC scheme
Delta Check with a Previous Result for a Patient
o Comparing a patient’s current test result to a previous result for the same analyte
o Most useful to detect mislabeled samples and samples altered by dilution with IV fluid during collection
from a patient
Clinical patient sample aliquots are assayed using each of two or more methods (or analyzers) to evaluate, and if necessary adjust the calibration as needed, to achieve agreement in results for patient samples
Round Robin
Automated approaches to determine the mean (or median) for groups of sequential patient results
Average of Normals (AON)/ Moving Average
A program to evaluate method performance by comparison of results versus those of other laboratories for the same set of samples
Proficiency Testing or External Quality Assessment
Proficiency Testing or External Quality Assessment
o Each laboratory assays the PT samples as if they were patient samples and reports the results for the PT samples to the PT provider for evaluation
o The PT provider assigns a target value to the PT samples and determines if the results for an individual laboratory are in close enough agreement with the target value
o PT allows a laboratory to verify that its results are consistent with those of other laboratories and to verify that it is using a method in conformance with the manufacturer’s specifications.
