Quality Control Flashcards
Ability of a method to measure the smallest concentration of analyte
Sensitivity
Ability of a method to measure only the analyte of interest
Specificity
Nearness to the true value
Accuracy
Determines how much of analyte can be identified in a sample
Recovery study
Determines a specific compound that affects the lab tests
Interference study
Used to assess the presence of error im actual patient sample
Sample comparison study
Repeated results that agree with one another
Precision
Degree by which a method is easily repeated
Practicablity
Ability of a method to maintain accuracy and precision over a period of time
Reliability
Ability of a method to detect the true positive
Diagnostic sensitivity
Ability of a method to detect the true negative
Diagnostic Specificity
Involves analyses of control samples together with the patient specimen
Intralab or Internal QC
Involves proficiency testing programs that periodically provide samples of unknown concentrations to participating laboratories
Interlab or External QC
Note: some proficiency tests are QUALITATIVE however Chemistry tests should be QUANTITATIVE
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Note:
In external QC, difference of greater than 2SD indicates that the lab is not in control
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Objectives of QC
Stability of machine, reagent and operator
Note:
Accuracy of any assay depends on the control solutions. Chemistry assay used 2 levels of control solutions while Immunoassay used 3 levels of control solutions
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The different control materials should be analyzed for how many days?
20 days
For highly precised assays such as blood gases, how many days is the analysis
5 days
Expected values represented by intervals of acceptable values with upper and lower limit
Control Values
How are control limits calculated
From the mean and SD
The ideal control/reference limit
(+/-) 2SD
Characteristic of ideal QC material
- Resembles human sample
- Inexpensive and stable for long period of time
- No communicable disease
- No matrix effect
- With known analyte cocentration
- Convenient packaging
Note:
QC materials should be available for 1 year (same lot number)
Bovine QC materials are used but not for immunochem, dye-binding and bilirubin assays
Stabilized frozen controls do not require reconstitution
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Gold standard for external qc testing
CAP Proficiency Testing
Note: Method Evaluation
- 40-100 samples should be ran by each method in duplicate on the same day over 8-20 days ideally within 4 hours
- If only 40 samples will be measured, daily analysis in duplicate of 2-5 specimens should be followed for atleast 8 days
- Most impt. characteristic of method evaluation is to DETERMINE IF THE TOTAL ERROR IS LESS THAN THE ALLOWABLE ERROR
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What are the types of error
Random, systematic, constant, proportional, clerical errors
Error that is due to chance; vary from sample to sample; due to instrument, operator and environmental condotions
Random error
Errors that influences observations consistently in one direction; detected as either positive or negative bias; related to calibration problems, deterioration of reagents etc
Systematic error
Refers to the difference between the target value and the assayed value; dependent on the sample concentration
Constant error
Exists when the difference between the test method and the comparative method values is proportional to the analyte concentration
Proportional / Slope / Percent Error
Highest frequency; occurs with the use of handwritten labels and requests
Clerical error
Note:
First step of method evaluation is the PRECISION STUDY which estimates RANDOM ERROR
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Most error free means of requestinh laboratory tests
Online computer output
Note: Allowable error
* is based on the quantity of error that will negatively affect the clinical decisions
- the long term precision is sufficient if the total imprecision is less than 1/3 of the allowable error
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It is the measure of central tendency
Mean
Measure of a the dispersion of values from the mean
SD
Most frequently used measure of variation
SD
Index of precision
Coefficient of Variation
Used to compare the means or SD of two groups of data
Inferential statistics
Used to determine whether there is a significant difference between the SD of two groups of data
F Test
Midpoint of distribution
Median
Most frequent observation
Mode
Difference between the highest and lowest score in the data
Range
Difference between the value of a data point and the mean value divided by the group’s SD
SD Index
Used to determine whether there is a significant difference between the mean of two groups of data
T-test
It is the system that ensures accuracy and precision in the laboratory
Quality Control System
Describes the distribution of data around the mean
SD
Is used to observe values of control materials over time
QC Chart
It is a population probabilty distribution that is symmetric about the mean; occurs when data elements are centered around the mean with most elements close to the mean
Gaussian curve / Bell-shaped curve
Calculates the difference between QC results and the target means; V-shaped; requires computer implementation
CUSUM
Used to compare results obtained on a high and low control serum from different laboratories; X and Y axis
Youden/Twin plot
Most widely used QC chart in the clinical laboratory
Shewhart Levey-Jennings Chart
Control values that either increase or decrease for six consecutive days
Trend
Control values that distribute themselves on one side of the mean for six consecutive days
Shift
Control values that are far from the main set of values
Outliers
Establish criteria for deciding whether an analytic process is out of control
Multirule
Warning rule
1(2)s
Rejection rule; One control exceeds 3SD; Needs correction
1(3)S
Last 2 controls exceed either mean +/- 2SD
2(2)S
Last or any four consecutive control results exceed either mean +/-1SD
4(1)S
Difference between the highest and lowest control result
R4S
Ten consecutive results are on the same side of the target mean
10x
Acceptable reference limit
+/- 2SD
99% confidence
+/- 3SD
68% confidence
+/- 1SD
95% confidence
+/- 2SD
Is the difference between two consecutive measurements of the same analytes on the same individual
Delta check
Are used to measure systematic errors or inaccuracy caused by substances other than the analyte
Ex: hgb, lipids, bilirubin
Interference experiments
Is the concentration range over which the measured concentration is equal tp the actual concentration without modification of the method
Linear range
Absurd value; detect sample contamination or dilution
Physiologic unit
Testing performed outside the confines of central laboratory, usually by non-laboratorian personnel; decentralized testing
Point of care testing
Is a systematic action necessary to provide adequate confidence that the laboratory services will satisfy the given medical needs for patient care
Quality assurance
It shows whether a method can measure all the analyte or just a part of it
Recovery experiment
It is the range of values into which 95% of nondiseased individuals will fall
Reference limit
