QUALITY ASSURANCE/CONTROL Flashcards
Overall program that ensures that the final
results reported by the laboratory are correct
Quality Assurance
➔ AKA External Quality Control
➔ Involves proficiency testing
programs
➔ Ex.: National External Quality
Assessment Scheme (NEQAS)
Interlab
➔ System of ensuring accuracy and precision in the lab
➔ Process of ensuring that analytical results are correct
➔ QC samples are measured periodically in the same manner as clinical samples
Quality Control
➔ AKA Internal Quality Control
➔ Analyses of control samples
together with patient specimens
➔ Important in daily monitoring
Intralab
Reagent without any analyte added
Reagent Blank
➔ Most specific analytical solution
➔ Value will tell the conc. of the
unknown
➔ Only one analyte
Standard
➔ Value will determine accuracy and
precision
➔ Resembles patient sample
➔ Many analyte
Control
Indicating a person has the disease when, in fact, he or she does not
False positive
-Done on apparently healthy
individuals
-Not a basis for treatment
-Less accurate, less expensive
Screening Test
-Done on sick or ill individuals
-Basis for treatment
-More accurate, more expensive
Diagnostic Test
Refers to how well the test measures what it is
supposed to measure. It refers to the accuracy of the test, meaning how close the result is to its true value.
Validity
Indicating a person has the disease when, in fact, he or she does
True positive
(TP)
Refers to how well the test performs in use over time.
It refers to the precision of the test, meaning how close are the results of a test on repetition or how close the
values are from one another.
Reliability
Indicating a person has the disease when, in fact, he or she
does not
False positive
(FP)
Indicating a person does not have the disease when, in fact, he or she
does
False negative
(FN)
Ability of test to detect the smallest amount of
analyte
SENSITIVITY
Indicating a person does not have
the disease when, in fact, he or she
does not
True negative
(TN)
Ability of test to identify correctly those who
have the disease (a) from all individuals with
the disease (a+c)
Diagnostic Sensitivity
Ability of the test to identify correctly those
who do not have the disease (d) from all
individuals free from the disease (b+d)
Diagnostic Specificity
Diagnostic Sensitivity formula
TPTP + FN
Ability of test to detect substances without
interferences
SPECIFICITY
Diagnostic Specificity formula
TNTN + FP
Measure of central
tendency
Mean
Middle value
Median
Most frequent value
Mode
Highest value – lowest value
Range
Measures of dispersion
Standard Deviation
and Variance
Predictable errors; an error that is constant
when measurements are made under the
same conditions
Causes: deterioration of reagents, improperly
made standard solutions, contaminated
solutions, calibration problems, failing
instrumentation
SYSTEMATIC ERRORS
Index of precision
Coefficient of
Variation
Unpredictable errors; often due to instrument,
operator, and environmental conditions
➔ Causes: pipetting error, mislabeling of
samples, temperature fluctuation, improper
mixing of sample and reagent
RANDOM ERRORS
-is the most
commonly used histogram in QC
➔ A histogram is/are sheets of rectangular
coordinate graphing paper where data for
sequential analysis are plotted to locate the
source of error
LEVEY-JENNINGS CHART
SYSTEMATIC ERRORS Two types:
Constant error & Proportional error
is a “multi-rule” system
➔ Developed by Dr. James O. Westgard
➔ Based on statistical concepts which is a
combination of decision criteria or rules to
assess if a system is in control
Wesguard Multirule System
Violated if the IQC value exceeds the mean by ±2SD
12SD
Violated if four consecutive IQC values exceed the same limit (mean ± 1SD)
41SD
Violated when the IQC value exceeds the mean by ±3SD
13SD
Violated when two consecutive IQC values exceed the
mean on the same side of the mean by ±2SD
22SD
Violated when one control exceeding the +2s and another exceeding the -2s
R4SD
➔ AKA Linear or Dynamic Range
➔ Range of analyte concentrations that can be directly measured without dilution,
concentration, or other pretreatment
Analytic Measurement Range (AMR)
Violated when 10 consecutive IQC values are on the
same side of the mean or target value
10x
How close the measured value is to the true
value due to systematic error, which can be
either constant or proportional
Accuracy
Ability of a method to detect small quantities
or small changes in concentrations of an
analyte
Analytic Sensitivity
➔ Difference between the true value and the
measured value
Bias
➔ AKA Cross-reactivity
➔ Ability of a method to detect only the analyte it is designed to determine
Analytic Specificity
➔ Type of systematic error in the sample
direction and magnitude; the magnitude of
change is constant and not dependent on the amount of analyte
Constant Error
➔ Effect of a compound/s on the accuracy of the detection of a particular analyte
Interference
➔ Substances that cause interference
Interferents
➔ Lowest amount of analyte accurately detected
by a method
Limit of Detection
➔ Lowest amount of analyte that can be
reported while achieving a precision target
(e.g. lowest concentration at which a CV of 10% may be achieved)
Limit of Quantitation
➔ Type of systematic error where the magnitude changes as a percent of the analyte present, error dependent on analyte concentrations
Proportional Error
➔ Error varies from sample to sample
➔ Causes include instrument instability,
temperature variations, reagent variation,
handling techniques, and operator variables
Random Error
➔ Body component (e.g. fluid and urine) in which the analyte is to be measure
Matrix
➔ Dispersion of repeated measurements about the mean due to analytic error
Precision
➔ Error always in one direction (may be constant
or proportional
Systematic Error
➔ Ability of an analytic test to measure a known amount of analyte
◆ A known amount of analyte is added
to real sample matrices
Recovery
➔ Random error + Systematic error
Total Error
➔ Refers to the difference between the
measured value and the mean expressed as a number of SDs
➔ An SDI = 0: value is accurate or in 100%
agreement
➔ An SDI = 3: 3 SDs away from the target (mean) and indicates inaccuracy
➔ SDI may be positive or negative
Standard Deviation Index
