Transcript Internal quality control

Quality Control
Internal QC
External QC
▪Internal quality control:
-Monitors a test's method precision
and analytical bias.
-Preparation of quality control
samples and their interpretation
which is handled within the
laboratory.
External quality control:
Involves the estimation of the test
methods accuracy by the analysis
of unknown samples sent to the
laboratory from outside sources.
the samples are sent to the
laboratory where thy analyzed and
returned to the agency that
supplied the results.
control material:
 Specimen or solution which is analyzed for quality
control purposes
 This specimen is treated as if it was were a patient
sample by being exposed to the same conditions as
the patient sample, as incubation time,
temperature,…etc.
 the range of values for the control specimen is usually
predetermined, and any deviation from the range
indicates a loss of the test method accuracy and/ or
precision
Characteristics of control material
Control materials should have
 The composition should be as similar to the patient
sample.
 The analyte concentration should be at medically
significant level.
 The concentration should be stable under storage for
long period of time.
 Material should be low vial-to- vial variability.
 After vial has been opened and material prepared, it
should remain stable for the period of use.
 The material should be reasonably priced (Not
expensive).
Standard specimen
 A substance that can be accurately weighed or measured to
produce a solution of an exactly known concentration.
 Primary standard: A reference material that is of fixed and
known composition and capable of being prepared in
essentially pure form; also, any certified reference material that
is generally accepted or officially recognized as the unique
standard for the assay, regardless of it is level of purity.
 Secondary standard: A reference material in which the analyte
concentration has been ascertained by reference to a primary
standard.
What statistics need to be calculated to
establish my own control limits?
1-Mean.
2-Standard deviation.
3-Coffecient of variation.
x is the mean
xi is an individual measurement
∑ is the operation of summation
n is the number of xi values in the
group.
What does the mean tell me about
method performance?
Any change in accuracy, such as a
systematic shift or drift, would be reflected
in a change in the mean value of the control.
Always keep in mind that the mean is related
to accuracy or systematic error.
standard deviation
or
What does the standard deviation tell
about method performance?
standard deviation is a measure of the
width of the distribution and is related
to imprecision or random error.
The bigger the standard deviation, the
wider the distribution, the greater the
random error, and the poorer the
precision of the method.
coefficient of variation (CV)
CV = (S/X )100%
Coefficient of variation describes
the standard deviation as a
percentage of the mean.
Why is a CV useful?
Because the CV reflects a ratio of the
standard deviation to the concentration,
it is often provides a better estimate of
method performance over a range of
concentrations.
How do you calculate control limits?
control limits are calculated:
X ± 2SD
For cholesterol where a control
material has a mean of 200 mg/dL
and a standard deviation of 4
mg/dL, the 2s control limits would
be 192 and 208 mg/dL,
Quality control charts
Control charts are
used to routinely
monitor quality
Shewart or Levey Jenings charts:
Is the most reliable type of QC. Chart,
now we will show how to use it:
A- selected appropriate control materials
B- analyzed those materials to
characterize method performance by
collecting a minimum of 20
measurements over at least 10 days
C- calculated the mean and standard
deviation of those data
Shift
Is defined as a drift of values from one
level of the control chart to another,
which may be sudden or gradual
and may be due to failure to
recalibrate when changing lot
numbers of reagent during an
analytical process.
Shift
Trend
Is the continuous movements in one
direction over six or more consecutive
values.
Trends are difficult to detect without
continual charting, and may start on one
side of the mean and move across it or
it can occur entirely on one side of the
mean
this problem is usually due to
deterioration of reagents or light source
of the instrumentations.
Trend
Dispersion
Values may be within the acceptable
range (2s and 3s) but are unevenly
distributed outside the ±1s limits.
This is indicating a loss of precision
and due to random error. A precision
problem, or increase in random error,
would cause the standard deviation to
increase, therefore widening the
expected distribution of control
Dispersion
Definition terms
 Sensitivity: method sensitivity refers
simply to the lowest level of analyte
that can be detected by a given
method with low CVs.
 Specificity: refers to how specific a
test is for a certain substance without
interferences.

 Assay: to analyze a sample of a
specimen to determine the amount,
activity, or potency of a specific
analyte or substance.
 Out of control: indicates that the
analysis of patient samples is
unreliable.
 Run: a period to time or series of
measurements within which accuracy
and precision of the measuring
system are expected to be stable.
 Range: the difference between the
largest and smallest observed value
of a quantitative characteristic or
statistical limits.