PN 177196BB
4-12
QUALITY ASSURANCE
INTERPRETING AND USING CELL CONTROL DATA
USE GRAPHS TO DETECT PROBLEMS AND POINT TO SOLUTIONS
Trend
r A trend is a gradual change in five or more consecutive
results, all going upward or all going downward.
r Systematic drift or trend in control values suggests that
a problem is progressively developing.
r Generally due to the deterioration of a reagent or control
material.
Shift
r A shift is a sudden or abrupt change in five or more
consecutive results all above or all below the mean.
r This type of systematic error may be associated with a
change in instrument performance or a malfunction of
the instrument.
Outlier
r An outlier is a single result that falls outside the upper
or lower limit.
r Chance probability for a value to be outside the limit is
about one time out of every 20 times control is run.
r In this example, run #6 (from the right) is an outlier;
notice the plot point is inside the upper double line.
t With computerized graphing, small double-lined area
typically serves as a limit for the plot point.
t Since the plot point for an outlier cannot extend
beyond this set boundary, the Operator must look at
the numeric results to determine if the result is
slightly outside the limit or greatly exceeds the limit.
t If the numeric result for the outlier is only slightly
beyond the limit, it may be a statistical outlier
(control value is out by chance).
r To confirm a statistical outlier, the Operator must run
the control one more time.
t If the result of the rerun is now in-control, the
analytic run is accepted and both runs (the statistical
outlier and the acceptable rerun) are to be included in
the control data.
t If rerun of control reflects the same parameters as
still being out-of-control, there may be a control or
system problem.
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