CLINICAL LAB SCIENEC

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ESSENTIALS OF CLINICAL LABORATORY SCIENCE
Table 10-1 Quality Control Terminology and Definitions (Continued)
Term
Sample
Shift
Shifts and trends
Standards
Systematic error
Trend
Definition
Subgroup of a population; such as women, Native Americans, etc.
An abrupt change from the established mean is a shift; may be positive to negative values, or vice versa,
but will all be on one side of the mean (e.g., all values for a glucose with a mean of 100 will provide
measurements of 98, 95, 96, 91, etc.).
Large number of assays more meaningful.
Ensures that results obtained are probably accurate, and that instruments and reagents are working adequately.
Primary standard—a substance that can be accurately weighed or measured to produce a solution of an
exactly known concentration that is free of impurities; NIST provides standard practices and quality control
for calibrating measuring devices and purity of solvent in which a substance is dissolved.
Related to instrument, reagent, and technological causes.
When a control value moves in the same direction (increases or decreases) for 6 consecutive days, it is
designated a trend.
Critical Reminder
Primary Measures of
Quality Assurance
Precision Refers to reproducibility
and is determined
by a set of
control samples
Accuracy Refers to any
deviation from the
true result, and is
measured by profi -
ciency testing that
is obtained from
an external comm
ercial companies
positive values (above and below the mean) should be seen over a period
of time, such as for the month.
• Twin-plot graph of Youden, modified by Tonks, uses two control values
by the same method or one control value using two methods. The variances
should be virtually the same for the two controls and for the one
control analyzed by two different methods.
• CUSUM graph is a cumulative method in which the mean result is
subtracted from each control result as it is obtained, producing either
a positive or a negative value each day. The resulting value is added to
the total of the previous days to give a cumulative difference from the
mean. Another similar method used in some laboratories is Bull’s moving
averages. These data are often managed cumulatively by automated
equipment, and deviations outside the norm would be brought to the
attention of the operator of the equipment.
Quality Control Concepts
The previous introduction to terms related to QC, a vital component
of quality assurance, laid the groundwork for the calculations to determine
precision. Precision relates to reproducibility of results when the
conditions under which the procedures are performed are the same in
all cycles of testing (from day to day, for example). An integral calculation
used in calculating standard deviations (SDs) for groups of
specimens relies on using the square root of the differences of results
from the mean value. Even the most basic calculators usually have a
key for easily calculating square roots.
Certain essential calculations are used in providing QC data to evaluate
the precision of test procedures on a daily basis. This might be done automatically
by the large autoanalytical instruments in use in most laboratories, but
it is essential that the laboratory worker be familiar with the statistical terms
shown in the Critical Reminder box on the following page.
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