CLINICAL LAB SCIENEC

CHAPTER 10: QUALITY ASSURANCE 251
answer when the test is performed repeatedly on the same specimen. In theory,
exactly the same answer should be obtained each time. But in actual practice
this does not happen due to equipment and human imperfections and differences
between operators. If all tests were 100% reproducible from day to day,
there would be no need for running control samples except at the initial use of
a piece of equipment. Variations are anticipated and accepted, hence, a “range”
for acceptable quality control results. A theoretical frequency distribution for a
set of data that are normally variable is often represented by a bell-shaped curve,
with data equally arranged on either side of the mean. This is sometimes called
a Gaussian distribution.
As was seen in the definitions of QC terminology, control specimens are used
for measuring precision, or reproducibility, of the QC system. Precision does not
mean the same thing as accuracy. Accuracy is provided through a PT program
where a commercial company provides specimens with known values. Each
laboratory reports its values, and all of the individual values from the various
procedures are measured against all other laboratories around the country that
use similar equipment and supplies. Both accuracy and precision are important.
One without the other is not a satisfactory condition (Figure 10-4).
Variation from the average (mean) value is expressed in terms of standard
deviation (SD). The laboratory frequently converts the SD figure to a percentage
of the mean value and calls this the coefficient of variation (CV). The majority
of tests in a good laboratory can be shown to have reproducibility expressed
as CV in the neighborhood of 4%. This means that two thirds of the values
3 Standard Deviations
2 Standard Deviations
1 Standard Deviation
Mean Value
1 Standard Deviation
2 Standard Deviations
3 Standard Deviations
FIGURE 10-4 Levey-Jennings chart with precise but inaccurate results.
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