CLINICAL LAB SCIENEC

242
ESSENTIALS OF CLINICAL LABORATORY SCIENCE
analytical instruments used in laboratory testing of control samples. This tool is
used not only in clinical laboratories but also in industrial plants as a component
of quality control programs. The control charts developed by Dr. Walter
Shewhart for detecting statistical control of a process are simpler to operate but
not as efficient in detecting small shifts in the mean of a process.
Measuring the small, sustained shifts in a process that would be important
in determining shifts and trends is simplified when using the CUSUM system.
Essentially, the system measures the totals of the positive movements above the
mean of a control sample and the totals of the negative movements. Over a
period of time, these totals should be virtually equal when equipment and supplies
are performing adequately. This process might be compared with the game
of golf, with par being the mean and each shot over is a +1 and each shot below
par is a -1, with a cumulative score being determined at the end.
Using the values provided in Example 10-2, a CUSUM determination would
be performed by calculating the cumulative sum of the variances from the mean
of 90, yielding the following data:
Value Variance
Cumulative
Sum Value Variance
Cumulative
Sum
92 +2 +2 88 -2 -2
93 +3 +5 91 +1 -1
92 +2 +7 90 0 -1
90 0 +7 88 -2 -3
89 -1 +6 87 -3 -6
87 -3 +3 93 +3 -3
88 -2 +1 90 0 -3
89 -1 0 92 +2 -1
88 -2 -2 92 +2 +1
92 +2 0 89 -1 0
Levey-Jennings Chart
The Levey-Jennings chart is the most common chart used in the laboratory. A
quick glance at the control results shows any shifts or trends developing, signaling
an impending systematic failure. These charts are seldom found on display
but can be visualized on computer screens of the various analyzers, especially for
chemistry procedures.
When Patient Results May Be Reported
Generally, patient results may be safely reported when the controls show no
signs of shifts or trends (Figure 10-2) or when none of the control values fall
outside the acceptable range. A student will find that a multitude of factors may
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