CLINICAL LAB SCIENEC

CHAPTER 10: QUALITY ASSURANCE 243
Example 10-3 Levey-Jennings Chart—Plotting of Daily Normal Control Values
Three levels of control samples were performed for a 20 day period. The following values are those from the normal control vial.
3 SDs (93) X X
2 SDs (92) X X X X X
1 SD (91) X
Mean (90) X X X
–1 SD (89) X X X
–2 SDs (88) X X X X
–3 SDs (87) X X
Day 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
For simplicity in depicting a typical Levey-Jennings chart, the standard deviations (SDs) shown were not calculated from any data
presented, but for ease of understanding, they were chosen arbitrarily as whole digits with some deviating from the norm by more than
2 SDs. The CUSUM determination is performed by subtracting each result from the mean; the above fi gures resulted in a +2 SD, which
is an acceptable value. In the CUSUM process, the lower the value of the sum of the differences of all the determinations, the better.
Optimally, there should be equal differences of both + and – values, indicating no shifts or trends.
affect the results of laboratory tests. Although
there are random errors at the rate of approximately
0.3% of tests performed, it is generally
safe to assume that patient results are
relatively accurate when all the controls are
within an established range. Again, random
errors are those for which no definite cause
can be established. Correlation of results and
knowledge of the patient’s medical condition
is often sufficient to resolve many of these
errors. Simply rerunning a patient sample
where all other test values are normal often
will provide a clinically consistent result.
Acceptability of Patient Testing
FIGURE 10-2 Levey-Jennings QC chart showing normal distribution for
days 1 through 6, a shift for days 7 through 9, and a trend for days 10
through 15.
Source: Delmar/Cengage Learning.
There are general and simple rules to follow
when determining if patient values are valid.
When all control values are within ±2 SDs, all data are accepted. If one or more
controls are within 2 and 3 SDs, repeat the errant sample using new control(s).
Frequently in laboratory medicine, both a normal and an abnormal control will be
analyzed. If both of these controls are out of range, the appropriate action would
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