CLINICAL LAB SCIENEC

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ESSENTIALS OF CLINICAL LABORATORY SCIENCE
require a rejection of all results followed by troubleshooting protocol. When a
control value moves in the same direction (increases or decreases) for 6 consecutive
days, it is termed a trend. An abrupt change from the established mean is a
shift. A shift may be positive to negative values, or vice versa, but it will all be on
one side of the mean.
QC records are evaluated under Westgard rules or possibly another similar
program. Many analytical instruments are capable of flagging results that fall outside
of the parameters established by the manufacturer of the controls. In the case
where unassayed controls are used, the values may require manually placing data
into the instrument after a period where sufficient values have been performed.
Westgard Rules (Multirules)
Westgard rules (sometimes called challenge rules) are a set of rules to determine
if a method is out of control. This occurs when at least two controls are not
within 2 SDs and the third is within 3 SDs. Preferably all three levels of controls
will be within 2 SDs. Results of laboratory tests must meet the following criteria
before they can be reported or placed on a patient’s chart.
Westgard Rules
1. The warning rule is broken when one control measurement exceeds the
±2 SD guideline. The other rules must now be tested by examining other
control values in the run (within run) and in previous runs (across runs)
before making a decision on the acceptability of the patient results. If any
of the remaining rules are violated, the laboratory professional must thoroughly
examine the test’s analytical procedure, troubleshoot steps in the
procedure, and or perform instrument maintenance, etc. before making any
decision about releasing the results or re-running the test.
2. The within-run only rule detects random error for a control but may also
point to systematic error. The run is considered out of control when one
control value exceeds the ±3 SD limits. But random errors may occur with
no warning and for unknown reasons. Repeating a control when it is out
of range often remedies the situation. Note that random errors for patient
specimens are not detected as this refers only to controls.
3. The two-consecutive control rule is designed to detect systematic error and
can be applied within and across runs. It is violated when two consecutive
control values exceed the same ±2 SD limit.
4. The range rule is applied within the same run only to detect random error.
It is violated when the SD difference between two consecutive control values
exceeds 4 SDs. The 4 SD spread may occur across the mean value and
include both a positive and a negative value from the mean.
5. This rule detects systematic errors and may be used both within and across
runs performed the same day or succeeding days. It is violated within the
control material when the last four control values of the same level exceed
the same ± SD limit. It is violated across control materials when the last four
consecutive controls for different levels exceed the same ±1 SD.
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