CELL-DYN
3000 System Operator’s Manual
7-15
9140240E — May 1995
Chapter 7
Quality Control
Westgard Rules
Introduction
A control rule tests the control result against control limits to determine
whether the CELL-DYN 3000 shows acceptable accuracy and precision.
The limits are derived from the mean and standard deviation of control
measurements obtained when instrument performance is stable and
acceptable. The most common rule used in hematology quality control is
the mean ± 2SD limits. Ninety-five percent of the control results should
fall within the ±2SD limits.
Quality control rules detect random or systematic error. Random error
may be defined as an increase in the SD. Systematic error may be defined
as a shift in the mean value. A multi-rule quality control procedure
combines several control rules to improve the detection of both types of
error.
multi-rule approach to evaluating quality
This approach has long been used in the
but is new in the hematology laboratory. A set of modified
Westgard rules may be used to monitor quality control results on the
CELL-DYN 3000.
CELL-DYN 3000 Westgard Rules
The modified Westgard Rules (Westgard’s nomenclature is given in
parentheses) available on the CELL-DYN 3000 are:
Rule 1 (1
3s
):
Value outside 3SD
A control result exceeded the mean ± 3SD
Rule 2 (2
2s
):
Two consecutive values outside the same 2SD
Two consecutive results fell outside 2SD on the same
side of the mean
Rule 3 (R
4s
):
Two consecutive values outside opposite 2SD
One result was greater than 2SD above the mean and
the next result was greater than 2SD below the mean.
Consequently, the range between the results is
greater than 4SD.
Rule 4
Two of three consecutive values outside same 2SD
(2 of 3
2s
):
Two of the last three results fell outside 2SD on the
same side of the mean
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