PN 4277248DD
3-19
OPERATION PRINCIPLES
XB ANALYSIS
3
3.8
XB ANALYSIS
Studies (Bull 1974, Koepke 1981) indicate that the red cell indices (MCV, MCH, and MCHC)
of patient populations are stable over time.
39,40
This stability characteristic of the indices is
the basis of a quality-control technique called XB Analysis. In a manually implemented
system, population means (target values) are established by analyzing as large a sample as
possible, at least 250, but ideally 1,000 blood samples. (The XB Analysis used in the
Workstation does all the calculating automatically.) Once the target values have been
established, the XB Analysis can be applied using quite small batches from the patient
population. A 20-patient sample batch is a typical size, and can be used in the LH
Workstation.
Here is the
XB formula.
Where:
The formula is easily implemented with a computer. Its function is to enable reliable
estimates of the values for these parameters to be made for a population from small samples
of that population. It is superior to the traditional moving average because it reacts quickly to
changes. Small batch sizes allow for more frequent, therefore tighter quality control. The
formula both trims the data by giving less weight to outliers, and smoothes it by
incorporating information from the previous patient batch in the analysis of the current
batch. As each sample is processed, the mean of the previous set of samples is subtracted from
each of the red cell indices. The square root of this deviation (difference between the means)
is stored. After 20 samples have been processed, the sum of the square roots is divided by 20.
The result is squared to recover the mean (average) deviation. The individual deviations carry
a positive or negative sign, so then it can be added to or subtracted from the corresponding
previous means. The resulting new mean is then used for the succeeding batch of 20 samples.
The hematology system is considered "in control" when the batch means are within
established limits of the target values. Using the XB Analysis, the direction and amount of
change due to the instrument, the reagent, flagged samples or sample handling can be
detected. Because of the characteristic appearance of the graphs of the XB results, it is also
often possible to identify changes.
X(B,i)
= ith XB value
X(B, i-1)
= (i-1)th XB value
X(j,i)
= the jth X value in the ith batch
SGN
= the arithmetic sign of number in parentheses
N
= number of samples in the batch
x
= symbol used to represent multiplication
X B i
X B i
SGN
SGN X j i
X B i
X j i
X B i
F
j
N
( , )
( ,
)
[ ( , )
( ,
)]
( , )
( ,
)
=
− +
−
−
×
−
−
∑
⎧
⎨
⎩
⎫
⎬
⎭
×
=
1
1
1
1
F
SGN X j i
X B i
X j i
B i
N
j
N
=
−
− ×
−
−
⎡
⎣⎢
⎤
⎦⎥
∑
⎧
⎨
⎩
⎫
⎬
⎭
=
( , )
( ,
)
( , ) ( ,
)
1
1
1
2
2
Summary of Contents for COULTER LH 750 System
Page 26: ...PN 4277248DD 1 12 USE AND FUNCTION MATERIAL SAFETY DATA SHEETS MSDS ...
Page 54: ...PN 4277248DD 3 22 OPERATION PRINCIPLES XB ANALYSIS ...
Page 84: ...PN 4277248DD 4 30 SPECIFICATIONS CHARACTERISTICS BAR CODE LABEL SPECIFICATIONS ...
Page 90: ...PN 4277248DD 5 6 HAZARDS DISPOSAL OF ELECTRICAL INSTRUMENTATION ...
Page 118: ......
