QUALITY CONTROL SIMPLIFIED
Quality control is a subject which is mainly
difficult to digest, because of the mathematical figures and formulas contained
in the calculations, and hence is many a times ignored by the
pathologists. As an honest and sincere
laboratory worker, everybody will be interested in providing correct
results. In this connection, we should
know the following words :
Quality Assurance
: This describes all the steps
taken , both in and outside the laboratory to achieve reliable results,
starting with preparation of the patient and ending with correct interpretation
of the results.
Quality Control describes the steps taken by the
laboratory to ensure that the tests are performed correctly. In simple words, this refers to minimization
of errors.
TYPES OF ERRORS :
Two types of errors can occur while performing various tests :
1.
Imprecision : Errors of scatter.
2. Inaccuracy : Errors of bias.
To understand these, the following experiment was done. Three different technicians were given a plasma to perform 20 tests on the same sample. They carried out glucose estimation on the same sample, with the same pipets and the autoanalyser, and the results were tabulated ( Tables A , B, & C ). In each case, the mean, bias and scatter were calculated. Bias is the difference between the actual level and the mean. Scatter means the standard deviation as calculated in the tables.
TABLE A : Glucose estimation by A
|
S NO |
GL MG % |
DIFF FROM |
|
|
S NO |
GL MG% |
DIFF FROM |
|
|
|
|
MEAN |
DIFF SQ |
|
|
|
MEAN |
DIFF SQ |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
1 |
131 |
2 |
4 |
|
11 |
136 |
3 |
9 |
|
2 |
130 |
3 |
9 |
|
12 |
139 |
6 |
36 |
|
3 |
127 |
6 |
36 |
|
13 |
139 |
6 |
36 |
|
4 |
130 |
3 |
9 |
|
14 |
128 |
5 |
25 |
|
5 |
129 |
4 |
16 |
|
15 |
132 |
1 |
1 |
|
6 |
142 |
9 |
81 |
|
16 |
129 |
4 |
16 |
|
7 |
134 |
1 |
1 |
|
17 |
131 |
2 |
4 |
|
8 |
135 |
2 |
4 |
|
18 |
135 |
2 |
4 |
|
9 |
142 |
9 |
81 |
|
19 |
133 |
0 |
0 |
|
10 |
131 |
2 |
4 |
|
20 |
130 |
3 |
9 |
Total of
columns 2 & 4 .2662 380
Mean 133
Bias 0
Scatter 4.5
<img
src=images/p1.gif>
SD = 380 / 19 = 20 =
4 . 5 2 SD = 9
CV = ( 4 . 5 / 133 ) X 100 = 3.38
TABLE B:
Glucose estimation by B
|
S NO |
GL MG% |
DIFF FROM |
|
|
S NO |
GL MG% |
DIFF FROM |
|
|
|
|
MEAN |
DIFF SQ |
|
|
|
MEAN |
DIFF SQ |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
1 |
123 |
0 |
0 |
|
11 |
124 |
1 |
1 |
|
2 |
122 |
1 |
1 |
|
12 |
122 |
1 |
1 |
|
3 |
122 |
1 |
1 |
|
13 |
123 |
0 |
0 |
|
4 |
124 |
1 |
1 |
|
14 |
122 |
1 |
1 |
|
5 |
126 |
3 |
9 |
|
15 |
122 |
1 |
1 |
|
6 |
122 |
1 |
1 |
|
16 |
123 |
0 |
0 |
|
7 |
123 |
0 |
0 |
|
17 |
123 |
0 |
0 |
|
8 |
123 |
0 |
0 |
|
18 |
122 |
1 |
1 |
|
9 |
123 |
0 |
0 |
|
19 |
124 |
1 |
1 |
|
10 |
123 |
0 |
0 |
|
20 |
124 |
1 |
1 |
TOTAL 2460 20
MEAN 123
BIAS - 10
SCATTER 1.02
2 SD = 2.04
<img
src=p2.gif>
_______ _____
SD = 20 /
19 = 1.05 = 1. 02
CV =- (1.02 / 123 ) X 100= 0.83
TABLE C
: GLUCOSE ESTIMATION BY C
|
NO |
GL MG% |
DIFF FROM |
|
|
S NO |
GL MG% |
DIFF FROM |
|
|
|
|
MEAN |
DIFF SQ |
|
|
|
MEAN |
DIFF SQ |
|
1 |
131 |
2 |
4 |
|
11 |
135 |
2 |
4 |
|
2 |
135 |
2 |
4 |
|
12 |
133 |
0 |
0 |
|
3 |
131 |
2 |
4 |
|
13 |
135 |
2 |
4 |
|
4 |
132 |
1 |
1 |
|
14 |
134 |
1 |
1 |
|
5 |
133 |
0 |
0 |
|
15 |
134 |
1 |
1 |
|
6 |
133 |
0 |
0 |
|
16 |
134 |
1 |
1 |
|
7 |
132 |
1 |
1 |
|