CLINICAL LAB SCIENEC

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ESSENTIALS OF CLINICAL LABORATORY SCIENCE
Critical Reminder
Statistically, in the normal
frequency distribution (Gaussian)
curve (Figure 10-1) approximately
95% of values should fall within
2 standard deviations of the
mean and 99.7% will fall within
3 standard deviations of the
mean. That means that 0.3% of
results will be attributed to random
error. This statement is true
for any procedure performed.
to obtain the square root of this total after dividing by n (the number of determinations)
minus 1. This gives 1 SD, so you would multiply the calculated
value by 2 to get 2 SDs, and by 3 to get 3 SDs. Look at the normal frequency
distribution curve, also called the Gaussian or bell-shaped curve, in Figure
10-1 for a graphic picture of the distribution of results when performing a
specific procedure on a number of different “runs” or sets of values that are
run simultaneously.
A Gaussian distribution curve enables one to readily see the distribution
of values for 1, 2, or 3 SDs. Slightly more than 95% of the
values obtained will fall within 2 SDs of an established mean average
for controls used to measure precision (see Figure 10-1), and approximately
99.7% will fall within 3 SDs. Each procedure is evaluated on
this basis. Most automated analyzers in the modern medical laboratory
perform these statistics through computations by each instrument’s
internal computer system. As samples are run over a period of time
and larger numbers of determinations are made, the SD values should
become narrower and narrower. This is particularly true for a laboratory
that conscientiously performs routine maintenance and cleaning
of equipment. Improvement in the value of SDs should be the goal of
every medical laboratory. As a general rule, a laboratory that performs
a procedure by the same method for a significant length of time will
realize that the SDs have grown progressively smaller.
Critical Reminder
Usually charts are drawn where
acceptable results for QC programs
will be within ±2 SDs.
The values for 3 SDs are often
included because 99.7% of results
will fall within 3 SDs. Observe the
Westgard rules for accepting or
rejecting patient results based on
QC results.
Spread of Data on the Scale
The term “spread of data” most often refers to the range incorporated
within 2 SDs and in most cases will include the 3 SD range,
because control values may occasionally extend into the 3 SD range
for a single determination, while the other controls are within 2 SDs.
This term is graphically seen on a Gaussian distribution curve.
Variance from Mean (SDs)
The acceptable variance from the mean value is 1 SD and this figure
doubled is the value for 2 SDs. This is the basic figure used to determine
precision based on QC samples for any test.
The variance (SD 2 ) of the samples in Example 10-1 is:
SD 2 = 35.625/18 (total values) – 1 or (n – 1) = 35.625/17 = 2.0956
The square root of 2.0956 = 1.4476, which is rounded to 1.45, or 1 SD
2 SDs = 2 × 1.45, or 2.9
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