CLINICAL LAB SCIENEC

CHAPTER 10: QUALITY ASSURANCE 255
the blood may be significantly different based on gender. So a female at the low
or high end of the normal range for females may fall slightly outside the normal
range. Normally the physician will discount any findings that are minimally outside
the normal range if the results are insignificant in value or if clinical findings,
physical findings, signs, and symptoms are absent.
Problems That Occur from the Use of Normal Ranges
Difficulties occur when the normal ranges are adhered to so closely that perfectly
normal patients may be required to undergo the expense and discomfort of further
testing because one test was outside the “normal range” by a small amount.
All living organisms have differences, and some individuals in a population will
be slightly different in their metabolic processes, which may be of a genetic or
environmental nature, rather than due to a disease. Possible misuses of normal
ranges include the following:
1. A supposedly normal group used to establish normal values would most
often contain a small but definite group of clinically normal persons with subclinical
manifestations of disease. Some who report themselves as “normal”
may have an undetected disease while nevertheless being included in the data
used to establish the values. This would have the effect of having abnormal
persons erroneously considered normal. This will affect the normal range
through the inclusion of test results from persons not suspected of having
any disease. But if large numbers of subjects are used to determine a mean
for normal values, a few diseased subjects may have only a minimal impact
on the population mean. In the same manner as including both males and
females and various population groups to determine the mean, the normal
limits would be widened with resultant overlap between normal and abnormal
persons. For example, tests for plasma glucose specimens from a small
group of clinically normal blood donors might be used to obtain the normal
range for a new instrument with reagents that differed from those previously
used. The range might be 75 to 101 mg/dL, and possibly very close to
the values listed in the package insert for the new reagents and instrument.
Subsequent test results from a larger group of donors that included those
with low normal values and high normal values may be influenced inordinately
if the expanded group had a larger number of subjects with a low
plasma glucose. Typically when developing a new population norm for any
analyte, each test is performed individually. Discarding abnormally low and
abnormally high results would yield a normal range very different from that
of a small group with a wide variation in values, which might be “normal”
for those individuals. Optimally, if the group had an equal number of high
values compared with low values, the results would provide a more realistic
mean.
2. Normal ranges should never be derived from a small number of individuals.
If normal ranges are calculated from a number of values too small to
be statistically reliable, a large percentage of individual patients may fall into
the abnormal range. This often occurs when a laboratory procedure is rarely
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