CLINICAL LAB SCIENEC

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ESSENTIALS OF CLINICAL LABORATORY SCIENCE
established range and those patients falling even slightly outside the range are of
great importance to the physician and the patient. The first step usually used to
establish normal ranges is to assume that all persons who do not demonstrate
clinical symptoms or signs of any disease are normal, even if one or more test
results fall outside the “normal” range. For some tests, normal is defined as the
absence of clinical evidence associated with one particular disease or chronic
condition. A second assumption commonly made is that test results from those
persons considered normal will have a random distribution. Factors that would
place a significant group of these values toward either the low or the high side
of the normal range may be present. Then a Gaussian (random) distribution that
resulted from a slight distortion of the mean from the previous level would yield
a new mean value located in the center (median) of the value distribution where
it is possible that no abnormality would be observed from results with slight
elevations or decreases in value.
In a truly random or Gaussian value distribution, 68% of the values will fall
within ±1 SD above and below the mean, 95% within ±2 SDs, and 99.7% within
±3 SDs (see Figure 10-1). The standard procedure is to select ±2 SDs from the
mean value as the limits of the normal range. Accepting ±2 SDs from the mean
value as normal will place 95% of clinically normal persons within the normal
range limits. Conversely, it also means that 2.5% of clinically normal persons
will have values above and below this range. Normal ranges created in this way
are a deliberately thought-out process.
A wider normal range of ±3 SDs would place almost all normal persons
within the normal range limits and this would require a wider swing from the
mean value for determining if results are abnormal. However, this would place
some diseased persons with only a small test abnormality into the expanded normal
range of ±3 SDs and thereby decrease test sensitivity for detection of disease.
On a practical basis, most patients who are suffering from a particular disease
will not have borderline results but will have results that may be several times
the normal mean rather than minimal changes in most analytical tests.
A patient may exhibit results that are outside the normal range and actually
be free of disease, at least relative to the test performed. There are several reasons
for this. For instance, genetic variations within a specific ethnic group may
be found, with no clinical reason or outward manifestation for the abnormal
analyte for a specific patient. Ranges have been established for indigenous population
groups from specific areas of the country or the world. Because the population
of the United States is quite homogeneous, normal values were developed
from a cross-section of the entire population. Racial differences may be averaged
into the mean. However, the mean value for that particular group may be low or
high within the normal range, or the numbers for each cultural or ethnic population
might have been insignificant and too small to affect the mean value to any
great extent, if at all.
Other factors that influence differences in the mean value for normal ranges
for most analytes relate to diet, culture, lifestyle, and gender. Long-term diets
rich in certain foods may cause a significant difference in values (e.g., the Northwest
with a diet rich in salmon or the Southeast with a diet rich in fried foods).
The normal ranges include both males and females where some constituents of
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