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Clinical Pathology Laboratory - Reference Intervals
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| The reference intervals provided by our laboratory were established
in our laboratory from healthy animals, using our own equipment with
specific methods and reagents. Because results are analyzer-, method-
and reagent- dependent, these intervals are only valid for results
from our laboratory. We have reference intervals for hematology and
chemistry for dogs, cats, horses, cattle and goats.We have blood gas
and electrolyte ranges for dogs, cats, horses and cattle only. For
species without established reference intervals, you should refer
to general publications and textbooks as a guideline only. With many
of our tests, (eg, immunology) we use the manufactures' intervals,
or do not provide intervals, however, we usually provide guidelines
for interpretation.
We establish reference intervals by collecting blood from at least
50 adult healthy animals. These healthy animals are obtained from
a variety of sources (eg, student- or faculty-owned). Therefore,
our reference intervals are only applicable for adult animals and
not young animals. Results from young animals may fall outside our
reference intervals because of age-dependent changes in their analytes.
For example, phosphate concentrations and alkaline phosphatase activity
are higher in young animals and decrease to within reference intervals
at about one year of age. Foals and calves have microcytic erythrocytes
until one year of age. Hematocrits are lower and lymphocyte counts
are higher in puppies and kittens under four months of age. Similarly,
immunoglobin concentrations are lower in animals under one year
of age.
We establish reference intervals using one of 2 techniques, based
on the distribution of the data:
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Normal distribution (gaussian): |
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| When data is normally distributed (a bell curve as illustrated below),
the mean ± 2 standard deviations is used to establish the reference
intervals. With this approach, confidence intervals on the upper and
lower limit of the reference interval can be provided. |
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Non-normal distribution (non-gaussian): |
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| If the data is significantly skewed, percentiles are used to establish
reference intervals. The 2.5 and 97.5 percentiles are the upper and
lower limits of the reference intervals and incorporate the middle
95% of the data. Since most of the variables that we test for are
not normally distributed, most of our reference intervals are based
on 95% percentiles. |
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| With either technique, the resulting interval then will include
95% of normal samples. As a result, up to 5% of normal animals may
have values that fall slightly outside (above or below) the reference
intervals for a given test. When numerous tests are run on the same
animal, the chances of obtaining one or more slightly "abnormal"
results on an animal that actually is normal rises (p = 1 - 0.95n).
For 12 tests, p = 0.46; for 21 tests, p = 0.66. |
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