CLINICAL LAB SCIENEC

256
ESSENTIALS OF CLINICAL LABORATORY SCIENCE
ordered in a particular laboratory and the range is calculated from a small
number of procedures spread over a significant time span. Most laboratories
now refer these seldom-performed procedures to commercial reference laboratories
that accept specimens from across the entire country, giving a more
accurate snapshot of what a normal range would be.
3. A number of different factors may affect results in nondiseased persons.
The population from which specimens are received for normal range determination
may not accurately represent the segment of the population that
is being tested. There may be differences due to age, sex, locality, race, diet,
sampling from an upright versus reclining position, the length of time the
tourniquet is left on the arm, specimen storage time, and many other factors,
such as condition of the sample. An example is the erythrocyte sedimentation
rate (ESR) in which the normal values by the Westergren method for persons
under age 60 years, after correction for anemia, are 0 to 15 mm/hr for men
and 0 to 20 mm/hr for women; for patients over age 60, normal values are
0 to 25 mm/hr for men and 0 to 30 mm/hr for women. There may even be
significant variation for certain times of the day and for some analytes from
day to day in the same person. One example of the effect of timing is that of
cortisol. Cortisol peaks around 8:00 am and falls to its lowest levels during
the late evening. This would be a preanalytical issue to consider when patient
results inexplicably are out of the normal range and differ from previous
results.
4. Normal values obtained by one analytical method may be inappropriate
when used with another method. For example, there are several wellaccepted
techniques for assay of serum albumin. The assay values differ
somewhat because the techniques do not measure the same thing. Dye-binding
methods measure dye-binding capacity of the albumin molecule, biuret
procedures react with nitrogen atoms, immunological methods depend on
antibodies against antigenic components, and electrophoresis is influenced
primarily by the electric charge of certain chemical groups in the molecule.
In fact, different versions of the same method may not yield identical results,
and even the same version and method, performed on different equipment,
may display variance.
5. Normal values supplied by the manufacturers of test kits frequently do
not correspond to the decrease in test values after change from upright to
supine position, or when a tourniquet has been left in place, constricting the
blood flow, for more than 30 seconds or so.
Erroneous Samples
A multitude of factors may affect the patient’s sample and alter the results. Some
may be easily identifiable and others may be virtually impossible to pinpoint.
While many of these problems are preanalytical in nature, they are not by any
means exclusively preanalytical. The quality of the sample with respect to timing
of the test, preparation of the patient for the test, use of the correct vacuum tube,
and drawing of the sample in the right order, as well as proper identification of
the sample, are all examples of erroneous samples.
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