CLINICAL LAB SCIENEC

CHAPTER 10: QUALITY ASSURANCE 257
Effects of Position on Laboratory Values
As pointed out previously, variations may be observed in common procedures
due to differences in anatomical positioning while the specimen is being collected.
Differences occur due to fluid shifts in the body that occur with various
positions. It takes as much as 20 minutes or more to equalize fluid shifts due to
changes in position. Keeping the tourniquet on for more than a minute will also
cause hemoconcentration and will increase certain values. This would be a preanalytical
issue to consider when results are out of range.
The position of the body for even a short period before blood is collected
and the position of the body during collection can be significant. Patients who
are having certain tests performed should be told to avoid prolonged standing
before the venipuncture. Standing for more than a few minutes will cause capillaries
in the tissues to allow only protein-free fluid or more watery substances to
pass through the small venules. A person who is lying supine and then returns to
a sitting or standing position will cause blood plasma to filter into surrounding
tissues, resulting in a decreased level of blood plasma of up to 10%. This would
result in an increase in values for those components normally tested for that
are bound to protein such as aldosterone (regulates sodium), bilirubin, calcium,
cholesterol, iron, and renin. Renin and aldosterone may affect blood pressure
directly, so there might be a change in blood pressure as well as laboratory results.
Potassium is also greatly increased if the patient had been standing during the
past 30 minutes, due to a release of intracellular potassium from muscle cells.
High-density lipoprotein (HDL) values may vary by up to 15% by going from a
supine position to that of standing.
Other Factors Affecting Laboratory Values
Pregnancy affects laboratory values because numerous physiological changes are
taking place in many of the body’s systems. Body fluid levels increase during pregnancy,
diluting red blood cells and other components and contributing to lower
than normal values. Smoking dehydrates the smoker, and the resulting hemoconcentration
may raise many values, particularly cholesterol, glucose, triglycerides,
and white blood cell counts. Smoking also lowers the levels of most of the immunoglobulins
(proteins that contribute to immunity). Youngsters who smoke may
also alter their thyroid-stimulating hormone (TSH) and growth hormone (GH).
Stress can cause increases in white blood cell counts and cortisol, and decreases
in serum iron. As one would predict, changes in temperature and humidity affect
body fluids. Acute heat will cause tissue fluids to move into the vascular system,
increasing volume and diluting components of the blood. Excessive sweating
will also cause hemoconcentration if fluid replacement is not provided. These
environmental factors account for differences in values for population groups in
diverse geographic locations with extremes in weather conditions.
Types and Characteristics of Samples
Samples vary by methodology and the test being performed and erroneous results
may be obtained if the wrong specimen or improper collection is performed.
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