Rational Use of Diagnostic Tests
distribution phase (e.g., digoxin levels drawn during the first 6 hours after an oral dose).
Substances that have a circadian variation (e.g., cortisol) can be properly interpreted only with
knowledge of the time of day the sample was drawn.
During specimen collection, other principles should be remembered. Specimens should not be
drawn above an intravenous line, as this may contaminate the sample with intravenous fluid.
Lysis of cells during collection of a blood specimen will result in spuriously increased serum
levels of substances concentrated in cells (e.g., lactate dehydrogenase and potassium). Certain
test specimens may require special handling or storage (e.g., blood gas specimens). Delay in
delivery of specimens to the laboratory can result in ongoing cellular metabolism and
therefore spurious results for some studies (e.g., low blood glucose).
Conclusion
Obtaining a proper medical history, performing a complete physical examination, and having
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