Strenuous or stressful physical or emotional activity
should be avoided. If drug therapy cannot be
suspended, the laboratory results must be interpreted
with consideration of the influence of the therapy
upon the measurement. (Drugs may interfere with
analytic methods as well as alter the physiologic
state of the patient.)
Analytic variability is the variability in labora-
tory measurement that can be attributed to the
analytic method generating the measurement. The
analytic method includes materials, equipment,
procedures, and personnel. Variable performance of
each of these components contributes to the total
analytic variability. Analytic variability is kept
within acceptable limits by both rigorous analytic
method assessment and ongoing surveillance of the
method using a quality assurance program.
Postanalytic variability in laboratory measure-
ment arises between the completion of the analytic
method and the assimilation of the measurement by
the clinician. A major source of postanalytic
variability is transcription error. Such errors may be
made by laboratory personnel, laboratory clerks,
ward clerks, medical students, resident physicians,
or attending physicians. The opportunity for such
error increases with increasing numbers of transcrip-
tions so the original laboratory report form is the
most reliable source for the measurement.
Laboratory error
Despite the efforts to eliminate mistakes in the
performance of laboratory studies, inevitably some
study results that reach the clinician will harbor an
error. The clinician must be vigilant for the
evidence of such errors. At the same time, he or she
must also remain open to the possibility that a study
with a suspicious result was performed correctly and
that the results, though surprising, are valid for the
specimen received.
Laboratory error should be considered when (1)
the result is unreasonable, unphysiologic, or impos-
sible; (2) the result is inconsistent with previous
results from the same patient or is incompatible with
the results of other studies performed upon the same
specimen; or (3) the result differs from that expected
on the grounds of the clinical impression. In the
third situation, the consideration of a laboratory
error is appropriate, but reevaluation of the clinical
impression is equally necessary. It may even be
advisable to confirm that the result really is inconsis-
tent with the impression.
When a laboratory error is suspected, the clini-
cian must act to confirm or refute the suspicion. It
is not enough simply to ignore the result. If the
result truly is in error, the laboratory must be made
aware of the problem so that steps may be taken to
prevent its recurrence. If the result is valid, the
clinician must confront the unpleasant fact that either
his or her interpretation of the study result was
faulty or that his or her clinical impression may be
incomplete or even frankly incorrect. The clinician
should evaluate the possible influences of known
sources of biologic and preanalytic variability upon
the laboratory study. Special attention should be
paid to the effects of drug therapy upon both the
physiologic state of the patient and the reliability of
the laboratory study. If laboratory error is still
suspected, he or she should request that the labora-
tory repeat the study upon the original specimen and,
if possible, upon a new specimen. These actions,
taken in the stated order, will detect the site of the
error in almost all cases in which an error exists. Of
equal importance, however, is that this regimen will
also reveal the explanation for a puzzling but valid
result and thereby facilitate patient care.
Variability and monitoring
When following, or monitoring, a patient by
repeatedly performing a laboratory study upon him
or her, it is essential that the clinician be able to tell
if a change in the results indicates a change in the
status of the patient or if it merely reflects variability
in the study's measurement. In order to make this
decision, the clinician must know the pattern of
systematic intraindividual variability of the analyte
(such as the diurnal oscillations in plasma iron
concentration) and the magnitude of the random
intraindividual variability of the analyte in the
patient. Techniques for characterizing systematic
variability, for quantifying random variability in
individual patients, and for the quantitative analysis
of serial laboratory data are discussed in later
chapters.
Variability and clinical classification
The central problem in the use of laboratory
studies for clinical classification is that of interindi-
vidual variability. Clinicians must be able to decide
if a given study result is better explained by the
variability in the measurement among persons
having a disorder or by the variability among
individuals who do not have the disorder. For
Laboratory-based Medical Practice
1-6
