LABORATORY MONITORING
A large fraction, perhaps even the majority,
of clinical laboratory studies are ordered not for the
purpose of diagnostic or prognostic classification
but, rather, to monitor patients; that is, to determine
if and by how much a study result has changed. The
clinical applications of monitoring are diverse,
embracing every aspect of patient management
(Table 5.1). Four general categories of clinical use
of monitoring studies can be identified: screening for
subclinical disease, monitoring physiologic status,
monitoring disease activity, and monitoring toxic
and therapeutic agents. Therapeutic drug monitoring
is discussed in a separate chapter.
SCREENING FOR SUBCLINICAL DISORDERS
As defined in Chapter 3, screening studies are
used to detect serious, treatable disorders in persons
who have the disorder but who have not manifested
the condition clinically. Screening studies can be
performed on a catch-as-catch-can basis, as exempli-
fied by the practice of ordering screening studies as
part of every hospital admission, or they can be
performed according to a screening program
designed to achieve certain performance criteria.
Screening programs have three components: (1)
a definition of the population to be screened, (2) a
rule indicating when to perform the first screening
study on an individual, and (3) a schedule for
performing subsequent screening studies. As
regards the population to be screened, it is obvious
that screening should be performed only on individu-
als who might have the disorder. For example, only
women develop endometrial cancer, so screening for
this cancer is performed only in women. The crite-
ria used to define the screening population for many
disorders are based upon the identification of
demographic or clinical groups in which overt
disease is found. This approach is improved by
broadening the criteria to include groups in which
subclinical disease is known to occur even if overt
disease is not seen.
Having defined the population for whom screen-
ing could be useful, the starting rule and schedule
for repeat testing are selected so as to assure that the
screening program has a reasonable probability of
detecting the disorder in a screened individual while
the disorder is in a treatable part of its subclinical
phase. In attempting to achieve this performance
goal, the program is constrained by many practical
considerations, the two most important of which are
the cost of performing the screening studies and the
need for adequate specificity.
The probability of detecting a disorder in its
subclinical phase depends upon many factors. These
include the natural history of the subclinical phase,
the diagnostic performance of the screening study as
the disorder develops within its subclinical phase,
and the timing of the first and subsequent repetitions
of the screening study (Provok
et al.
1981).
Consider a screening program consisting of a
single study performed at some prescribed time in
one's life. An example of such a design is a single-
study screening for a genetic disorder in the neonatal
period. For a single-study screening program, the
probability of detecting subclinical disease equals the
sensitivity of the screening study for the subclinical
disease times the probability of being in the subclini-
cal phase at the time the screening study is
performed. The probability of being in the subclini-
cal phase is determined by the natural history of the
disorder and the sensitivity of the screening study is
determined by the study's critical value. In order to
assure that the study has adequate specificity, the
Monitoring
5-1
Chapter 5
MONITORING
© 2001 Dennis A. Noe
Table 5.1
Clinical Uses of Monitoring Studies
1. Detect the development of a disorder while the disorder is
still subclinical
2. Assess changes in organ function, metabolic activity, or
macro- or micronutritional status
3. Assess changes in organ function, metabolic activity, or
macro- or micronutritional status in response to therapy
4. Detect changes in the level of activity of a disorder
5. Detect changes in the level of activity of a disorder in
response to therapy
6. Follow the time course of intoxicants and poisons
7. Follow the time course of pharmacologic agents
