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a Chapter 24 Doppler Velocimetry for Fetal Surveillance: Adverse Perinatal Outcome and Fetal Hypoxia 365
Diagnostic Efficacy of Doppler Velocimetry
Among High-Risk Pregnancies
The efficacy of fetal Doppler investigations for predicting
adverse perinatal outcome in complicated
pregnancies has been widely investigated. A selection
of these studies is summarized in Table 24.1, some of
which are also discussed later.
Most studies indicated that the Doppler results
were efficacious for identifying the fetus at risk in
complicated pregnancies. There are, however, wide
variations in the performance of the technique, with
the sensitivity ranging from 21% to more than 90%
and the specificity from 63% to 97%. This range may
not be surprising, as the studies were heterogeneous
in several ways, including the population selection
criteria, the method of using Doppler surveillance
(e.g., the frequency of examination), the diagnostic
threshold value of the test result, and the outcome
parameters. As is evident, the investigators were not
uniform in their selection criteria of an adverse perinatal
outcome, which included fetal smallness for gestational
age (SGA), operative delivery for fetal distress,
Apgar score, the need for admission to the neonatal
intensive care unit (NICU), and various other
conditions. These variations are not surprising as the
criteria for morbid perinatal outcome remain controversial.
Many of the traditional measures of morbidity
are now known to be of little significance for
long-term prognosis of the infant. Despite these limitations,
traditional measures of perinatal morbidity
have not yet been replaced by any more insightful alternatives.
In addition, some of these studies were deficient in
their experimental approach. For example, many investigators
did not employ a blind technique, which might
have compromised the studies' validity as the clinicians'
preconceived notion about the efficacy of the test
would inevitably introduce bias. If the physician was already
favorably disposed toward the test, the results
could be erroneously affirmative; on the other hand,
the chances of obtaining false-negative results would
increase if the physician had no confidence in the test.
Thus blind evaluations significantly enhance the validity
of the study results. In one of the first studies reported
on the diagnostic efficacy of Doppler velocimetry,
the investigators [2] did not state whether the clinicians
were blind to the Doppler results. Moreover, in
one of the largest studies on Doppler efficacy, which
also included a nonstress test (NST) and biophysical
Fig. 24.2. Principle of receiver operating characteristic
(ROC) curve analysis for assessing the efficacy of a diagnostic
test. D (±) disease-free, D (+) diseased, T (±) test negative,
T(+) test positive, TP true positive, FP true negative. Top:
Distribution pattern of the diseased and disease-free populations
along the test value represented by the horizontal
line in the middle of each panel. Bottom: ROC curves. Note
that with a useless test there is an equal chance of being
diseased or disease-free at any given value of the test (A1);
the corresponding curve (A2) is the diagonal from the lower
left to the upper right corner. In contrast, a perfect test
completely discriminates between the two populations
(B1). The arrow points to the most discriminatory value of
the test. The corresponding ROC curve (B2) is represented
by the left and upper margins of the graph with no false
positives. The upper left corner corresponds to only truepositive
and no false-positive results and therefore represents
the absolute discriminatory value of the test. A useful,
but less than perfect, test predominantly separates the
two populations with some degree of overlap that contributes
to the false-positive and false-negative results (C1).
The arrow, as in the previous case, points to the most discriminatory
value of the test. The ROC curve covers an area
(black shaded) that is a measure of the test's overall efficacy
(C2). The test value nearest to the upper left corner is
the most efficacious