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Calf Sonography for Detecting Deep Venous
Thrombosis in Symptomatic Patients:
Experience and Review of the Literature
Ronald H. Gottlieb, MD,1 Jani Widjaja, MD,2 Lili Tian, MA,3 Deborah J. Rubens, MD,1
Susan L. Voci, MD1
1
Department of Radiology, University of Rochester Medical Center, 601 Elmwood Avenue, Box 648,
Rochester, New York 14642
2
Department of Radiology, Rochester General Hospital, 1425 Portland Avenue, Rochester, New York 14621
3
Department of Biostatistics, University of Rochester Medical Center, 601 Elmwood Avenue, Box 630,
Rochester, New York 14642
Received 2 December 1998; accepted 27 April 1999
ABSTRACT: Purpose. We determined the accuracy of
sonography in the detection of isolated calf deep venous thrombosis (DVT) and the rate of indeterminate
ultrasound examinations in patients with physical
signs or symptoms suggestive of DVT.
Methods. We reviewed the medical literature
(MEDLINE) to determine the accuracy of sonography
and the frequency of indeterminate studies in detecting isolated calf DVT in patients with physical signs or
symptoms suggestive of DVT. A meta-analysis was
used to derive summary measures of sensitivity,
specificity, and accuracy from studies in which 5 or
more isolated calf DVT were identified. Frequencies of
indeterminate examinations were recorded for studies in which these data were provided, and we pooled
these results with our own data for 196 patients.
Results. The meta-analysis revealed that sonography correctly identified isolated calf DVT in 49 of 53
extremities (sensitivity, 92.5%; 95% confidence interval, 81.8–97.9%) and correctly identified the absence
of calf DVT in 157 of 159 extremities (specificity,
98.7%; 95% confidence interval, 95.5–99.9%), yielding
an accuracy of 97.2% (95% confidence interval, 93.9–
99.0%) for ultrasound examinations considered diagnostic. However, when evaluating our patient population and the literature, we found a substantial number
of indeterminate studies (overall rate of 54.6% in 463
extremities), with a wide variation in the reported frequency of indeterminate studies (9.3–82.7%).
Conclusions. Sonography is highly accurate in detecting isolated calf DVT in symptomatic patients,
Correspondence to: R. H. Gottlieb
© 1999 John Wiley & Sons, Inc.
VOL. 27, NO. 8, OCTOBER 1999
CCC 0091-2751/99/080415-06
but indeterminate studies occur frequently, with a
wide range of reported rates. Each ultrasound laboratory should evaluate its own rate of indeterminate
studies. © 1999 John Wiley & Sons, Inc. J Clin Ultrasound 27:415–420, 1999.
Keywords: lower extremities; thrombosis; ultrasonography
M
ore than 600,000 cases of pulmonary embolism occur in the United States each year,
resulting in 100,000 deaths.1 Efforts have been
directed toward diagnosing and treating thrombi
in the deep venous system of the lower extremities because approximately 80% of emboli to the
lungs arise from these veins.2 However, clinical
assessment is unreliable in detecting deep venous
thrombosis (DVT) in the lower extremities.3 Contrast venography is the gold standard against
which other methods of diagnosing lower extremity DVT are compared. Sonography, being noninvasive and portable, has replaced contrast venography as the primary imaging modality in the
evaluation of patients with clinically suspected
DVT in the lower extremities. Sonography is generally acknowledged to be very accurate in detecting thigh DVT (88–100% sensitive and 92–100%
specific).4–7 There is not complete agreement,
however, regarding the accuracy of sonography in
detecting calf DVT, with better results reported in
symptomatic compared with asymptomatic patients.8
415
GOTTLIEB ET AL
Most patients who present to our ultrasound
laboratory to rule out lower extremity DVT have
physical signs or symptoms suggestive of this condition, but the majority of our ultrasound evaluations of the calf are either negative or indeterminate for DVT. We believed it would be useful to
perform a meta-analysis to evaluate the accuracy
of calf sonography in detecting isolated DVT and
the rate of nondiagnostic ultrasound examinations in symptomatic patients. Knowledge of the
effectiveness of sonography in the calf could assist
in determining the need for additional confirmatory imaging (eg, contrast venography), establishing protocols for evaluating the lower extremity
(whether or not to include the calf), and guiding
treatment decisions.
PATIENTS AND METHODS
Retrospective Study
We retrospectively evaluated the reports of all patients (n ⳱ 397) presenting to our ultrasound
laboratory for lower extremity ultrasound examinations between August 1, 1996, and March 30,
1997. Inclusion criteria for our study population
were lower extremity physical findings (eg, swelling, palpable cord) or symptoms (eg, pain). Patients were excluded if they had positive or indeterminate findings for DVT on thigh sonography
or if no specific indication for the examination
could be identified. Patient information was obtained by both medical record review and review
of our radiology information system. Our study
population (n ⳱ 196) had 81 male and 115 female
patients with a mean age of 54 years (range, 1–93
years). Risk factors for DVT in our patients are
summarized in Table 1. Our institutional review
board approved our study protocol without requiring the use of informed consent because there was
no active patient intervention.
All ultrasound examinations were performed
by experienced sonographers and checked by radiologists with special expertise in sonography. A
DVT was considered to be present if a deep vein in
TABLE 1
Risk Factors for Deep Venous Thrombosis in 196 Patients
Risk Factor
Malignancy
Surgery
Trauma
Previous deep venous thrombosis
Birth control pills
None
416
No. Patients (%)
39 (19.9)
54 (27.6)
22 (11.2)
23 (11.7)
3 (1.5)
55 (28.1)
either the thigh or calf was noncompressible, as
described previously.9–12 For an examination to
be considered negative in the thigh, all of the
deep thigh veins had to be compressible. A negative calf examination required all the deep calf
veins to be compressible or visualized with color
Doppler imaging. If there was uncertainty as to
the presence of thigh or calf DVT or all the deep
veins in the thigh or calf were not visualized, the
examination was considered indeterminate. Examinations positive for DVT in the thigh or calf
were not considered indeterminate even if all of
the other deep veins could not be visualized. Color
Doppler imaging was used in all cases to identify
the paired deep calf veins (anterior tibial, posterior tibial, and peroneal) and also facilitated
identifying the other veins in the calf, including
the intramuscular branches and the superficial
veins. Examinations were categorized as negative
for DVT, positive for DVT, or indeterminate for
DVT in the calf or thigh based on review of the
radiology reports as gathered from our radiology
information system. We compared the results of
sonography with those of contrast venography
(considered the gold standard) performed within
48 hours of the sonographic examination whenever contrast venography results were available.
Literature Review
A MEDLINE search of all English-language papers published from January 1982 to January
1998 was performed with the following keywords:
calf thrombophlebitis, ultrasound, venography,
and phlebography. Relevant references from
these papers were also reviewed. Studies were selected4,13–16 for the meta-analysis if they met the
following criteria for adequacy:
1. The population studied was specified accurately (eg, symptomatic or asymptomatic).
Only patients with lower extremity physical
signs or symptoms suggestive of a DVT were
considered in the analysis.
2. The study detected at least 5 cases of calf DVT.
3. The ultrasound methodology was described accurately and was acceptable.
4. The paper defined a positive sonographic result for DVT as noncompressibility of a deep
calf vein. A negative result was defined as
compressibility of all visualized deep veins of
the calf or adequate visualization of all of the
deep calf veins with color Doppler imaging.
5. Only studies considering the accuracy of sonography for detecting isolated DVT in the calf
were included (accuracy in the calf was evaluated separately from accuracy in the thigh).
JOURNAL OF CLINICAL ULTRASOUND
DEEP VENOUS THROMBOSIS OF CALF
6. Indeterminate studies were defined as those
examinations in which segments of the deep
calf veins were not visualized or the interpretation was inconclusive for the presence of a
DVT.
7. The gold standard for accuracy in the calf was
contrast venography, with a filling defect in a
deep calf vein considered positive for DVT and
the absence of a filling defect (with all of the
deep calf veins visualized) negative for DVT.
The study had to require venographic correlation within 48 hours of the ultrasound examination.
8. The readers of the ultrasound studies were
blinded to patient outcome as well as the results of all alternative tests for DVT or pulmonary emboli.
Statistical Methods
We performed a meta-analysis using the method
of Midgette et al17 to summarize the sensitivity,
specificity, and accuracy of all studies from the
literature meeting our selection criteria. The sensitivity, specificity, and accuracy figures from our
own patient population were not included in the
meta-analysis since we had too few cases of calf
DVT confirmed by contrast venography. The chisquared test18 was used to determine that the results of all studies were homogeneous. The homogeneity of results between studies allowed us to
pool the results of all of the studies in the metaanalysis.
We also recorded the frequency of indeterminate examinations (if this was reported) from
each of the studies included in the meta-analysis.
We calculated a combined frequency of indeterminate calf ultrasound studies from the studies extracted from the literature as well as our own
patient population. The extreme variability in reported frequency of indeterminate examinations
prevented us from pooling these results using a
meta-analytic technique.
Our unit of analysis was the extremity (rather
than the patient) in calculating sensitivity, specificity, accuracy, and frequency of indeterminate
ultrasound examinations for the pooled studies.
RESULTS
A total of 249 extremities were considered in our
study population (143 unilateral examinations
and 53 bilateral examinations). Sixteen isolated
calf DVT (6.4%) were identified, 27 examinations
(10.8%) were read as negative with adequate visualization of all of the deep calf veins, and 206
examinations (82.7%) were considered indeterminate in the calf.
VOL. 27, NO. 8, OCTOBER 1999
We also evaluated the results of our study
population on a per patient basis (n ⳱ 196). We
found 160 patients (81.6%) to have indeterminate
examinations: 41 patients had bilateral indeterminate examinations, 115 patients had unilateral
indeterminate examinations in which only 1 extremity was evaluated, and 4 patients had unilateral indeterminate examinations in which the
other extremity was negative. Twenty patients
(10.2%) had completely negative examinations, of
which 17 were unilateral examinations and 3
were bilateral examinations. Sixteen patients
(8.2%) had positive unilateral examinations.
There were no bilateral positive examinations.
Corresponding contrast venograms were obtained in 6 patients. Four of these patients had
indeterminate calf ultrasound examinations; contrast venography identified an isolated calf DVT
in 1 and was negative in 3. One patient had an
isolated calf DVT identified by ultrasound and
confirmed by contrast venography. The remaining patient had a negative calf ultrasound examination and a negative contrast venogram.
Table 2 summarizes the sensitivity, specificity,
and accuracy of calf sonography in detecting isolated calf DVT in symptomatic patients from
studies meeting our selection criteria. All were
prospective studies in which the results of calf
sonography were confirmed with venography.
The denominator in computing sensitivity, specificity, and accuracy in all of the studies was the
number of extremities evaluated, rather than patients, since some patients had both calves studied.
Table 3 summarizes the frequency of indeterminate calf ultrasound examinations in our patient
population as well as in studies that met the selection criteria for our meta-analysis. Two studies in
our meta-analysis did not report their frequency of
indeterminate calf ultrasound examinations.
The total number of extremities differed for
calculations of accuracy (Table 2) and frequency
of indeterminate examinations (Table 3). Only examinations with a gold standard of contrast venography were considered in calculating accuracy, whereas all examinations (both with and
without a corresponding contrast venogram) were
used in calculating the frequency of indeterminate examinations.
DISCUSSION
Sonography is generally acknowledged to be
highly accurate in detecting thigh DVT.4–7 However, there remains uncertainty as to the effectiveness of sonography in detecting calf DVT.
Prior studies of calf sonography have evaluated
relatively small patient populations. We therefore
417
GOTTLIEB ET AL
TABLE 2
Accuracy of Calf Sonography for Detecting Deep Venous Thrombosis in Symptomatic Patients
First Author
16
Bradley
Rose
4
Yucel15
Study Type
Gold Standard
Sensitivity*
Specificity*
Accuracy*
Prospective
Venography
Prospective
Venography
100%
(16/16)
80%
(4/5)
88%
(15/17)
88%
(7/8)
100%
(7/7)
92.5%†
(49/53)
100%
(50/50)
100%
(24/24)
96%
(26/27)
98%
(48/49)
100%
(9/9)
98.7%‡
(157/159)
100%
(66/66)
97%
(28/29)
93%
(41/44)
96%
(55/57)
100%
(16/16)
97.2%§
(206/212)
Prospective
Venography
13
Prospective
Venography
Simons14
Prospective
Venography
Atri
Pooled totals
*Sensitivity [true positives/(true positives + false negatives)], specificity [true negatives/(true negatives + false positives)], and accuracy [(true
positives + true negatives)/total examined] were calculated based on number of extremities evaluated.
†
95% confidence interval, 81.8–97.9%.
‡
95% confidence interval, 95.5–99.9%.
§
95% confidence interval, 93.9–99.0%.
TABLE 3
Frequency of Indeterminate Calf Ultrasound Examinations
First Author
Study Type
Rose4
Simons14
Atri13
Current study
Pooled total
Prospective
Prospective
Prospective
Retrospective
Frequency of Indeterminate
Examinations*
42% (21/50)
29% (16/56)
9.3% (10/108)†
82.7% (206/249)
54.6% (253/463)
*The frequency of indeterminate calf ultrasound examinations was
the percentage of extremities in which all of the deep calf veins were
not visualized or the examination was considered nondiagnostic. The
denominator was the number of extremities examined.
†
In this study, the frequency of indeterminate calf ultrasound examinations was the percentage of calf examinations considered as
such in all patients (not just in patients with a negative thigh ultrasound examination).
considered it useful to perform a meta-analysis of
the existing studies4,13–16 of calf sonography in
patients suspected of having lower extremity
DVT. This analysis included assessing the sensitivity, specificity, and accuracy of the examination in detecting isolated calf DVT as well as the
frequency of indeterminate ultrasound studies.
This information could help ultrasound laboratories to (1) decide whether to incorporate the calf
as part of routine protocols of assessing the lower
extremity for DVT and (2) determine the need for
additional confirmatory imaging in patients who
have a negative thigh ultrasound study for DVT
and a negative, positive, or indeterminate calf
study for DVT.
We decided to focus on patients with physical
signs or symptoms suggestive of DVT rather than
combining this group with patients without such
findings (eg, high-risk postoperative patients or
those with suspected pulmonary embolus) for the
418
following reasons: (1) physical signs or symptoms
suggestive of a DVT are our most common indication for sonography to detect DVT, (2) concentrating on this group of patients allowed us to
evaluate a relatively homogeneous patient population with a defined presentation, and (3) Doppler sonography has been found more accurate in
detecting DVT in symptomatic than asymptomatic patients.8
Our analysis found sonography to be very accurate (comparable to rates reported in the thigh)
in detecting isolated calf DVT when ultrasound
studies were considered diagnostic. However, indeterminate studies in the calf occur relatively
frequently, with a wide variation in reported
rates.4,13,14
There are many potential causes for indeterminate calf ultrasound examinations for DVT. Some
of the causes are correctable, whereas others are
not. Eliminating the correctable causes could reduce the rate of indeterminate calf examinations.
Distending the deep veins by dangling the
leg(s) over the side of the bed could increase visualization of the deep veins and reduce the rate of
indeterminate examinations. In our analysis of
the 3 studies from the literature reporting rates of
indeterminate examinations, the study13 with the
lowest reported rate (9.3%) used leg dangling as
part of the routine protocol, whereas the other 2
studies4,14—with higher rates (42% and 29%, respectively)—did not.
As in other fields of sonography, some variability in examination results is due to the experience
level and ability of the individual sonographer.
Dedicated vascular laboratories with sonographers performing a high volume of calf ultrasound examinations would be expected to have a
JOURNAL OF CLINICAL ULTRASOUND
DEEP VENOUS THROMBOSIS OF CALF
lower rate of indeterminate examinations than
laboratories with lower volumes. However, we
have observed in our laboratory that this source
of variation is reduced considerably after an initial learning curve. The learning curve can be
shortened by feedback from quality assurance review and intense individual training.
A factor that affects the rate of indeterminate
examinations but that cannot be corrected is differences in patient populations. All of the deep
calf veins frequently will not be seen, even with
the best ultrasound technique, in patients with
large hematomas, open wounds, extensive swelling, casts, or obscuring bandages. Also, it may not
be possible to position immobile patients optimally to visualize the calf veins; large numbers of
immobile patients will likely increase the frequency of indeterminate calf examinations. However, innovative or persistent scanning techniques can reduce the rate of indeterminate
examinations even in difficult-to-scan patients.
Probably the most important source of variation in the reported frequency of indeterminate
calf examinations is differences between laboratories in what is considered visualization of “all of
the deep calf veins.” Some laboratories require
complete visualization (at least by color Doppler
imaging) of all segments of the deep calf veins,
leading to higher rates of indeterminate studies.
Other laboratories may accept lesser degrees of
visualization of all of the deep calf veins, potentially leading to lower rates of indeterminate
studies. Requiring visualization of the relatively
small anterior tibial veins, which are believed by
some not to be an important isolated source of
pulmonary emboli, would raise the rate of indeterminate studies reported.19
A uniformly agreed-upon definition of a negative calf ultrasound examination for DVT would
help reduce the variation in reported rates of indeterminate studies. Such a definition would include (1) whether visualization of the anterior
tibial veins was required and (2) whether visualization of segments of specified lengths of the
paired deep calf veins without identification of a
DVT would be sufficient to call a calf examination
negative. Power Doppler imaging20,21 or ultrasound contrast agents22 may also reduce the rate
of indeterminate calf examinations.
In our population, the exceptionally high rate
of indeterminate studies was likely mainly due to
our strict requirement that all paired deep calf
veins, including the anterior tibial veins, be seen
in their entirety by color Doppler imaging and be
compressible along visualized segments. Leg dangling was performed intermittently at the discreVOL. 27, NO. 8, OCTOBER 1999
tion of the sonographer and was generally not
performed in less mobile inpatients; this may also
have contributed to our high rate of indeterminate examinations.
Our retrospective study design forced us to
evaluate examination reports. Examination reports that did not state that all the deep calf veins
were seen were considered indeterminate. However, in our practice, most lower extremity ultrasound studies are performed by sonographers,
and the hard copy images are reviewed by radiologists who scan any questionable areas identified by the sonographers. The dictated report usually reflects the sonographer’s description of the
findings, and the sonographers usually do not visualize the deep calf veins in their entirety. The
physician receiving a report stating that the calf
veins were not completely visualized is most
likely going to consider their evaluation incomplete and the study therefore indeterminate for
the presence of a DVT. Despite the high rate of
indeterminate calf examinations in our patient
population, only 4 patients with indeterminate
studies underwent contrast venography, implying
that our referring physicians do not treat isolated
calf DVT. It is somewhat sobering that subsequent contrast venography in 2 studies from our
meta-analysis detected calf DVT in 17%13 and
19%4 of patients with indeterminate calf examinations.
Indeterminate ultrasound examinations were
not considered in calculating diagnostic accuracy
and were evaluated separately. We followed this
approach since referring physicians and radiologists deal with clinically indeterminate calf ultrasound examinations differently than diagnostic
examinations. Indeterminate examinations are
considered to be nondiagnostic for the presence or
absence of DVT and may motivate further evaluation (eg, contrast venography or follow-up sonography of the leg).
We were unable to provide meaningful figures
for sensitivity, specificity, and accuracy of calf sonography in detecting DVT in our patient population because so few patients underwent contrast venography. In our practice, and most likely
throughout the nation, the trend is for referring
physicians to accept the results of calf sonography
in excluding DVT without requiring confirmatory
imaging. This philosophy will, because of extreme
verification bias, make future prospective studies
evaluating the accuracy of sonography in detecting calf DVT very difficult to perform.
We used the extremity as the unit of analysis
in our determination of sensitivity, specificity, accuracy, and frequency of indeterminate examina419
GOTTLIEB ET AL
tions. We recognize that in patients with bilateral
leg examinations, the results of each leg would
not be completely independent. We were forced to
use the extremity as the unit of analysis because
2 of the 5 studies4,13 in our meta-analysis included patients with bilateral examinations and
did not describe the results of these patients separately. However, only a small percentage of patients (8.7% and 12.5%) in each of these studies
had bilateral examinations. Therefore, the results
of these studies would most likely be very similar
using the patient or extremity as the unit of
analysis. The rate of indeterminate examinations
was very high in our population using the patient
(81.6%) or extremity (82.7%) as the unit of analysis.
We recognize that our meta-analysis is limited
by the small number of studies included in the
evaluation. However, these were the only available studies that met our criteria for acceptability. We believed that quantitatively summarizing
the existing information would be useful. The limited number of studies prevented us from constructing a summary receiver operating characteristic curve. However, since the sensitivity and
specificity were fairly consistent for all studies,
we were able to summarize this information with
mean values and associated confidence intervals.
In summary, sonography is highly accurate in
detecting DVT in the calf in patients with lower
extremity symptoms or physical signs suggestive
of DVT. However, indeterminate examinations
occur relatively frequently, with a wide variation
in reported rates. Each ultrasound laboratory
should review its own rate of indeterminate studies in the calf.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
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