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2270 Part XII: Hemostasis and Thrombosis Chapter 133: Venous Thrombosis 2271

OBJECTIVE TESTING FOR PULMONARY with high predictive value in most patients, and withholding anticoagu-
lant therapy based on a negative CTA alone is associated with a low rate
EMBOLISM of subsequent VTE on followup. Objective testing for DVT is useful
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in patients in whom the CTA image is of poor quality or inconclusive,
The objective diagnostic imaging tests include CT, CTA, radionuclide and in patients who also have symptoms suggesting DVT. CTV has the
lung scanning, selective pulmonary arteriography, and objective testing advantage of being easily performed at the time of CTA, but incurs the
for DVT. Measurement of plasma D-dimer is useful as an exclusion test risk of added radiation exposure for the patient. Compression ultra-
in patients with an unlikely or intermediate clinical probability.
sonography can also be used, and avoids added radiation exposure and
can be performed serially if needed.
D-DIMER ASSAY
The assay for plasma D-dimer is useful as an exclusion test, provided an RADIONUCLIDE LUNG SCANNING
appropriately validated test is available. A negative result by the rapid Radionuclide lung scanning continues to have a role in the diagnosis of
quantitative ELISA for D-dimer has a negative likelihood ratio similar suspected PE A normal perfusion lung scan excludes the diagnosis of
to that of a normal perfusion scan. A positive D-dimer result is not clinically important PE. 15,45 A normal perfusion lung scan is found in
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useful diagnostically. Several management studies have found that PE approximately 10 percent of all patients with suspected PE seen at aca-
can be excluded without performing imaging studies in patients with demic health centers or tertiary referral centers. A high-probability lung
a low, intermediate, or unlikely clinical probability. When combined scan result (i.e., large perfusion defects with ventilation mismatch) has
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with pretest clinical probability assessment, the use of an age-adjusted a positive predictive value for PE of 85 percent and provides a diagnos-
D-dimer cutoff value instead of a fixed D-dimer cutoff of 500 mcg/mL, tic end point to give antithrombotic treatment in most patients. 15,46,47 A
improves the utility of the test, and enables more patients to have the high-probability lung scan is found in approximately 10 to 15 percent of
diagnosis of PE safely excluded. 39
symptomatic patients. For patients with a history of PE, careful compar-
ison of the lung scan results to the most recent lung scan is required to
COMPUTED TOMOGRAPHY IMAGING ensure the perfusion defects are new. Further diagnostic testing is indi-
AND ANGIOGRAPHY cated for patients with a high-probability lung scan who have a “low”
pretest clinical suspicion, and in those who are at high risk for major
CT imaging is the primary imaging test for the diagnosis of PE in most bleeding, to reduce the likelihood of a false-positive diagnosis.
centers. Single-detector spiral CT is highly sensitive for large emboli The major limitation of lung scanning is that the results are incon-
(segmental or larger arteries), but is much less sensitive for emboli in clusive in most patients, even when considered together with the pretest
subsegmental pulmonary arteries 16,40 ; such emboli may be clinically clinical probability. The nondiagnostic lung scan patterns are found in
15
important in patients with severely impaired cardiorespiratory reserve. approximately 70 percent of all patients with suspected PE. 13,15,47 These
Therefore, a negative result by single-detector spiral CT should not be lung scan results have historically been called “low probability” (match-
used alone to exclude the diagnosis of PE. A filling defect of a segmental ing ventilation–perfusion abnormalities or small perfusion defects),
or larger artery on single-detector spiral CT is associated with a high “intermediate probability,” or “indeterminate” (because the perfusion
probability (>90 percent) of PE. 40 defects correspond to an area of abnormality on chest radiograph).
The development of multidetector row CT, together with the use Further diagnostic testing is required in most of these patients because,
of contrast enhancement, has established CT as the preferred diagnos- regardless of the pretest clinical suspicion, the posttest probabilities of
tic imaging test in most patients. 41–43 Contrast-enhanced CTA has the PE associated with these lung scan results are neither sufficiently high to
advantage of providing clear results (positive or negative), with a low give antithrombotic treatment nor sufficiently low to withhold therapy.
rate of nondiagnostic test results, good characterization of nonvascu- The uncommon exception is the patient with a low clinical suspicion
lar structures for alternate or associated diagnoses, and the ability to and a so-called low-probability lung scan result. However, even in these
simultaneously evaluate the deep venous system of the legs (computed patients, objective testing for DVT with ultrasonography and/or mea-
tomographic venography [CTV]). surement of plasma D-dimer is without risk for the patient and may
The accuracy and clinical utility of multidetector CTA and com- provide added diagnostic value (see “Objective Testing for Deep Vein
bined CTA-CTV were evaluated in the Prospective Investigation of Pul- Thrombosis” below). A randomized trial has established that CTA is not
monary Embolism Diagnosis (PIOPED) II study. Among 824 patients inferior to using ventilation–perfusion lung scanning for excluding the
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with a reference diagnosis and a completed CT study, CTA was incon- diagnosis of PE when either test is used in an algorithm together with
clusive in 51 (6 percent) because of poor image quality. The sensitivity of venous ultrasonography of the legs. 48
CTA was 83 percent and the specificity was 96 percent. CTA-CTV was In centers where CTA is available, the major role for lung scanning
inconclusive in 87 (11 percent) of 824 patients because the image quality is in select patients; for example, in younger women to reduce radiation
of either CTA or CTV was poor. Multidetector CTA-CTV had a higher exposure to the breast. Lung scanning can be useful in such patients
sensitivity (90 percent) than CTA alone (83 percent), with similar spec- who are less likely to have comorbid cardiorespiratory disorders and
ificity (~95 percent for both testing techniques). Positive results on CTA therefore a higher proportion of diagnostic scan results (normal or high
in combination with a high probability or intermediate probability of probability).
PE by the clinical assessment, or normal findings on CTA with a low
clinical probability had a predictive value (positive or negative) of 92 to
96 percent. Such values are consistent with those generally considered MAGNETIC RESONANCE ANGIOGRAPHY
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adequate to confirm or rule out the diagnosis of PE. Additional testing is The accuracy of magnetic resonance angiography for diagnosing PE,
necessary when the clinical probability is discordant with CTA or CTA- with or without the addition of magnetic resonance venography, was
CTV imaging results. 43 evaluated in the PIOPED III study. This was a prospective study of
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Figure 133–2 summarizes the approach to diagnosis of suspected 371 adults with suspected PE recruited from seven hospitals and their
PE using CTA or CTA-CTV as the primary imaging test. A high-quality emergency services. Magnetic resonance angiography was technically
image by CTA is sufficient to establish or exclude the diagnosis of PE inadequate in 25 percent of patients (92 of 371); this rate ranged from

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