Spiral Volumetric CT to Diagnose Pulmonary Embolism

Abstract & Commentary

Synopsis: Of 11 published investigations of spiral volumetric computed tomography in the diagnosis of pulmonary embolism, none met all 11 standards applied by the authors for studies of diagnostic tests, and only five articles met five standards. Spiral CT can be helpful for "ruling in" pulmonary embolism, but not for excluding this diagnosis.

Source: Mullins MD, et al. Arch Intern Med 2000; 160:293-298.

The use of spiral volumetric computed tomography (SVCT) to diagnose pulmonary embolism (PE) was first reported in 1992. Since that time, this technique has become more widely used to evaluate patients suspected of having PE. This study sought to determine whether current enthusiasm for SVCT in this context is justified by rigorously examining the available published studies of its use.

Mullins and associates at the University of Virginia performed a MEDLINE search for articles published up to mid-1998. They only included articles reporting comparisons of SVCT to the results of pulmonary arteriograms or another reference standard (such as a high-probability ventilation-perfusion scan in the presence of a high clinical suspicion for PE). To the published articles that met these criteria, Mullins et al applied 11 standards, adapted from accepted methods for scrutinizing studies of diagnostic tests (see Table).

Eleven studies were identified in the English literature prior to July 1998 that met the requirements of Mullins et al for inclusion. These studies came from six different countries, and reported on between 10 and 185 examinations. None of them met all 11 standards shown in the table. In fact, only five of them met five or more standards. Mullins et al conclude that there is not yet an established place for SVCT in the diagnostic evaluation of patients suspected of having acute PE. They point out that SVCT may be relatively sensitive and specific for diagnosing central pulmonary artery PEs, but that it is insensitive for diagnosing subsegmental clots.

COMMENT BY DAVID J. PIERSON, MD, FACP, FCCP

Enthusiasm for and widespread clinical use of a new diagnostic technique typically precedes appropriate scientific studies of its validity and cost-effectiveness. This is particularly true for conditions that are potentially life threatening, and for which the existing diagnostic armamentarium is far from ideal. PE is one such condition. A killer of hundreds of thousands of people annually, PE is notoriously difficult to diagnose, particularly in the presence of underlying cardiopulmonary disease. Ventilation-perfusion scanning is highly accurate in otherwise healthy patients when the history is typical and there are neither coexisting medical problems nor technical difficulties. When these conditions are not met, however, as tends to be the case in the ICU, the ventilation-perfusion scan all too often yields unhelpful "indeterminant" results. The currently popular ultrasound studies of the lower extremities are highly operator dependent and cannot say whether a clot is present in the lungs. The gold standard for diagnosing PE, pulmonary angiography, is invasive and not always easy to obtain.

Table-Standards by Which Published Studies of SVCT Were Examined
1. Clear description of SVCT technique
2. Clear criteria for positive or negative result
3. Assessment of interpretation reliability by comparing independent (blinded) readings
4. Assessment of reliability of SVCT by having some patients undergo repeated testing with comparisons of both tests
5. Sufficient description of patient selection process so that similar patients could be studied
6. Sufficient description of patients themselves for reader to make comparisons with his/her own patients
7. Sufficient description of eligible patients who were not enrolled in study
8. Sufficient description of extent of disease so that results could be stratified by location or severity of PE
9. Sufficient detail on non-PE diagnoses so that inference of discriminative ability of SVCT for patients without PE possible
10. Referral of patients for SVCT and reference standard regardless of results of either
11. Results of SVCT and reference standard interpreted independently

It is no wonder that SVCT has been so enthusiastically embraced by clinicians and radiologists alike. It is easy and quick to perform, and more and more reliance is being placed on its findings. In many institutions SVCT has rapidly become the initial diagnostic study of first choice for patients suspected of having PE. But is this trend justified?

This systematic review points out how incomplete the current database is with respect to using SVCT to diagnose PE. Especially, this test cannot be used to exclude this diagnosis with confidence. SVCT is highly specific for PE, and if it shows a big clot in a central pulmonary artery, the diagnosis is made. However, when a clot is not seen on SVCT, PE has definitely not been excluded. Thus, as Mullins et al point out, SVCT may have a role as a "rule-in" test for large central emboli, but additional research is required to establish its place in clinical practice.

In the diagnostic evaluation of patients suspected of having pulmonary embolism, spiral volumetric computed tomography:

a. is the gold standard.

b. can rule in large central emboli.

c. has replaced the ventilation-perfusion scan.

d. is useful in excluding small peripheral emboli.

e. All of the above