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Pulmonary Embolism Can Be Excluded Without High Exposure to Radiation
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Key Point |
Acute pulmonary embolism can be excluded with a very low probability interpretation of ventilation/perfusion lung scan plus a low probability clinical assessment. This process may be an alternative for patients for whom CT may be disadvantageous. |
Clinicians may soon be able to avoid using computed tomographic angiography (CTA) to screen for pulmonary embolism (PE) in many patients by using a combination of ventilation/perfusion (V•/Q•) lung scans and an objective clinical assessment instead. Alexander Gottschalk, MD, a Professor of Diagnostic Radiology at Michigan State University in East Lansing, and colleagues used data from the Prospective Investigation of Pulmonary Embolism Diagnosis II (PIOPED II) to verify that “low probability” findings on both measures reliably excluded PE in patients. The results were published in the September Journal of Nuclear Medicine.
“Our study is important for doctors—and their patients—since tests show that high radiation doses to the breast from CT, especially important in young women, may increase their risk of breast cancer,” said coauthor H. Dirk Sostman, MD, in a released statement. Dr. Sostman is Chair of Radiology and Senior Associate Dean for Clinical Affairs at Weill Medical College of Cornell University, New York City.
TWO ASSESSMENTS ARE NEEDED
All PIOPED II participants were seen at the eight participating medical centers between September 2001 and July 2003 and had clinically suspected acute PE; the participants were seen as either inpatients or outpatients, although the researchers noted that PIOPED II was “heavily weighted toward outpatients.” Among the 824 patients with suspected PE, 460 (56%) had a “very low probability” consensus interpretation of their V•/Q• scans from two independent readers. “The importance of this finding is that it increases the number of patients whose V•/Q• scan result enables enough diagnostic certainty for definitive clinical management,” said the researchers. They noted that while V•/Q• scans were sensitive and specific for the diagnosis or exclusion of PE in the original PIOPED study, “only a minority of patients (approximately 25% of the whole patient sample) had a reading that enabled clinical management without further testing.”
CTA or digital subtraction angiography of diagnostic quality was obtained in 440 patients of the patients with very low probability V•/Q• scans; 430 of these patients also had objective clinical assessment using the Wells Clinical Probability of PE scoring system. The positive predictive value of a very low probability interpretation was 32 of 430 patients (7.4%); however, among those with a low probability clinical assessment (Wells score < 2), the positive predictive value was 8 of 262 (3.1%). Intermediate (Wells score 2 to 6) and high probability (Wells score > 6) clinical assessments had positive predictive values of 12.6% and 29.4%, respectively.
A CHANGE IN CLINICAL CARE
“Because V•/Q• scans have been suggested by some to be the imaging test of choice in women of reproductive age, we analyzed the positive predictive value of a very low probability interpretation of the V•/Q• scan in women [40 and younger],” stated Dr. Gottschalk and colleagues. Overall, the positive predictive value was seven of 77 women (9.1%) with both a very low probability V•/Q• scan and an objective clinical assessment. Stratified by probability of the clinical assessment, the positive predictive values were one of 50 women with low probability (2%), five of 21 women with intermediate probability (23.8%), and one of six women with high probability (16.7%).
V•/Q• scintigraphy is advantageous because it uses a lower radiation dose than CTA and does not require iodinated contrast material, noted the researchers, and they suggested that clinicians reappraise its role in suspected acute PE. They acknowledged that while a high probability interpretation allows the V•/Q• scan to be interpreted as “PE present” with high specificity and positive predictive value, the sensitivity is low, as reported in the original PIOPED study in JAMA in 1990. Instead, the study authors recommended that normal perfusion and very low probability interpretations combined allow a “PE absent” interpretation, with “PE uncertain” indicated by all other interpretations. “We believe this is comparable to the diagnostic choices usually used when interpreting the CT angiogram as well as to their diagnostic accuracy,” they said.
—Jessica Dziedzic
Suggested Reading
Gottschalk A, Stein PD, Sostman HD, et al. Very low probability interpretation of V•/Q• lung scans in combination with low probability objective clinical assessment reliably excludes pulmonary embolism: data from PIOPED II. J Nucl Med. 2007;48(9):1411-1415.
Value of the ventilation/perfusion scan in acute pulmonary embolism: results of the prospective investigation of pulmonary embolism diagnosis (PIOPED). The PIOPED Investigators. JAMA. 1990;263(20):2753-2759.
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