Oral Contrast for Abdominal CT: How Important Is It and How Long Does It Take?

Joseph R. Lex Jr., MD

Disclosures

April 16, 2008

Question

How important is oral contrast material to the accuracy of an abdominal computed tomography (CT), and how long does it really take to reach its intended target?

Response from Joseph R. Lex Jr., MD

 

Joseph R. Lex Jr., MD
Associate Professor, Department of Emergency Medicine, Temple University School of Medicine, Philadelphia, Pennsylvania


Although CT is a relatively recent technology, there have been great improvements in the equipment. The early, slow "Step and Scan" machines required long image acquisition times, resulting in a greater chance for movement artifact from respirations and bowel peristalsis. It was necessary to give high-volume oral contrast to opacify the small bowel in order to maximize imaging accuracy. Also, there were few studies comparing oral contrast with no oral contrast, as contrast material was low-cost and there were no direct adverse events for the patient.

CT technology has evolved, and image acquisition with the newer helical technology has practically eliminated movement issues due to respiration and peristalsis. Many radiologists who read these scans now feel very comfortable interpreting them without oral contrast, and the literature supports this practice.

Inflammatory conditions often manifest with abnormalities of the adjacent fat of the peritoneal cavity and omentum, which are detectable without oral contrast. Abscesses are also detected without oral contrast. Bowel wall pathology may be better interpreted with bowel distention, but oral contrast does not distend the colon, and the detection of pneumatosis is not improved by oral contrast. In fact, recent studies indicate that oral contrast adds very little to the accurate diagnosis of nontraumatic abdominal pain.

Lee and colleagues[1] did a prospective study of 100 emergency department (ED) patients with abdominal pain. These patients were initially scanned without oral contrast and then again 90 minutes after oral contrast with identical scanning parameters. Experienced radiologists were given no information about medical history before they interpreted the noncontrast scans; the same group of radiologists interpreted scans performed after ingestion of oral contrast, but they did not have access to earlier matched scans or interpretations of those scans.

At first glance the results weren't terribly impressive: For 21 patients there was clinically significant disagreement between contrast and noncontrast interpretations: 11 had normal noncontrast but abnormal contrast studies, 6 had abnormal noncontrast studies but normal contrast studies, and 4 had abnormal studies with and without contrast but a disagreement in the abnormality by the interpreting radiologist. However, on careful post-hoc analysis, only 2 cases were discrepant primarily because of the addition of oral contrast; 18 were due solely to interobserver variability (1 turned out not to be discrepant). This 18% interobserver variability is not significantly different from other published studies, which show discrepancy rates of up to 38%.[2]

There is also the ever-present problem of self-referential bias -- that is, the final diagnosis is that given by the interpretation of the CT scan.

When looking at the accuracy of abdominal CT with and without contrast and specifically looking for appendicitis, the results are surprising. A meta-analysis of 23 studies showed that unenhanced CT sensitivity was similar (95% vs 92%) as was specificity (97% vs 94%) and accuracy (97% vs 89%).[3]

The patient must wait 60-90 minutes after drinking a contrast agent before it is adequately distributed in the bowel. Studies have shown that by requiring oral contrast, the patient's stay in the ED is extended by far more than 90 minutes. In one study there was 173-minute increase in time from registration to disposition in patients who received oral contrast prior to CT vs those who did not, representing an 83-minute extension over and above the 90-minute total bowel opacification time.[4]

Conclusion

Oral contrast takes at least 90 minutes to adequately opacify the bowel and increases length of stay in the ED by almost double that amount of time, but it adds little, if anything, to the accuracy of diagnosis in patients with nontraumatic abdominal pain. Your local radiologist, however, may not be comfortable interpreting CT scans without the contrast and might have the final say. Discuss this controversy with your radiologists to reach an agreement that satisfies everyone.

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