Is Unenhanced CT Accurate for Diagnosing Acute Abdominal Pain in the ED?

Unenhanced CT was about 30% less accurate than contrast-enhanced CT for the evaluation of acute abdominal pain in the emergency department (ED), a retrospective study showed.

Of 201 adult patients, overall accuracy of unenhanced CT was 70% -- faculty: 68% to 74%; residents: 69% to 70% -- when compared with a reference standard reached by majority rule among three blinded radiologists using contrast-enhanced CT, reported Matthew S. Davenport, MD, of Michigan Medicine in Ann Arbor, and colleagues.

Of six blinded radiologists from three centers -- three specialist faculty members and three residents -- who interpreted the unenhanced CT scans, faculty had higher accuracy than residents for primary diagnoses (82% vs 76%; adjusted OR 1.83, 95% CI 1.26-2.67, P=0.002), but lower accuracy for secondary diagnoses (87% vs 90%; OR 0.57, 95% CI 0.35-0.93, P<0.001), they noted in JAMA Surgery.

They said this was likely because faculty made fewer false-negative primary diagnoses (38% vs 62%; OR 0.23, 95% CI 0.13-0.41, P<0.001) but more false-positive actionable secondary diagnoses (63% vs 37%; OR 2.11, 95% CI 1.26-3.54, P=0.01).

"The consistent results we observed across three centers suggest that the substantial diagnostic penalty we observed is likely to be related to the removal of contrast medium rather than to radiologist idiosyncrasy," Davenport and team wrote.

Contrast medium is often administered in the ED for CT of the abdomen and pelvis, but is sometimes withheld because of sensitivity to the iodinated contrast medium, severe kidney disease, or lack of availability. When contrast media is withheld, it's unclear how much diagnostic accuracy is lost -- and the consequences of under- or misdiagnosis can be serious, Davenport told MedPage Today.

"In many patients, the risk of withholding iodinated contrast medium may be higher than the risk of administering it," the authors concluded.

The study serves to reinforce the relative safety of newer IV contrast and "better informs that risk-benefit decision making, and in general would probably push people in those gray areas especially to give the IV contrast and accept those other risks for contrast reaction and kidney problems, just because the diagnostic penalty is so large," Davenport said.

He noted that around two decades ago, "people thought that IV contrast was a very common cause of kidney injury. And there was sort of a revolution that happened in the last 10 years in which that idea has been revised."

"But like many things in healthcare, when a new guideline comes out, or new ideas come out, it takes a long time for that to reach everybody," he added. "Doctors who treat the patients have long suspected that when you take away the IV contrast, it's harder to interpret the scan, but no one's really measured that. And so I was surprised at how much of a diagnostic penalty there was."

In an invited commentary, Mayur B. Patel, MD, MPH, of Vanderbilt University Medical Center in Nashville, Tennessee, and colleagues noted that "the risk of kidney injury varies based on pre-CT kidney function: it approaches 0% with normal kidney function but is higher with chronically impaired kidney function."

"In the low-risk patient, complications of IV contrast is often negligible, and the benefits of routine IV contrast far outweigh the risks," they concluded.

For this multicenter study, Davenport and colleagues included 201 consecutive adult ED patients who underwent dual-energy contrast-enhanced CT for the evaluation of acute abdominal pain from April 1-22, 2017. Mean age was 50.1, 54% were women, and mean body mass index was 25.5.

There were 104 primary diagnoses in 98 patients (six patients had two primary diagnoses) and 17 secondary diagnoses in 17 patients. Diagnoses included appendicitis, colitis, urinary tract infections, small bowel obstruction, and diverticulitis, among others.

A simple diagnostic accuracy analysis, which evaluated accuracy by a radiologist at the level of the CT examination, was calculated as sensitivity (faculty: 78% to 89%; residents: 67% to 87%), specificity (faculty: 73% to 86%; residents: 75% to 92%), positive predictive value (faculty: 79% to 88%; residents: 81% to 92%), and negative predictive value (faculty: 77% to 87%; residents: 70% to 83%).

In the detailed diagnostic accuracy analysis, overall accuracy was higher for faculty when only primary diagnoses were considered (faculty: 85% to 90%; residents: 76% to 79%) and higher for residents when only secondary diagnoses were considered (faculty: 81% to 89%; residents: 88% to 92%).

Davenport and colleagues said their study was limited by its restrospective design, but they noted that they attempted to reduce bias by using consecutive patients for whom dual-energy CT was routinely used, using a robust reference standard, blinding the radiologists, using radiologists from different institutions, and not limiting the study to a specific diagnosis.

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