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CT with Contrast for Kidney Stones?

May 26, 2021

Written by Sam Parnell

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Contrast-enhanced CT has a very high negative predictive value (100%) for obstructive urolithiasis and appears to accurately and safely exclude obstructing ureteral calculi for patients with acute flank pain. In addition, IV contrast improves the diagnostic yield for other acute abdominopelvic pathology and perhaps should be the test of choice for patients presenting with acute flank pain.

Why does this matter?
Flank pain is a very common chief complaint encountered in the acute care setting. Historically, a non-contrast CT of the abdomen/pelvis (NCCT) has been the gold standard for diagnosing acute obstructing urolithiasis. However, 54-67% of patients presenting to the ED with flank pain do not have urolithiasis, and there are several conditions that mimic renal colic such as diverticulitis, appendicitis, cholecystitis, ovarian cyst rupture, etc. where IV contrast would improve the diagnostic accuracy of the test. But can a contrast-enhanced CT (CECT) safely and accurately exclude obstructing ureteral calculi?

When in doubt, light em’ up!
This was a retrospective cohort analysis of patients with acute flank pain who received CT abdomen/pelvis imaging. A total of 1286 CT scans were reviewed, and the prevalence of obstructive urolithiasis was 44.0% (351/797) in the NCCT group and 18.7% (86/459) in the CECT group. Based on 200 consecutive studies in each of the two groups, negative predictive values (NPV) were calculated based on follow up abdominal imaging within 7 days from original presentation. NPV for obstructive urolithiasis was 99.5% in the NCCT group and 100% in the CECT group. This study had some limitations including the retrospective design and use of re-presentation for repeat imaging to determine the NPV. However, it appears that CECT is just as good as NCCT for excluding obstructing urolithiasis and has the additional benefit of improved diagnostic accuracy for a variety of other conditions.

Furthermore, CECT has additional advantages compared to NCCT for diagnosis of ureteral obstruction including secondary signs of obstruction such as a delayed nephrogram and increased ureteral enhancement. What’s more, the American College of Radiology and the National Kidney Foundation recently released a consensus statement in 2020 stating the risk of acute renal injury after IV contrast administration is far lower than previously believed, and the available evidence is reassuring that significant acute kidney injury, death, and need for renal replacement therapy are rare after IV contrast administration. Of course, the study we covered yesterday is one of the most convincing yet that contrast does not cause long term kidney injury.

I don’t know about you, but based on this study, a contrast-enhanced CT will be my test of choice the next time I order a CT scan for a patient who presents with acute flank pain.

Can obstructive urolithiasis be safely excluded on contrast CT? A retrospective analysis of contrast-enhanced and noncontrast CT. Am J Emerg Med. 2021 Mar 22;47:70-73. doi: 10.1016/j.ajem.2021.03.059. Online ahead of print.