Author: Dr. Tucker Matthews 

We see a whole lot of flank pain in the ED, and more and more, we go for non-contrast CT scans (19% of acute flank pain encounters in 2000 had NCCTs, whereas 45% did in 2008). And these NCCTs are pretty damn effective at detecting urolithiasis. Unfortunately, however, a lot of these patients presenting with acute flank pain don’t actually have stones. Like, 54-67% of them according to some studies. 

It’s actually kind of difficult to figure out how necessary IV contrast is for detection of non-stone pathologies, but there’s definitely some benefit, and contrast also can actually better evaluate for secondary signs of ureteral obstruction. This is in opposition to the traditional way of thinking that non-con scans are more effective at detecting stones. 

Which leads us to this study’s question: 

How effective is contrast-enhanced CT (CECT) versus non-contrast CT (NCCT) at ruling out obstructive urolithiasis? 

The way they chose to investigate that is through a retrospective cohort analysis, where they looked at patients who presented with acute flank pain and received a CT scan, and then looked to see if they received follow up imaging. If they didn’t get follow up imaging, it was deemed a true negative original scan. 

Arguably the best thing about this paper is how ridiculously simple the results are. They only found 1 false negative out of all 400 of the patients, and it actually happened to be a patient who had a NCCT. And importantly, this wasn’t because of disease scarcity; they actually found decently high levels of obstructive urolithiasis (44.0% in the NCCT group and 18.7% in the CECT group). 

So what does this all mean? 

To me, it means a number of things. 

First: we should stop just reflexing to a NCCT in these patients we thing have a kidney stone. There’s really no benefit to detecting a stone from a treatment perspective. All we do is give some toradol and try and discharge them. Instead, what we really want to investigate is twofold: if they have a stone, do they have either infection or obstruction (both of which are evaluated with other means); or second, is there actually something else going on that we don’t want to miss (i.e. AAA or diverticulitis or appendicitis; all of which would probably benefit from some of that good ol’ contrast). 

Second—and what could be the subject of an entire separate blog post/presentation—we should stop caring about contrast damaging the kidneys. This isn’t really a thing (except maybe in specific and relatively rare instance), and in the cases where we feel the need to get a CT, that should almost by definition be high enough necessity to say that the benefits of the contrast outweigh the risks of the kidney damage. 

Third: we should not be getting multiple abdominal CT scans. That’s too much radiation. 

Fourth: nonetheless, it isn’t totally clear how much we need contrast to reliably rule out those other bad things. So we should really study that with some high quality research, especially now that our CT scanners are getting better and better. Because maybe we can just broadly cut down on our contrast overall and this will become an unnecessary discussion. 

So my takeaway is that I’ll probably start trying to press for contrast in my scans more and more. But I also think it’s important to really think about what we’re looking for in the scan. It’s extremely lazy (and spoiled) medicine to get CT scans on all patients we think might have a kidney stone, just so someone can tell us they have a stone. So I think it’s time we (as residents) start pressing our attendings on why they keep ordering these scans, and if they want to order them, why or why not to use contrast enhancement. 

Ultimate one-liner: Contrast-enhanced CT scans can safely exclude obstructive urolithiasis just as well as noncontrast CTs. 

Citation: Lei B, Harfouch N, Scheiner J, Demissie S, Hayim M. Can obstructive urolithiasis be safely excluded on contrast CT? A retrospective analysis of contrast-enhanced and noncontrast CT. Am J Emerg Med. 2021 Sep;47:70-73. doi: 10.1016/j.ajem.2021.03.059. Epub 2021 Mar 22. PMID: 33774453.

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