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During our GU board review this week, I got to thinking about kidney stones and the frustrating stepwise approach to the patient with equivocal flank pain and a working diagnosis of nephrolithiasis. Most cases are easy since renal colic is common and presents so characteristically, but there are some patients with equivocal presentations. These patients with mushy histories, maybe some mild flank pain, maybe not, possible dysuria, and subjective fevers present diagnostic challenges. Is this an episode of renal colic, renal colic with obstruction and superinfection, pyelonephritis, or something else in the back or belly? This is when the first groan happens because the realization is the that the patient is possibly going to need 2 CT scans: one without IV contrast and then another one with. But maybe that’s not necessary because pyelo can be seen on a non-contrasted CT right? Something about fat stranding? Maybe we can just get away with the single scan, and if we don’t see the stone, we’ll find something to hang a weak diagnosis of pyelo on.

A quick review of nephrolithiasis. The most general approach to these patients is pain control followed by urinalysis. As was mentioned at conference, pain control is best achieved with combination therapy using an NSAID such as ketorolac and narcotic such as morphine (PMID: 16953530). If the clinical presentation is correct and urinalysis shows blood, imaging is not necessary. Here’s the first problem: the “classic” finding of hematuria can be absent up to 15% of the time (PMID: 7747369).

In cases which the diagnosis is not as clear, non-contrasted CT scan of the abdomen is indicated and proves to be a very good test for nearly all renal stones except for those secondary to HIV protease inhibitors such as indinavir (PMID: 9230000). So that’s great because unless we are dealing with that specific situation, the non-contrasted CT scan should certainly find the stone. But invariably that sometimes doesn’t happen, which brings up the second problem: what now?

At this point the arrow starts to point further down the differential and a diagnosis of pyelonephritis comes to mind given the patient’s bloody urine, positive leukocyte esterase, and mild flank tenderness. While pyelo is a clinical diagnosis supported by a characteristic urine, often muddled histories and unconvincing exams can leave the diagnosis in doubt. Non-contrasted CT scans can show an enlarged kidney or perinephric fat stranding indicative of a pyelo, but they can also be normal in the setting of pyelo. Thus in the patient with an unclear diagnosis based on urine and history and non-diagnostic CT scan, a contrasted CT scan should be pursued as the next step(PMID: 16937102, 15486235). Besides giving a much better image of the kidney in cases where the diagnosis is in doubt, a contrasted scan will highlight vascular diseases or renal infarcts whose presentations mimic that of renal colic and would not be seen on a non-contrasted scan. These are rare entities but can be missed easily. Bottom line: slog through getting 2 CT scans when kidney stones are not seen on the initial non-contrasted CT and the clinical picture is not a slam dunk for pyelo - the correct imaging for pyelo should be with IV contrast.