Ultrasound for kidney stone has always been confusing.  If we do the ultrasound and find no hydro, don’t we need the CT to rule an alternate diagnosis?  If we do the ultrasound and find hydro, don’t we need a CT to see if the stone would be too large to pass?  While these questions still haunt me, there are some cases where US might be enough.
1.  In a patient with a known history of kidney stones who presents with similar symptoms and you’re not worried about an alternative diagnosis. Consider getting and US and if there is not large hydro, high WBC, elevated Cr, consider discharge with urology follow up.
2. In younger and low-risk patients in whom you’re not worried about alternative diagnosis and appear well.  These are the patients who potentially suffer most from radiation exposure.
Remember to think about septic stones, appendicitis, AAA, diverticulitis, pyelo, etc especially if the patient appears toxic/febrile or has VS abnormalities.
A NEJM article by Smith-Bindman, R et al. Ultrasonography versus computed tomography for suspected nephrolithiasis. N Engl J Med. 2014 Sept 18;371(12):1100-10 of randomly assigned 2,759 patients with possible kidney stones showed  “Initial ultrasonography was associated with lower cumulative radiation exposure than initial CT, without significant differences in high-risk diagnoses with complications, serious adverse events, pain scores, return emergency department visits, or hospitalizations.”
Le’ts talk it through.