Ultrasound First for Kidney Stones: What the STONE Trial Showed

The STONE trial, published in the New England Journal of Medicine in 2014, asked whether emergency clinicians could begin the workup of a suspected kidney stone with ultrasound instead of going straight to CT. Across 15 U.S. emergency departments and 2,759 patients, starting with ultrasound matched CT on the outcomes that matter most: serious complications from a missed or delayed diagnosis, pain scores, return emergency visits, and hospital admissions were statistically indistinguishable between the arms. The groups that started with ultrasound accumulated meaningfully less radiation over the following six months, about 10 mSv against roughly 17 mSv. The catch sits in one word, "first," because ultrasound first was a starting point rather than a ban on CT, and its lower sensitivity for actually seeing a stone means it does not fit every patient.

This is educational content, not medical advice.

What the trial actually tested

The Study of Tomography Of Nephrolithiasis Evaluation, or STONE, was a pragmatic comparative-effectiveness trial led by Rebecca Smith-Bindman and colleagues. Adults aged 18 to 76 who arrived at the emergency department with symptoms suggesting a kidney stone were randomized to one of three initial imaging strategies: point-of-care ultrasound performed by the emergency physician, formal ultrasound performed in the radiology department, or abdominal CT. The design point that is easy to miss is that this compared whole strategies, not individual scans. Clinicians in the ultrasound arms stayed free to order CT afterward if the ultrasound was unrevealing or the clinical picture demanded it. The question was never "which scan sees stones better." It was whether opening with the lower-dose test leads to worse patient outcomes.

What it found

On the primary safety outcome, high-risk diagnoses with complications that could plausibly trace back to a missed or delayed diagnosis within 30 days, the trial found no meaningful difference. Eleven patients, about 0.4 percent of the whole cohort, had such events, and they were spread evenly across the arms. Serious adverse events occurred in roughly 11 to 12 percent of patients in each group, a spread that did not reach statistical significance, and pain scores, return emergency visits, and hospitalizations tracked together as well. Where the arms genuinely diverged was radiation. Cumulative exposure over the next six months averaged about 10 mSv in the two ultrasound groups versus roughly 17 mSv with initial CT, a gap the authors reported as highly significant. Beginning with ultrasound, in plain terms, bought a real reduction in radiation without buying a measurable increase in harm.

Why "first" carries the whole result

Ultrasound is a less sensitive test for a stone than CT, and STONE did not hide it. Within the trial, diagnostic sensitivity for a stone was about 54 percent for point-of-care ultrasound and 57 percent for radiology ultrasound, against 88 percent for CT. If the trial had forced an ultrasound-only pathway, those numbers would predict missed stones and missed alternative diagnoses. It did no such thing. About 40 percent of the point-of-care ultrasound patients and about 27 percent of the radiology ultrasound patients went on to CT anyway. The strategy worked precisely because it was a triage step: ultrasound answered the question in many patients, and the clinician escalated to CT in the rest. Reading STONE as "ultrasound replaces CT" inverts what it showed. It showed that ultrasound is a reasonable place to start, with CT held in reserve.

Where the lower-dose pathway runs out

A pragmatic trial earns its conclusions inside the population it enrolled, and that is exactly where the limits live. The patients were selected as probable stones, skewing younger and lower risk for the dangerous mimics, so the low rate of catastrophic missed diagnoses partly reflects who walked through the door. When the differential includes appendicitis, a leaking abdominal aortic aneurysm, bowel pathology, or another surgical emergency, ultrasound-first logic weakens, and CT's sensitivity for those conditions becomes the whole point. Ultrasound also measures stone size and location less reliably, which matters when a urologist is choosing between watchful waiting and intervention. Body habitus degrades the images. A single prior normal ultrasound does not settle a patient with worsening pain or fever. The comparison also predates the broader adoption of ultra-low-dose CT protocols, which shrink the very radiation gap that made an ultrasound-first approach so appealing. The trial supports starting low-dose, not staying there when the picture changes.

Reading the result without overreading it

STONE has aged into guideline territory. Professional bodies now describe ultrasound as a reasonable first imaging test for many patients with suspected stones, with CT reserved for diagnostic uncertainty, larger or complex stones, or surgical planning. That is a modest, durable message, and it is a clean example of how a well-built pragmatic trial changes practice. It changed practice by validating a sequence that lowers cumulative harm while keeping an escape hatch open, rather than by crowning one technology. For a patient, the practical translation is a question, not a demand. Asking whether ultrasound is a reasonable place to begin is fair. Insisting on it regardless of the clinical scenario is not, because the same evidence that endorses ultrasound first also documents when CT is the right tool.