"It's actually quite surprising that ultrasound is just as good as CT scanning when you look at patient outcomes," said Dr. Rebecca Smith-Bindman of the University of California, San Francisco, chief author of the report in the New England Journal of Medicine.

Dr. Charles D Scales, Jr. of Duke University Medical Center in Durham, North Carolina, called it ‘a really provocative study’ adding, “it should make doctors and patients think about what we do when a kidney stone may be causing a patient's pain.”

"It doesn't necessarily say patients should not get a CT scan," said Scales, who was not connected with the research, "but I think the main message is that an ultrasound is the best place to start."

Kidney stones account for nearly a million emergency room visits in US each year at a cost of nearly a billion dollars. One in 11 Americans says they have had one.

For years, an abdominal CT scan has been the standard method for detecting stones because it makes the stones easier to see than regular X-rays. But other calcium deposits in the body can be mistaken for stones, leading to unnecessary treatment.

"The CT scan has become the standard of care without really any evidence to support it. So it was initially difficult to get funding for this project because people said, 'You can't use ultrasound. That's crazy,'" Smith-Bindman said.

In the USD 9 million three-year study, 2,759 adult patients at 15 emergency departments who were suspected of having a kidney stone were randomly assigned to ultrasound performed by an emergency physician or a radiologist, or to a conventional CT scan.

Beyond finding kidney stones themselves, one reason doctors might be reluctant to give up CT scanning for a suspected kidney stone is the fear that ultrasound might miss a serious problems, such as appendicitis or a ballooning blood vessel, that a CT scan can pick up.

"So we came up with a list of bad things we didn't want to miss," said Smith Bindman.

Only 11 patients were found to have a high-risk diagnosis with complications; six where the emergency room doctor performed ultrasound, three where a radiologist performed ultrasound and two in the CT group, an insignificant difference among thousands of patients.

The odds of the patient returning to the emergency department or being admitted to the hospital within six months of their initial visit were not significantly affected by whether the patient initially received ultrasound or a CT scan.

Overall, the researchers calculated that kidney stones would be correctly detected in 54 percent of ultrasounds done in the emergency room, in 57 percent of the ultrasounds done by a radiologist and in 88 percent of the CT scans.

However, they found that the CT scans falsely indicated a stone when there wasn't one in 42 percent of the cases, compared with rates of 27-29 percent with ultrasounds.

And the ultrasound patients received only about 60 percent of the radiation that CT patients did. As a group, the ultrasound patients received as much radiation as they did because many doctors who were supposed to try ultrasound first subsequently ordered a CT scan nonetheless, just to be sure.

"It may be a challenge for physicians to get comfortable with this," said Scales.

"Among the patients in this study who started with an ultrasound, two out of five ended up getting a CT scan at the doctor's request anyway," he added.

Smith-Bindman said that as the experiment progressed, "There was a real shift among both physicians and patients in preferring ultrasound, so I'm hoping the study will persuade them that ultrasound is a better way to start," she said.

She said the study carries an important lesson for patients, who have a higher risk of developing pain from a subsequent stone after they've had an initial visit to an emergency department.

"If you have symptoms of a kidney stone like back pain, or flank pain, or groin pain going to the scrotum, and you're going to the emergency department, talk to the doctor about what they think may be going on with it," Smith-Bindman said.

"If kidney stones are on the list, ask if CT scanning is necessary or ultrasound might be a way to start," she added.

She said patients who got ultrasound got to leave the emergency department sooner and that may encourage doctors to use the technique because it helps reduce waiting times for patients.

The costs for patients who got ultrasound first "was a little less, but there wasn't a huge cost difference," she said.

"One of the fears when we started was that if we start with ultrasound and then we do CT that will be more expensive because patients will be getting two tests instead of the fancier test to begin with. In fact, that was not the case. With most patients you could just start with ultrasound and end with ultrasound," she added.

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