Ultrasound may be a viable alternative to CT scanning as an initial investigation for suspected nephrolithiasis, the findings from a study in the United States suggest.

A total of 2,759 patients aged 18-76, presenting to an emergency department with suspected renal colic were randomly assigned to either initial diagnostic ultrasound carried out by an emergency physician (908), ultrasound by a radiologist (893), or abdominal CT scan (985).

There were three primary outcome measures: high-risk diagnoses with complications that could be related to missed/delayed diagnosis (e.g. ruptured abdominal aortic aneurysm, appendicitis with rupture, diverticulitis with abscess formation etc), cumulative radiation exposure from imaging, and total costs (not reported in this paper).

No significant difference was seen between groups in terms of high-risk diagnoses with complications. There were six cases (0.7%) in the point of care ultrasound group, three (0.3%) in the other ultrasound group and two (0.2%) in the CT group (P = 0.30).

Cumulative radiation exposure over six months following presentation was significantly lower in the two ultrasound groups (10.1 mSv and 9.3 mSv respectively) compared with initial CT scanning (17.2 mSv; P < 0.001). There was also no difference seen in the numbers of serious adverse events between the groups: 12.4%, 10.8% and 11.2% respectively; P = 0.50).

Diagnostic accuracy was also assessed as a secondary outcome measure. Within six months of randomisation, the proportion of patients with a confirmed diagnosis of kidney stones was similar in the three groups – 34.5%, 31.2% and 32.7% respectively.

Overall, 40.7% of patients who underwent ultrasound by an emergency physician subsequently received a CT scan, compared with 27.0% of patients in the radiology ultrasound group. Only 5.1% of patients who initially had a CT scan had ultrasound later on.

At initial test, ultrasound had lower sensitivity and higher specificity than CT. The sensitivity was 54%, 57% and 88% for the three arms, and specificity 71%, 73% and 58% respectively.

This study, published in the New England Journal of Medicine, begins with the premise that ‘there is a lack of consensus about whether the initial imaging method for patients with suspected nephrolithiasis should be CT or ultrasound’.

Ultrasound is simple, safe, cheap and non-invasive but may lack sufficient accuracy to be a useful diagnostic test. CT, on the other hand, may be the gold standard in terms of diagnostic sensitivity and specificity, but can be difficult to arrange urgently, is significantly more expensive and involves a significant dose of radiation to the patient.

The study was conducted in an emergency department setting, so caution must be exercised in extrapolating the findings to a UK primary care population.

So, what test should a GP request when a patient presents with suspected renal colic?

Clearly, local radiology guidelines will reflect the preferences of the radiologists and will be different according to local expertise and availability of the imaging modalities.

British and European urology guidelines stress that CT is the definitive investigation, but that ultrasound is a reasonable alternative, particularly in thinner patients where good images can be obtained, both in terms of direct visualisation of the calculus and secondary signs such as dilation of the ureter which may suggest an obstructing stone.

This study emphasises that ultrasound appears to be a reasonable initial investigation, with the potential for reducing unnecessary radiation exposure in those who do not subsequently require CT scanning.

Dr Jonathan Rees

• Smith-Bindman R, Aubin C, Bailitz J et al. Ultrasonography versus computed tomography for suspected nephrolithiasis. N Engl J Med 2014;371:1100-10