Ultrasonography versus CT for Suspected Nephrolithiasis
Michael Macias
By Matthew O'Connor
In an oft-cited article from 1998 in the Journal of Clinical Urology, unenhanced helical CT began to be called the new gold-standard imaging modality in the diagnosis of nephrolithiasis. This study primarily evaluated the sensitivity and specificity of CT as compared with IVP, did not include ultrasonography, and did not evaluate potential harm. Over time, we have come to realize the risk associated with CT (particularly with multiple CT scans), with estimates of around 2% of all new cancers in the U.S. (some 29,000 cases) caused by these imaging studies.
Meanwhile, we are also learning more about the capabilities of ultrasound as a diagnostic tool. A recent study compares CT with ultrasonography as the initial imaging modality in suspected nephrolithiasis, focusing on high-risk diagnoses, radiation exposure, and adverse events in addition to sensitivity and specificity. With over 70 million CT scans performed each year, and the comparative safety and cost-effectiveness of ultrasound, it would seem that there's ample opportunity to improve outcomes, reduce unnecessary risk, and save money. And we are so coming to love our ultrasounds, are we not? Anyway, let’s get down to brass tacks.
Now onto the study
Smith-Bindman, Aubin, et al, "Ultrasonography versus Computed Tomography for Suspected Nephrolithiasis," NEJM, 371;12, Sep. 18, 2014.
Study Design: Multi-center, randomized comparative trial. (15 academic centers, 4 of which were safety-net hospitals)
Population: 2759 patients who presented between October 2011 and February 2013 with abdominal or flank pain in whom the treating physicians sought imaging to rule in or out nephrolithiasis.
Intervention Protocol: Assignment to one of three imaging modalities: point-of-care ultrasonography by EM physician, radiology ultrasonography, or CT. Intention-to-treat was followed, and any further imaging or intervention was left to the discretion of the treating physicians.
Outcome Measures:
-Primary: High-risk diagnosis with complication, and radiation exposure, measured over the entire course of the patients' care.
-Secondary: Serious adverse events, related serious adverse events, length of stay, return ed visits, hospital admission, pain score, and sensitivity/sensitivity for diagnosis.
Results: After 30 days, the primary groups showed no significant difference amongst themselves in terms of the high-risk diagnoses listed. Readmission rates, adverse events, and pain scores likewise showed no statistically significant differences between the three groups, while point-of-care ultrasonography showed statistically significantly lower radiation exposure and marginally lower cost (~$25 per patient).
Primary & Secondary Outcomes
Interpretation: Ultrasonography seems to have comparable patient outcomes and reduced radiation exposure when compared with CT as the initial imaging modality utilized in the diagnosis of nephrolithiasis.
Comments:
The patient population is well-described and the groups are comparable. The adverse outcomes were pre-defined and are described clearly. Less well-defined is "diagnostic accuracy." This seems to take into account the fact that some 40% of patients randomized to initial point-of-care ultrasound went on to get a CT (27% in the radiology-ultrasound group), while the raw sensitivity of point-of-care ultrasound was lowest (54%, vs 57% for radiology & 88% for CT), and specificity higher than CT (71% vs 73% vs 58% respectively). This may suggest US as a better rule-in, and perhaps the best go-to test if nephrolithiasis is strongly suspected.
The 1998 study took into account high-risk diagnoses ,other than nephrolithiasis, made using helical CT, and this is cited as a strong advantage over ultrasound for initial evaluation. This new study directly addresses this, with "high-risk diagnoses with potential complications" as a primary outcome along with radiation exposure and total costs. Maybe closer to our bottom line, rates of adverse events in general were comparable between groups.
One shortcoming is that the rate of high-risk diagnoses with complications was small--12 total out of the 2700 randomized. There is a possibility that this study is underpowered to detect a difference among these groups with regards to this outcome, leaving us with an unknown number needed to harm. Likewise, we still do not have good data on how dangerous an extra 7mSv per patient turns out to be over many thousands of scans. Nevertheless, this study may provide some of the best evidence yet that ED US over CT in suspected nephrolithiasis may be not only an acceptable, but the more responsible choice.
For more:
- EM: RAP Interview with the author
- Opinion piece on CTs co-authored by the first author
- Cases Based approach to new study at emDocs
- Study breakdown at EM Literature of Note
- How to perform renal US at Ultrasound Podcast
References:
-Smith-Bindman, Aubin, et al, "Ultrasonography versus Computed Tomography for Suspected Nephrolithiasis," NEJM,371;12, Sep. 18, 2014.
-Miller, Nicole; Lingeman, James; "Management of Kidney Stones," BMJ. 2007;334:468-Redberg, Smith-Bindman, "We Are Giving Ourselves Cancer," http://www.nytimes.com/2014/01/31/opinion/we-are-giving-ourselves-cancer.html? _r=0, New York Times, Opinion, Jan. 30, 2014.
-Schottenfeld, David; "Current Perspective on the Global and Unites States Cancer Burden Attributable to Lifestyle and Environmental Risk Factors," Annual Review of Public Health, Vol. 34, pgs 97-117, 2013
-Teichman, Joel, "Acute Renal Colic from Ureteral Calculus," NEJM 2004;350:684-693, Feb 12, 2004
-Viewweg, Teh, et. al, "Unenhanced helical Computerized Tomography for the Evalutation of Patients with Acute Flank Pain," The Journal of Urology, Sep. 1998