Use of and Regional Variation in Initial CT Imaging for Kidney Stones

Discussion

In this cross-sectional study that included clinics and hospitals throughout the United States, CT was the most frequently ordered initial imaging study for children during a kidney stone episode. On average, >60% of children had CT as the initial imaging study during a kidney stone episode. In some states, nearly 80% of children underwent CT as the first or only imaging study. Girls, in whom abdominal and pelvic radiation has a higher lifetime attributable risk of cancer than boys, were more likely to have CT.¹⁶ Only 24% of children with nephrolithiasis underwent ultrasound as the initial study. The high prevalence of initial CT and low prevalence of ultrasound indicate that current imaging practices deviate substantially from recently published guidelines that recommend ultrasound as the first imaging study for children with suspected nephrolithiasis.

The rationale for using ultrasound as the initial screening modality for children with suspected nephrolithiasis is that ultrasound accurately localizes most clinically important stones in children^17,18 and CT can be reserved for children with a nondiagnostic ultrasound for whom the clinical suspicion for stones remains high. Passerotti et al¹⁸ observed that the sensitivity and specificity of ultrasound for 50 children with suspected nephrolithiasis were 70% and 100%, respectively, when compared with CT. Of the stones not visualized on ultrasound, 25% were in the ureter and the remainder were nonobstructive stones in the kidney. Prompted by the good sensitivity of ultrasound in detecting clinically significant stones, the radiation risk associated with CT, and the low diagnostic yield for alternative diagnoses,¹⁹ the American Urological Association in 2013 and the European Society for Pediatric Radiology in 2009 developed imaging protocols for children with suspected nephrolithiasis that recommend CT only if an initial ultrasound is nondiagnostic.^5,6 The Image Gently Alliance^7,8 also recommends limiting radiation when possible and promotes alternative imaging modalities such as ultrasound for children who need diagnostic imaging.

Ideally, this study should be conducted in children in whom nephrolithiasis is suspected before obtaining imaging rather than those found to have stones after imaging. Unfortunately, we cannot identify these patients with the use of claims data. Given that use of CT in the ED for abdominal pain, which is the most common presenting symptom for kidney stones,²⁰ also varies substantially by region,¹¹ it is unlikely that the imaging patterns we identified would differ significantly among children in whom nephrolithiasis is suspected but not confirmed. However, because CTs in this study may have been obtained to evaluate for kidney stones or other conditions on the differential diagnosis (eg, ovarian torsion, ureteropelvic junction obstruction), the diagnostic yield of CT to identify kidney stones and alternative diagnoses in children with abdominal and/or flank pain should be considered. Persaud et al²⁰ reported that most children (72%; 244 of 339) who underwent helical CT for suspected nephrolithiasis did not have stones. Furthermore, few (9%) of 244 children without stones had clinically important alternative diagnoses identified by CT. Eshed and Witzling¹⁹ also evaluated the diagnoses of children with abdominal pain and suspicion for ureteral stones who underwent CT. In 20 children suspected to have nephrolithiasis, 3 were found to have stones as the source of the pain and CT confirmed an alternative diagnosis in only 4 more. Of note, the alternative diagnoses were lobar nephronia, ureteropelvic junction obstruction, ovarian torsion, and dermoid cyst, all of which can be identified by using ultrasound.

The public health implications of the high prevalence of initial CT use are heightened by the rising incidence of kidney stones among children. Over the past 25 years, the incidence of nephrolithiasis among adolescents has increased by 6% to 10% annually.^21,22 The potential harms of radiation are further increased for patients with nephrolithiasis because multiple imaging studies are often performed for the same kidney stone episode. Alarmingly, Routh and co-workers demonstrated that 79% of children had ≥2 CTs for each stone episode treated at PHIS hospitals, with some children undergoing 8 CTs.⁹ Furthermore, some patients in whom nephrolithiasis begins in childhood will receive additional imaging for recurrent stones and will be exposed to radiation during surgical treatment, both of which increase lifetime radiation doses.^9,23,24

We observed substantial geographic variability in initial CT utilization across the United States. Midwestern and southern states had the highest initial CT utilization, and the northeast states had the lowest initial CT utilization. However, even in states where initial CT use was relatively low, >40% of children underwent CT alone or CT before ultrasound. The observed regional variation was not due to differences in patient characteristics or temporal trends in CT utilization because adjustment for age, gender, year of presentation, and type of insurance resulted in very small and statistically insignificant changes in the association between geographic location and CT utilization. We also observed heterogeneity of initial CT utilization within census divisions. For example, within the Pacific division, California had relatively low CT utilization whereas 77% of children living in Washington underwent CT as the first imaging study. Generally, however, there was greater variation in CT utilization across versus within divisions. These results may help focus efforts to disseminate and implement imaging guidelines in areas of the country with high rates of initial CT use.

Variability in initial CT utilization is analogous to observed regional differences in per capita Medicare spending,^25,26 which has been attributed to geographic differences in physician evaluation and management practices. Similar to variations in diabetes care among adults,²⁷ we hypothesize that CT use to evaluate children with nephrolithiasis differs across the country due to engrained practice patterns or resources specific to local medical communities. Ultrasound may be less available in certain areas and some practitioners may be less comfortable with ultrasound evaluation given its operator and interpreter dependence. Additionally, it is possible that different specialists involved in the management of children with nephrolithiasis (eg, ED physicians versus pediatric urologists) may be more or less likely to order CT as the initial imaging study. Accordingly, geographic differences in the prevalence of specialists across the country may contribute to the variability in initial CT utilization we observed. An ancillary hypothesis is that the rapidly changing epidemiology of nephrolithiasis^22,28,29 has contributed to wide variations in practices. Because CT is traditionally used to evaluate adults with suspected nephrolithiasis, many physicians may not be aware that ultrasound can be used to effectively diagnose kidney stones in children, who are now presenting with nephrolithiasis with increasing frequency.

Clinical guidelines have been shown to reduce variability and improve processes of care as well as outcomes.³⁰ Future studies should assess awareness of these guidelines and determine the physician, institutional, and community characteristics that serve as barriers to implementation of recommended imaging practices. Additionally, interventions to decrease initial CT use should be developed. Miller et al³¹ established a collaborative network of urology practices to improve compliance with national guidelines on diagnostic imaging use for men with prostate cancer. They observed that providing data back to urologists on their imaging practices reduced inappropriate imaging utilization.³² Additionally, the Centers for Medicare and Medicaid Services Physician Quality Reporting System uses quality of care measures related to overuse of imaging in staging men with localized prostate cancer. Similar measures for overuse of CT imaging in children with nephrolithiasis could be developed and used by private payors, perhaps in the setting of hospitals participating in accountable care organizations established by these payors. Finally, systems that integrate clinical decision support into computerized order entry systems to provide evidence for or against imaging studies at the time of order entry may facilitate implementation of these measures.

We acknowledge the limitations to this study. First, we used claims dates to determine the order of imaging and claims. Misclassification of study order may exist because claims are not necessarily made on the day of service and 16% of imaging studies in this study had the same claims date. However, to mitigate bias, we classified all ultrasounds with the same claim date as a CT, IVP, or KUB as the first imaging study performed. It is also unlikely that any discrepancy between the claims date and date of service would alter our results given that ultrasound would rarely, if ever, be performed after CT for kidney stones. Second, as described previously, we were not able to determine imaging utilization for children for whom nephrolithiasis was suspected but who were ultimately found not to have stones. Third, we were not able to differentiate recurrent stones from incident stones. It is possible that ultrasound may be used more often as a screening study for children with a history of nephrolithiasis. However, this possibility should not affect our results unless recurrence rates differ by geographic region. Fourth, we were unable to measure race and socioeconomic level, which have been associated with the probability of nephrolithiasis and thus could influence the choice of initial imaging. However, it is unlikely that any association between race and socioeconomic level and CT utilization could explain the large geographic variability we observed. Finally, although MarketScan contains data from all 50 states, it is a sample of the population with commercial insurance and does not represent the overall population. Our results should not be extrapolated to uninsured or Medicaid/Medicare-insured subjects.