Ultrasound vs. Computed Tomography for Severity of Hydronephrosis and Its Importance in Renal Colic

Matthew Wood, MS4

The article: Leo et al. “Ultrasound vs. Computed Tomography for Severity of Hydronephrosis and Its Importance in Renal Colic.” Western Journal of Emergency Medicine; June 2017: Vol 18: 559-568

The Idea: CT has been considered the gold standard imaging standard for diagnosis of renal colic with sensitivities from 91-97% and specificities of 91-100% for diagnosing kidney stones. Many patients who suffer from renal colic and nephrolithiasis do so recurrently, having multiple episodes throughout their lifetime. Using CT to diagnose during each instance leads to increased costs, increased length of stay, and increased radiation exposure. While CT may have benefits over ultrasound such as being able to accurately determine size and location of ureteral stones, using a surrogate finding such as hydronephrosis that is easily determined by US may give insight into the presence of a renal calculi and subsequent dilation of the renal pelvis and calyces. US has been shown to have sensitivities ranging from 72-87% and specificities between 73-83% in the detection of hydronephrosis when compared to CT. The goal of this study was to determine if emergency physician performed US can detect severity of hydronephrosis in ED patients with suspected renal colic when compared to CT.

The Study. This was a prospective, observational cohort study of a convenience sample of ED patients with suspected renal colic from November 2010 to March 2014 at Boston Medical Center.

Design: After consent was obtained, an investigator (blinded to CT results) performed an US on the patient with suspected renal colic that had an abdomen and pelvis CT w/o contrast ordered. Long and short axis US views were obtained. US was used to determine presence of hydronephrosis and was graded as none, mild, moderate, or severe. Each patient also received an abdominal CT, findings recorded were hydronephrosis and/or hydroureter (none, mild, moderate, severe); renal stone location and size; and any additional pathological findings. Results of US and CT findings were compared as described below. Medical records were reviewed for disposition, hospital discharge diagnosis and return events to assess predictors of 30 day morbidity. If there was no return outcome in 30 days, patients had a follow-up phone call.

Inclusion criteria: age >21; CT of the abdomen and pelvis without contrast ordered; and ability to provide a telephone number for 30-day follow-up.

Exclusion criteria: prisoners, non-English speaking patients and those unable to provide informed consent (defined as medically unstable, those who had dementia, altered mental status, or deemed mentally incompetent by the treating physician). Participants were excluded if an US could not be completed prior to discharge from the ED.

Primary goals: 

  • Comparing EP-performed US with CT in identifying the severity of hydronephrosis (none, mild, moderate, severe),

  • Assess the ability of US to correctly classify those patients with and without hydronephrosis, using CT as the criterion standard.

  • Calculate the diagnostic test characteristics (sensitivity, specificity, positive predictive value [LR+), and negative predictive value, [LR-]) of any degree of hydronephrosis on EP-performed US for the presence of any ureteral stone or ureteral stone size > 5mm on CT.

Secondary endpoints:

  • Assessing predictors of 30- day events in participants with confirmed renal colic

    • Tracked outcomes of admitted to the hospital on the day of enrollment due to renal colic; or return visit for pain, infection, the need for a urologic procedure, or hospital admission related to renal colic

Results: 297 patients were analyzed.

  • 166 (55%) had a diagnosis of renal colic based on our study definition and 136 had an alternate diagnosis by CT

  • EP-performed US can detect the severity of hydronephrosis when compared to CT as the gold standard, (chi-square p<0.001)

  • The detection of any hydronephrosis on EP-performed US had a sensitivity of 85%, a specificity of 71% of detecting a renal stone of any size

  • For the presence of a ureteral stone >5mm on CT, the detection of any hydronephrosis by EP- performed US had a sensitivity of 86%, a specificity of 37%

  • Ultrasound under-classified the severity in 9% of participants, over-classifying in 13%, and correctly classifying in 78%. The majority of misclassified degrees of hydronephrosis by US were off by one degree of severity

  • Any degree of hydronephrosis on EP US makes the presence of a ureteral stone on CT more likely (PPV 88%)

  • Absence of hydronephrosis on EP US is good for ruling out the presence of stones > 5mm (NPV 88.5%, LR- 0.39) and may reassure the provider that a large stone is not present.

  • Found a significant association only for the presence of a ureteral stone > 5mm. Renal stones > 5mm had an OR of 2.30 for a 30-day event compared to smaller stones, 20 out of 40 vs. 33 out of 109 (95% CI [1.10, 4.84]; p=0.03).

The takeaway: US is a reasonable first line screening tool for suspected renal colic. Emergency physician performed US can reliably identify the severity of hydronephrosis when compared to CT as the criterion standard. Any degree of hydronephrosis on US is a good predictor for the presence of a ureteral stone on CT. Absence of hydronephrosis is good for ruling out the presence of stones >5mm. It is unclear if severity of hydronephrosis provides predictive information, the study not find that any degree of hydronephrosis or presence of moderate/severe hydronephrosis were predictive of a 30-day event. Only found that a stone >5mm on CT was predictive. Limitations include that this study was done at a single center, and cannot necessarily be generalized. Inclusion criteria included patients who had/were to get a CT abdomen, which may have introduced selection bias by excluding patients who had no imaging.