Diagnostic Performance of Non-Contrast CT for Hydronephrosis

Sensitivity and Specificity

The American College of Radiology recommends non-contrast CT for detecting hydronephrosis, with a sensitivity of 88.0% (84.2-91.1%) and specificity of 85.0% (80.9-88.5%) for identifying hydronephrosis associated with ureteric and obstructing renal pelvic calculi 1
Non-contrast CT has a positive predictive value of 85.7% and negative predictive value of 87.4% for detecting hydronephrosis 1

The presence of any degree of hydronephrosis on imaging makes ureteral stone presence more likely, with a positive likelihood ratio of +2.91 1
Moderate to severe hydronephrosis provides definitive evidence of obstruction in patients with moderate or high risk of ureteric calculi 1
The absence of hydronephrosis does not rule out stone disease, with a negative predictive value of only 65% when used alone 1

The American College of Radiology suggests that non-contrast CT is the initial imaging study of choice for suspected renal colic with concern for hydronephrosis, and will reliably detect hydronephrosis if present 1
If hydronephrosis is present with stone identified, diagnosis is confirmed, and management is based on stone size, location, and clinical symptoms 1
If no hydronephrosis and no hematuria are present, ureteral stone is highly unlikely, with a negative predictive value of 96.4% 1

Non-contrast CT may reveal alternative diagnoses, including vascular emergencies, malignancies, and gastrointestinal conditions, in 13-19% of patients 1

If non-contrast CT shows hydronephrosis but excludes obstructing stones, additional imaging such as MR urography (MRU) is preferred over CT urography (CTU) in patients with renal impairment 2

The absence of hydronefrosis on imaging studies makes the presence of a symptomatic obstructive calculus less likely, with a sensitivity of 88% and specificity of 85% for predicting ureteral obstruction, according to the American College of Radiology 3, 4
The combined negative predictive value of absence of hydronefrosis and absence of hematuria is 96.4% for ruling out obstructive ureteral calculus, as recommended by the American College of Radiology 3, 4

Small calculi (<5mm) have a lower probability of causing significant obstruction and severe symptoms, as reported by the American College of Radiology 3, 5

The absence of hydronefrosis in the first 2 hours of presentation can be falsely negative because secondary signs of obstruction may not have developed yet, according to the American College of Radiology 6

The European Urology guideline notes that symptoms develop in 7-77% of cases of small non-obstructing renal stones, even without obstruction 7
The American College of Radiology recommends non-contrast CT as the reference standard for evaluating urolithiasis, with a sensitivity as high as 97% 8
The American College of Radiology suggests that ultrasound may be used as an initial imaging modality, but has lower sensitivity (24-57%) for stone detection compared to CT, and the absence of hydronephrosis on ultrasound makes the presence of a larger obstructing stone (>5mm) less likely but does not rule out symptomatic non-obstructing stones 9