Will a non-contrast computed tomography (CT) scan detect hydronephrosis in a patient with suspected renal colic and a history of kidney stones?

Non-Contrast CT Reliably Detects Hydronephrosis

Yes, a non-contrast CT scan will catch hydronephrosis with high sensitivity (88%) and specificity (85%), making it an excellent imaging modality for detecting this finding. 1

Diagnostic Performance of Non-Contrast CT

Non-contrast CT demonstrates robust performance characteristics for hydronephrosis detection:

• Sensitivity of 88.0% (84.2-91.1%) for identifying hydronephrosis associated with ureteric and obstructing renal pelvic calculi 1
• Specificity of 85.0% (80.9-88.5%) for the same conditions 1
• Positive predictive value of 85.7% and negative predictive value of 87.4% 1

The high negative predictive value is particularly clinically useful: when combined with absence of hematuria, the lack of hydronephrosis on non-contrast CT has a 96.4% negative predictive value for ruling out ureteral stones 1, 2

Clinical Context and Limitations

When Hydronephrosis is Present

• Any degree of hydronephrosis on imaging makes ureteral stone presence more likely (positive likelihood ratio +2.91) 1
• In one study, 49 of 50 patients with symptomatic stones demonstrated some degree of hydronephrosis 1
• Moderate to severe hydronephrosis provides definitive evidence of obstruction in patients with moderate or high risk of ureteric calculi 1

When Hydronephrosis is Absent

• Critical pitfall: Absence of hydronephrosis does NOT rule out stone disease, with a negative predictive value of only 65% when used alone 1
• Hydronephrosis may not yet be visible in the first 2 hours of presentation because secondary signs of obstruction take time to develop 2
• Some obstructing stones, particularly smaller ones, may not cause visible hydronephrosis initially 1

What Non-Contrast CT Actually Shows

Beyond detecting hydronephrosis itself, non-contrast CT provides comprehensive evaluation:

• Identifies the stone causing obstruction in the vast majority of cases 1, 3
• Detects secondary signs of obstruction including perinephric stranding and ureteral dilation in 76% of cases 4
• Reveals alternative diagnoses in 13-19% of patients, including vascular emergencies, malignancies, and gastrointestinal conditions 1, 5

When Non-Contrast CT is Insufficient

If non-contrast CT shows hydronephrosis but excludes obstructing stones, additional imaging is necessary to determine the cause:

• CT urography (CTU) with contrast is recommended as the next step if renal function is normal 6
• MR urography (MRU) is preferred over CTU in patients with renal impairment 1, 6
• Do not repeat ultrasound after CT has already been performed, as it will not provide additional etiologic information 6

Practical Algorithm

For suspected renal colic with concern for hydronephrosis:

1. Order non-contrast CT as the initial imaging study—it will reliably detect hydronephrosis if present 1, 3
2. If hydronephrosis is present with stone identified: Diagnosis confirmed; manage based on stone size, location, and clinical symptoms 1
3. If hydronephrosis is present without stone: Proceed to CTU (if normal renal function) or MRU (if impaired renal function) to identify non-calculous causes 6
4. If no hydronephrosis and no hematuria: Ureteral stone is highly unlikely (96.4% NPV) 1, 2

Non-contrast CT remains the reference standard for evaluating suspected urolithiasis and will detect hydronephrosis when present, though functional imaging may be needed to assess the degree of obstruction 4