Gold Standard for Diagnosing Nephrolithiasis
Non-contrast CT of the abdomen and pelvis is the gold standard for diagnosing nephrolithiasis, with a sensitivity of 97% and specificity of 95-100%. 1, 2, 3
Why Non-Contrast CT is Superior
Non-contrast CT directly visualizes stones throughout the entire urinary tract from the kidney to the bladder, regardless of stone composition, size (down to 1-2 mm), or location 1, 4, 2
CT detects secondary signs of obstruction including hydronephrosis, periureteral inflammation, perinephric fluid, and ureteral dilation that confirm clinical significance 1, 2, 3
Low-dose CT protocols (<3 mSv) maintain 97% sensitivity while reducing radiation exposure to levels comparable to or lower than traditional intravenous urography 2, 3
CT outperforms all other modalities: Studies comparing CT to intravenous urography (IVU) showed CT sensitivity/specificity of 96%/100% versus IVU's 87%/94% 1
Alternative Imaging When CT is Not Appropriate
Ultrasound should be first-line only in pregnant patients and children to avoid radiation exposure 2, 3:
Ultrasound has poor sensitivity (24-57%) for direct stone visualization but excellent sensitivity (up to 100%) for detecting hydronephrosis as an indirect sign of obstruction 1, 4, 3
Within the first 2 hours of symptom onset, secondary signs may not have developed, making ultrasound even less reliable 1, 4
Combining ultrasound with plain radiography (KUB) improves sensitivity to 79-90% when CT cannot be used 2, 3
Ultrasound is also appropriate for patients with renal impairment to avoid nephrotoxic contrast agents, though non-contrast CT requires no contrast 4
Inferior Imaging Modalities
Plain radiography (KUB) alone has only 29% overall sensitivity, detecting only 72% of large stones (>5 mm) in the proximal ureter and missing most small stones entirely 1, 3
Intravenous urography (IVU), once considered the reference standard, is now obsolete with sensitivity of only 75-87% compared to CT's 96-97% 1, 3
MRI has limited utility for stone detection (50% sensitivity for direct visualization) and should only be considered when both CT and ultrasound are contraindicated 1, 3
Common Pitfalls to Avoid
Do not order contrast-enhanced CT as first-line imaging because enhancing renal parenchyma obscures small stones within the collecting system 2, 3
Do not rely solely on ultrasound for detecting small stones (<5 mm) as sensitivity decreases dramatically with decreasing stone size 4, 3
Beware of motion artifact on CT which can cause warping distortion making stones appear larger than actual size; correlate with KUB or ultrasound when planning surgical intervention 5
Ultra-low-dose CT protocols (10% of standard dose) may have unacceptably high false-positive and false-negative rates due to impaired soft tissue resolution, making accurate placement of calcifications difficult 6