What is the recommended diagnostic workup for suspected nephrolithiasis?

Diagnostic Workup for Suspected Nephrolithiasis

Non-contrast CT of the abdomen and pelvis is the gold standard imaging test for suspected nephrolithiasis, with 97% sensitivity and 95% specificity, and should be the first-line imaging modality in most patients. 1, 2

Primary Imaging Recommendation

Obtain non-contrast CT abdomen and pelvis as the initial imaging study for suspected kidney stones. 3, 1, 2

• Non-contrast CT detects virtually all kidney stones regardless of composition, including stones as small as 1 mm, without requiring IV contrast 1, 2
• CT provides accurate stone size measurements (critical for treatment planning), precise stone location, and identifies secondary signs of obstruction including hydronephrosis and periureteral inflammation 1, 2
• Use low-dose CT protocols (<3 mSv) which maintain the same 97% sensitivity while substantially reducing radiation exposure 1, 2
• CT imaging must include the pelvis to detect stones in the distal ureters and bladder 3, 1

When Ultrasound May Be Appropriate

Reserve ultrasound as first-line imaging only for specific populations where radiation avoidance is critical: 1

• Pregnant patients - ultrasound is the imaging tool of choice due to radiation concerns 1, 2
• Pediatric patients - to avoid radiation exposure 1
• Patients with renal impairment - to avoid nephrotoxic contrast agents (though non-contrast CT uses no contrast) 1

Critical Limitations of Ultrasound

• Ultrasound has poor sensitivity (24-57%) for directly detecting kidney stones compared to CT's 97% sensitivity 1
• Sensitivity for ureteral stones is even worse (45-61%), though specificity remains 100% 1
• Ultrasound significantly overestimates stone size in the 0-10 mm range, leading to inappropriate management recommendations in 22% of cases 4
• When ultrasound suggests observation, CT may reveal stones requiring intervention in 39% of cases 4

When Ultrasound Findings May Suffice

• In patients with moderate to severe hydronephrosis on ultrasound AND moderate or high clinical risk for ureteric calculi, ultrasound may provide sufficient diagnostic certainty without requiring CT 2
• However, absence of hydronephrosis on ultrasound does NOT rule out stones (negative predictive value only 65%) 3, 2
• If clinical suspicion remains high despite negative ultrasound, proceed to non-contrast CT 1

Imaging Modalities to Avoid for Initial Diagnosis

Do not use these imaging tests as first-line for suspected nephrolithiasis:

• Plain abdominal radiography (KUB) - detects only 8% of stones ≤5 mm and 78% of stones >5 mm; most useful for monitoring known stones, not acute diagnosis 1, 5
• Contrast-enhanced CT - the enhancing renal parenchyma may obscure stones within the collecting system, and it has lower sensitivity than non-contrast CT for small stones 1, 2
• MRI/MRU - poor accuracy for detecting small urothelial calculi; detected stones in only 50-79% of patients compared to 91-100% with CT 1
• Intravenous urography (IVU) - not beneficial for initial evaluation 3

CT Technical Specifications for Optimal Detection

Request the following CT parameters to maximize stone detection: 2

• Thin (1-1.5 mm) axial slice images preferred over thick (5 mm) slices 2
• Coronal reformations for accurate stone measurements 1, 2
• Bone window settings for optimal stone visualization 1, 2
• Magnified views to improve measurement accuracy 1, 2

Common Pitfalls to Avoid

• Do not rely on bladder distension protocols - stone visualization depends on the imaging modality's inherent sensitivity, not urinary tract opacification 1
• Do not assume a negative KUB excludes stones - KUB fails to detect 92% of stones ≤5 mm and 22% of stones >5 mm 1
• Do not order contrast-enhanced CT for stone detection - it adds unnecessary cost, radiation, contrast-related risks, and actually reduces sensitivity for small stones 1
• Do not use ultrasound alone to guide management decisions for residual or asymptomatic calculi - one in five patients may be inappropriately counseled when using ultrasound alone 4