What is the recommended work-up for a patient with suspected kidney stones?

Kidney Stone Work-Up

For suspected kidney stones, obtain a low-dose non-contrast CT of the abdomen and pelvis as the first-line imaging study, combined with immediate laboratory testing including urinalysis (dipstick), serum creatinine, uric acid, ionized calcium, sodium, potassium, complete blood count, and C-reactive protein. 1

Initial Imaging Approach

Low-dose non-contrast CT is the gold standard imaging modality with 97% sensitivity and 95% specificity, providing precise stone size measurement, exact anatomical location, and detection of secondary signs like hydronephrosis and perinephric stranding. 1, 2, 3 This imaging modality detects virtually all stone types regardless of composition and is critical for determining whether conservative management versus intervention is appropriate. 1, 2

Radiation Dose Considerations

• Use low-dose CT protocols (≤3 mSv) rather than conventional dosing, which maintains diagnostic accuracy (93.1% sensitivity, 96.6% specificity) while minimizing radiation exposure. 1, 2
• The radiation reduction does not compromise clinical decision-making for stone detection and management. 1

Alternative Imaging in Special Populations

For pregnant patients, ultrasound is the mandatory first-line imaging modality, with MRI without contrast as second-line if ultrasound is inconclusive, and low-dose CT only as a last resort. 1

For pediatric patients, start with ultrasound, followed by KUB radiography or low-dose non-contrast CT only if ultrasound fails to provide necessary diagnostic information. 1

When Ultrasound May Be Considered

• Ultrasound alone has limited sensitivity (24-57%) for direct stone visualization but excellent sensitivity (up to 100%) for detecting obstruction through hydronephrosis. 2, 3
• Combining ultrasound with KUB radiography improves sensitivity to 79-90% and provides an acceptable alternative to CT for select patients when radiation exposure is a concern. 1, 2, 3
• However, ultrasound significantly overestimates stone size in the 0-10 mm range and may lead to inappropriate management recommendations in 22% of cases. 4

Laboratory Work-Up

Immediate Testing (All Patients)

Every patient presenting with suspected kidney stones requires:

• Urinalysis (dipstick) to detect hematuria, infection, and pH. 1
• Serum creatinine to assess renal function. 1
• Uric acid, ionized calcium, sodium, potassium for metabolic assessment. 1
• Complete blood count and C-reactive protein to evaluate for infection/sepsis. 1

When to Omit Certain Tests

If no intervention is planned, measurement of sodium, potassium, CRP, and coagulation studies can be omitted. 1

Stone Analysis

Perform stone analysis on all first-time stone formers (ideally on all stone formers) to guide prevention strategies. 1 Stone composition determines specific metabolic work-up and prevention protocols.

Extended Metabolic Evaluation (High-Risk Patients Only)

Perform comprehensive 24-hour urine collection and additional metabolic testing for patients with:

• Recurrent stones (≥2 episodes). 1
• Bilateral stone disease. 1
• Strong family history of stones. 1
• Solitary kidney. 1
• Inherited or metabolic disorders. 1

Critical Red Flags Requiring Immediate Evaluation

Urgent decompression via percutaneous nephrostomy or ureteral stenting is mandatory for:

• Sepsis with obstructed kidney (fever + obstruction). 1
• Anuria in an obstructed kidney. 1
• Solitary kidney with obstruction. 1

Definitive stone treatment must be delayed until sepsis resolves. 1

Common Pitfalls to Avoid

• Do not rely on ultrasound alone for small stones (<5 mm) as sensitivity decreases dramatically and size estimation is unreliable, potentially leading to inappropriate counseling in one in five patients. 2, 4
• Do not use contrast-enhanced CT as first-line imaging because enhancing renal parenchyma obscures stones within the collecting system, particularly small calculi. 2
• Absence of hydronephrosis on ultrasound does not rule out kidney stones (negative predictive value only 65%). 5
• Plain KUB radiography alone is inadequate (sensitivity only 44-77%) and has an appropriateness rating of only 3 (usually not appropriate) due to inability to visualize radiolucent stones and alternative diagnoses. 1

Recurrent Stone Disease

For patients with known current stones and recurrent symptoms:

• Review previous imaging to understand stone history and location. 1, 5
• Consider limiting CT scan field to the area of interest (e.g., bladder only) to reduce cumulative radiation exposure. 1, 5
• Low-dose non-contrast CT remains the preferred modality with appropriateness rating of 7. 5