What is the appropriate workup to evaluate for nephrolithiasis?

Workup for Suspected Nephrolithiasis

Begin with ultrasound as the primary imaging modality, followed by non-contrast CT if ultrasound is inconclusive, combined with urinalysis and basic serum chemistry to guide diagnosis and treatment decisions. 1, 2

Initial Imaging Strategy

First-Line: Ultrasound

• Ultrasound is the primary diagnostic tool and should be performed first in all patients with suspected kidney stones 1, 2
• Provides 45% sensitivity with 94% specificity for ureteral stones and 88% specificity for renal stones 1
• Advantages include no radiation exposure, ability to detect hydronephrosis, and visualization of radiolucent stones 2
• Should not delay emergency care in unstable patients 1

Second-Line: Non-Contrast CT

• Non-contrast CT is the standard second-line imaging when ultrasound is inconclusive or additional anatomic detail is needed 1, 2
• Achieves 93.1% sensitivity and 96.6% specificity with low-dose protocols 1, 2
• Provides critical information on stone location, size, density, and anatomic variations 1
• Low-dose CT protocols should be used to minimize radiation exposure while maintaining diagnostic accuracy 1, 2

Adjunctive: KUB Radiography

• Plain kidney-ureter-bladder (KUB) radiography helps differentiate radioopaque from radiolucent stones 1, 2
• Sensitivity ranges from 44-77% for stone detection 1
• Most useful for follow-up imaging after initial diagnosis rather than primary detection 1, 2

Laboratory Evaluation

Urinalysis (Essential for All Patients)

• Perform dipstick urinalysis immediately to assess for hematuria, pH, signs of infection, and crystals 1, 2
• Microscopic examination should identify red blood cells, white blood cells, bacteria, and crystal types 2
• Urine pH is critical: pH <5.5 suggests uric acid stones, while pH >7.0 suggests infection stones 3, 4

Serum Chemistry Panel

• Obtain creatinine, uric acid, ionized calcium, sodium, potassium, blood cell count, and C-reactive protein (CRP) 1, 2
• These tests identify metabolic abnormalities and assess for complications like acute kidney injury or infection 1
• CRP and blood coagulation studies can be omitted if no intervention is planned 1

Urine Culture

• Obtain urine culture if urinalysis suggests infection or patient has history of recurrent UTIs 2
• Critical for identifying infection stones requiring urgent intervention 1

Stone Analysis

• Stone analysis should be performed for all first-time stone formers when material is available 1, 2
• Instruct patients to strain urine to capture passed stones for compositional analysis 2
• Stone composition guides preventive therapy and identifies metabolic disorders 1, 4

Metabolic Workup (For Recurrent or High-Risk Patients)

24-Hour Urine Collection

• Reserved for recurrent stone formers, bilateral stones, or high-risk first-time formers 2, 4, 5
• Measure total volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium, and creatinine 2, 4, 5
• Identifies specific metabolic abnormalities in approximately 96% of recurrent stone formers 4, 6

Key Metabolic Abnormalities to Identify

• Hypercalciuria (most common, found in 60.9% of patients) 4, 6
• Hyperuricosuria (35.8% of patients) 4, 6
• Hyperoxaluria (8.1% of patients) 4, 6
• Hypocitraturia (28% of patients) 4, 5, 6
• Low urine volume <1 L/day (15.3% of patients) 4, 6

Special Population Considerations

Pregnant Women

• Use ultrasound as first-line imaging, MRI as second-line, and low-dose CT only as last resort 1, 2
• This hierarchy minimizes fetal radiation exposure while maintaining diagnostic capability 1

Children

• Ultrasound is strongly recommended as first-line imaging 1
• Follow with KUB or low-dose non-contrast CT only if ultrasound is insufficient 1
• Detailed history should include family history of stone disease, hearing loss, and metabolic disorders 1

Genetic Testing Indications

• Consider genetic testing for patients ≤25 years old, those with suspected inherited disorders, recurrent stones (≥2 episodes), bilateral disease, or strong family history 1
• Combine with metabolic evaluation and provide genetic counseling before testing 1

Critical Red Flags Requiring Immediate Evaluation

• Fever or signs of infection with obstruction (requires urgent decompression) 1, 2
• Solitary kidney with obstruction 1
• Bilateral obstruction or anuria 1
• Intractable pain despite analgesia 2
• Acute kidney injury 2

Common Pitfalls to Avoid

• Do not use contrast-enhanced CT as first-line imaging—it obscures stones in the collecting system and provides no advantage over non-contrast CT 1
• Do not skip urinalysis; it provides essential information about stone type and complications that imaging cannot reveal 2
• Do not perform extensive metabolic workup on first-time stone formers unless they are high-risk (young age, bilateral stones, family history) 2, 4, 5
• Do not assume hematuria resolution after treating presumed non-stone causes without repeat urinalysis to confirm 1