Kidney Stone Workup
Initial Diagnostic Evaluation
For suspected kidney stones, begin with ultrasound as the primary imaging modality, followed by non-contrast CT if ultrasound is inconclusive, combined with basic laboratory testing including urinalysis, serum electrolytes, calcium, creatinine, and uric acid. 1, 2
Imaging Strategy
Ultrasound is the first-line imaging tool with 45% sensitivity and 94% specificity for ureteral stones (88% specificity for renal stones), and should not delay emergency care in acute presentations 3, 1, 2
Non-contrast CT scan serves as the gold standard second-line modality when ultrasound is inconclusive or additional anatomic detail is needed, providing 93.1% sensitivity and 96.6% specificity while assessing stone location, size, density, and anatomic abnormalities 3, 1, 2
Low-dose CT protocols should be utilized to minimize radiation exposure while maintaining diagnostic accuracy 3, 1
KUB radiography helps differentiate radiopaque from radiolucent stones and is useful for monitoring stone passage during conservative management 3, 1
Laboratory Workup
Basic screening for all patients:
Urinalysis with dipstick and microscopic examination to assess pH, hematuria, signs of infection (pyuria, nitrites), and crystal identification 3, 1, 2
Serum chemistry panel including sodium, potassium, calcium, creatinine, and uric acid to identify metabolic abnormalities and assess renal function 3, 1, 2
Complete blood count and C-reactive protein if infection is suspected based on fever or urinalysis findings 3, 2
Urine culture if infection is suspected or the patient has recurrent UTI history 2
Stone Analysis
Stone material should be analyzed for all first-time stone formers when available by instructing patients to strain their urine and collect any passed stones 1, 2
- Stone composition guides both acute management and long-term prevention strategies, as calcium oxalate (61%), calcium phosphate (15%), and uric acid (12%) stones have different treatment approaches 4
Advanced Metabolic Evaluation
Comprehensive metabolic testing with 24-hour urine collection is indicated for high-risk patients, not all stone formers 1, 2
High-Risk Criteria Requiring Metabolic Workup:
- Recurrent stone formers (≥2 episodes) 3, 1
- First-time stone formers with multiple or bilateral stones 1, 2
- Nephrocalcinosis on imaging 1
- Stones composed of cystine, uric acid, or struvite 1
- Strong family history of stone disease 3
- Solitary kidney 1
- Suspected inherited or metabolic disorders 3
24-Hour Urine Collection Parameters:
One or two 24-hour urine collections should analyze: total volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium, and creatinine 3, 1, 2
- Collections should be obtained at least 6 weeks after an acute stone episode to avoid acute phase alterations 3
- Follow-up collections are essential to evaluate the impact of dietary or pharmacologic interventions 3, 1
Special Population Considerations
Pregnant Women:
- Ultrasound is strongly recommended as first-line imaging, followed by MRI as second-line, with low-dose CT only as last resort 3, 1, 2
Children:
- Ultrasound is first-line, followed by KUB or low-dose CT only if additional information is required 3, 1, 2
Patients with Suspected Genetic Disorders:
- Genetic testing should be considered for early-onset disease, recurrent stones, bilateral disease, or strong family history, with family screening offered if confirmed 3, 2
Immediate Evaluation Priorities
Urgent assessment is mandatory in specific clinical scenarios to prevent irreversible renal damage 3, 1, 2:
- Solitary or transplanted kidney
- Fever with obstructing stone (suggests infected hydronephrosis requiring emergency decompression)
- Bilateral obstruction
- Acute kidney injury
- Intractable pain or vomiting
- Anuria
Common Pitfalls to Avoid
Delaying imaging in patients with fever, solitary kidney, or suspected obstruction can lead to irreversible renal damage and sepsis 1, 2
Failing to obtain stone analysis when material is available misses critical information for prevention strategies 1, 2
Inadequate metabolic evaluation in high-risk or recurrent stone formers results in preventable recurrences, as rates can reach 30-50% within 5 years 3, 1
Confusing stone passage with treatment failure when patients pass pre-existing stones after implementing dietary changes 3
Neglecting to assess for associated systemic conditions including obesity, hypertension, diabetes, and metabolic syndrome, which are both risk factors and potential consequences of stone disease 1, 4, 5
Incomplete evaluation in patients with infection stones (struvite), as complete stone removal is necessary to prevent recurrent UTIs and progressive renal damage 1