Diagnostic Testing for Kidney Stones in Primary Care
Start with ultrasound as your primary imaging tool, obtain urinalysis with dipstick and microscopy, and order basic serum chemistries (creatinine, calcium, uric acid, electrolytes) for all patients presenting with suspected kidney stones. 1, 2
Immediate Laboratory Testing
Blood Work
- Serum creatinine to assess renal function 1
- Ionized calcium to screen for hyperparathyroidism 1
- Uric acid to identify metabolic abnormalities 1
- Electrolytes (sodium, potassium) for comprehensive metabolic assessment 1
- Complete blood count and C-reactive protein if infection or emergency presentation is suspected 1
Urine Testing
- Urinalysis with dipstick and microscopic examination to assess pH, detect infection indicators, identify crystals, and quantify hematuria 2, 3
- Urine culture if urinalysis suggests infection or patient has recurrent UTI history 2, 3
Imaging Strategy
Ultrasound is your first-line imaging modality with 45% sensitivity and 88% specificity for renal stones, though sensitivity drops to 45% with 94% specificity for ureteral stones. 1, 2 This lower sensitivity means you'll miss some stones, but ultrasound avoids radiation and provides adequate initial assessment.
Order non-contrast CT scan if ultrasound is inconclusive or you need definitive assessment of stone location, size, burden, and density—this is the gold standard with 93.1% sensitivity and 96.6% specificity. 1, 2 Use low-dose CT protocols to minimize radiation while maintaining diagnostic accuracy. 1, 2
Plain KUB radiography (44-77% sensitivity) helps differentiate radiopaque from radiolucent stones and aids follow-up, but should not replace ultrasound or CT for initial diagnosis. 1, 2
Stone Analysis
Send all stone material for compositional analysis whenever available—this is critical for guiding future prevention strategies and should be done for all first-time stone formers. 4, 2, 3 Stone composition (calcium oxalate 61%, calcium phosphate 15%, uric acid 12%) directly determines preventive therapy. 5
When to Pursue Extended Metabolic Evaluation
Reserve 24-hour urine collection for high-risk patients, not routine first-time stone formers. 2, 3, 6 High-risk criteria include:
- Recurrent stone formers (≥2 episodes) 2, 3
- Young patients (age ≤25 years) 1, 2
- Bilateral stones or multiple stones 1, 2
- Strong family history 1, 2
- Solitary kidney 6
- Comorbidities (chronic kidney disease, metabolic syndrome, diabetes, hypertension) 5, 6
24-Hour Urine Parameters
When indicated, analyze for: total volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium, and creatinine. 2, 3 Two collections are preferred over one for accuracy. 4
Measure urinary cystine in patients with known/suspected cystine stones or family history of cystinuria. 2, 3
Suspect primary hyperoxaluria when urinary oxalate exceeds 75 mg/day in adults without bowel dysfunction. 4, 3
Special Situations Requiring Immediate Action
Obtain blood coagulation studies only if intervention is planned—you can omit these if conservative management is the strategy. 1, 4
Check serum intact parathyroid hormone if serum calcium is high or high-normal, suggesting primary hyperparathyroidism. 2
Consider genetic testing for children, adults ≤25 years, or those with recurrent stones, bilateral disease, or strong family history—monogenic forms occur in 12-21% of children/young adults and 1-11% of adults. 1
Common Pitfalls
Don't assume normal-sized kidneys on ultrasound exclude chronic kidney disease—stone formers as a group have decreased renal function compared to normal individuals, even common calcium oxalate stone formers. 4
Don't skip stone analysis when material is available—stone composition may change over time, and repeat analysis is justified in patients not responding to treatment. 3
Don't order extensive metabolic workup on every first-time stone former—reserve 24-hour urine collections for high-risk patients to avoid unnecessary testing and cost. 6