How do you check for kidney stones?

Diagnostic Evaluation for Kidney Stones

Ultrasound should be used as the primary diagnostic tool for kidney stones, followed by non-contrast CT scan if more detailed information is needed about stone location, burden, and density. 1

Initial Diagnostic Approach

• A screening evaluation for kidney stones should include a detailed medical and dietary history, serum chemistries, and urinalysis 1
• Ultrasound is the recommended first-line imaging modality with 45% sensitivity and 94% specificity for ureteral stones and 88% specificity for renal stones 1
• Non-contrast CT is the standard follow-up modality for acute flank pain after ultrasound, offering superior diagnostic accuracy (93.1% sensitivity, 96.6% specificity) 1
• Low-dose CT reduces radiation exposure while maintaining high diagnostic accuracy 1

Laboratory Evaluation

• Serum chemistries should include electrolytes, calcium, creatinine, and uric acid to identify underlying medical conditions associated with stone disease 1, 2
• Urinalysis should include both dipstick and microscopic evaluation to assess urine pH, indicators of infection, and identify crystals that may indicate stone type 1, 2
• Urine culture should be obtained if urinalysis suggests infection or in patients with recurrent UTIs 1, 3
• When available, stone analysis should be performed at least once to determine composition and guide preventive measures 1

Comprehensive Metabolic Evaluation

• Additional metabolic testing is recommended for high-risk or interested first-time stone formers and all recurrent stone formers 1
• Metabolic testing should consist of one or two 24-hour urine collections analyzed for:
  • Total volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium, and creatinine 1, 2
  • Urinary cystine should be measured in patients with known or suspected cystine stones 1
• Serum intact parathyroid hormone level should be obtained if primary hyperparathyroidism is suspected (when serum calcium is high or high normal) 1

Imaging Studies

• Kidney-ureter-bladder (KUB) radiography helps differentiate between radiopaque and radiolucent stone types and aids in follow-up, with 44-77% sensitivity 1
• CT scan can classify stones according to their density, inner structure, and composition, which affects treatment decisions 1
• Imaging studies should be reviewed to quantify stone burden, as multiple or bilateral renal calculi may indicate higher risk of recurrence 1
• Nephrocalcinosis on imaging suggests underlying metabolic disorders (e.g., renal tubular acidosis, primary hyperparathyroidism) 1, 2

Special Considerations

• For pregnant women, ultrasound is strongly recommended as first-line imaging, followed by magnetic resonance imaging, and low-dose CT as the last option 1
• For children, ultrasound is also recommended as first-line imaging 1
• Genetic testing should be considered for patients with early-onset stone disease (≤25 years), suspected inherited disorders, recurrent stones, bilateral disease, or strong family history 1

Common Pitfalls

• Ultrasound may overestimate stone size in the 0-10 mm range, potentially leading to inappropriate management decisions in approximately 22% of cases 4
• Relying solely on ultrasound may result in missed stones due to its lower sensitivity (54%) compared to CT 4
• Combining ultrasound with KUB radiography increases sensitivity to 78%, but may still lead to inappropriate management in some cases 4
• Stone analysis should be repeated in patients not responding to treatment, as stone composition may change over time 2

By following this diagnostic algorithm, clinicians can accurately diagnose kidney stones and identify underlying risk factors to guide appropriate management and prevention strategies.