How do I evaluate a patient for kidney stones?

Evaluation of a Patient for Kidney Stones

Non-contrast CT is the gold standard for diagnosing kidney stones with sensitivity and specificity exceeding 95%, and should be the first-line imaging modality for evaluating patients with suspected kidney stones. 1

Initial Diagnostic Approach

History and Physical Examination Focus

• Assess for acute flank pain, radiation to groin, hematuria, nausea/vomiting
• Evaluate risk factors: family history, prior stones, dehydration, dietary habits
• Check vital signs for fever (suggesting infection)

Laboratory Testing

• Urinalysis: Look for hematuria, pyuria, pH abnormalities
• Urine culture: Especially if infection is suspected 1
• Serum chemistries: Electrolytes, calcium, creatinine, uric acid 1

First-Line Imaging

• Non-contrast CT of abdomen and pelvis: Gold standard with >95% sensitivity and specificity 2, 1
  • Accurately identifies stone size, location, and density
  • Can detect complications and alternative diagnoses
  • Disadvantage: Radiation exposure

Special Populations

Pregnant Patients

• Ultrasound is first-line: No radiation risk, ~45% sensitivity, ~88% specificity 2, 1
• If ultrasound is inconclusive:
  • MRI without contrast (second trimester onward) 2
  • Limited IVU as last resort 2

Children and Young Adults

• Low-dose CT protocol: Reduces radiation by ~50% 1
• Ultrasound: Consider as initial screening

When Initial CT is Inconclusive

If non-contrast CT is inconclusive for stones (e.g., unclear if calcification is in ureter or adjacent structure):

• CT with IV contrast: Helps differentiate ureteral stones from phleboliths and vascular calcifications 2
• CTU (CT urography): Confirms ureteral location of calculi and degree of obstruction 2

Follow-Up Imaging

• Ultrasound: Preferred for follow-up to reduce radiation exposure 1, 3
• KUB (kidney, ureter, bladder X-ray): Useful for monitoring known radiopaque stones 2, 3
  • Note: KUB has limited sensitivity (47.6%) compared to CT 4

Additional Evaluation for Recurrent Stone Formers

• 24-hour urine collection: Analyze for volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium, and creatinine 1
• Stone analysis: Essential when stone is available to direct preventive measures 1

Common Pitfalls and Caveats

1. Ultrasound limitations:

   • Low sensitivity (54%) compared to CT 5
   • Tends to overestimate stone size in the 0-10mm range 5
   • May lead to inappropriate management decisions in 22% of cases 5

2. CT radiation concerns:

   • Consider low-dose protocols to minimize exposure 3
   • Balance diagnostic accuracy with radiation risk

3. Mimics of ureteral stones:

   • Phleboliths and vascular calcifications can be mistaken for stones 2
   • IV contrast can help differentiate these entities

4. Microscopic hematuria workup:

   • Rule out contamination, UTI, and stones before considering more invasive testing 2
   • Evaluate for stones with CT urogram 2

By following this evidence-based approach to kidney stone evaluation, clinicians can accurately diagnose stones, determine appropriate treatment strategies, and minimize unnecessary radiation exposure while maintaining diagnostic accuracy.