What is the best approach to diagnose a kidney stone?

Diagnosing Kidney Stones

Non-contrast CT is the most accurate diagnostic test for kidney stones, with sensitivity up to 97% and specificity of 95%, and should be the first-line imaging modality for suspected urolithiasis when obstructive urolithiasis is a primary concern. 1

Diagnostic Approach Algorithm

First-Line Imaging

1. Non-contrast CT scan

   • Gold standard for diagnosis of kidney stones 1, 2
   • Detects virtually all renal calculi regardless of composition
   • Identifies secondary signs of obstruction and complications
   • Consider low-dose protocols to reduce radiation exposure (as low as 0.57 mSv) 1

2. Ultrasound

   • Alternative first-line option, especially for:
     • Pregnant patients (imaging modality of choice due to lack of radiation) 1
     • Children
     • Follow-up of known stones
   • Lower sensitivity (24-57%) compared to CT 1
   • Adding color Doppler with twinkling artifact can improve sensitivity to up to 99% for stones <5mm (but with high false-positive rate) 1
   • Tends to overestimate stone size, particularly for smaller stones (≤5 mm) 1

Second-Line or Adjunctive Imaging

1. KUB (Kidney, Ureter, Bladder) Radiography

   • Sensitivity: 53-62%, Specificity: 67-69% 1
   • Most useful for follow-up of known radiopaque stones 1
   • Combining with ultrasound improves sensitivity to 78-90% 1

2. MRI

   • Limited for direct stone visualization
   • Useful for secondary signs of obstruction
   • Consider when CT is contraindicated 1

Clinical Evaluation

• Key symptoms to assess:

  • Renal colic (severe, intermittent pain)
  • Flank pain radiating to groin
  • Hematuria
  • Dysuria
  • Urinary frequency 3
  • Fever (may indicate infection)

• Risk factors to identify:

  • Previous stone history (50% recurrence within 5 years) 4
  • Family history
  • Anatomic abnormalities of kidneys or ureters
  • Obesity
  • Metabolic syndrome
  • Diabetes
  • Hypertension 3

Important Considerations

• Stone size and location are critical factors in determining management approach and likelihood of spontaneous passage 1

  • Smaller and more distally located stones have higher rates of spontaneous passage
  • Larger and more proximally located stones are associated with lower rates of spontaneous passage 5

• Radiation exposure concerns:

  • Use low-dose CT protocols when possible 1
  • Consider ultrasound for follow-up imaging to reduce cumulative radiation exposure
  • For recurrent stone formers, limit scan area to region of concern 1

• Common pitfalls to avoid:

  1. Using standard-dose CT when low-dose protocols would be sufficient 1
  2. Relying solely on ultrasound for small stones (<5mm) without additional imaging
  3. Failing to look for secondary signs of obstruction (hydronephrosis, perinephric stranding) when primary findings are unclear 1
  4. Missing complete obstruction, which requires urgent urological intervention 1

• Special populations:

  • Pregnant patients: Ultrasound is first-line imaging 1, 3
  • Children: Ultrasound findings of calicectasis may warrant investigation for vesicoureteral reflux 1
  • Recurrent stone formers: Consider low-dose CT or KUB for follow-up 1

Diagnostic Performance Comparison

Modality	Sensitivity	Specificity	Key Advantages	Key Limitations
Non-contrast CT	Up to 97%	95%	Highest accuracy, detects all stone types	Radiation exposure
Ultrasound	24-57%	Variable	No radiation, good for follow-up	Lower sensitivity, operator-dependent
KUB	53-62%	67-69%	Good for radiopaque stone follow-up	Poor for small or radiolucent stones
MRI	Limited for stones	Good for obstruction	No radiation	Limited stone visualization, costly

While CT remains the most accurate diagnostic tool, the American College of Radiology guidelines recognize that the choice of imaging should consider radiation exposure, especially in younger patients and those requiring repeated imaging 5, 1.