What are the signs and tests for diagnosing kidney stones?

Signs and Tests for Kidney Stones

Non-contrast CT scan is the first-line diagnostic test for suspected kidney stones, with sensitivity and specificity exceeding 95%. 1

Clinical Presentation

Kidney stones typically present with the following signs and symptoms:

• Renal colic: Severe, intermittent pain that radiates from the flank to the groin
• Hematuria: Blood in the urine (microscopic or gross)
• Urinary symptoms: Dysuria, frequency, urgency
• Nausea and vomiting: Often accompanying severe pain
• Fever: May indicate infection (complication)
• Flank tenderness: On physical examination

Diagnostic Testing Algorithm

Initial Evaluation

1. Urinalysis (dipstick and microscopic)

   • Assesses urine pH
   • Identifies hematuria (present in 85% of cases)
   • May reveal crystals pathognomonic of stone type:
     • Hexagonal crystals for cystine
     • Coffin-lid shaped for struvite
     • Envelope-shaped for calcium oxalate dihydrate 1
   • Evaluates for signs of infection (leukocyte esterase, nitrites)

2. Urine culture

   • Obtain if urinalysis suggests infection or in patients with recurrent UTIs 2

3. Serum chemistries

   • Electrolytes, calcium, creatinine, and uric acid
   • May suggest underlying medical conditions associated with stone disease 2

Imaging Studies

1. Non-contrast CT scan

   • First-line imaging test for suspected kidney stones
   • Sensitivity and specificity >95%
   • Low-dose protocol recommended to reduce radiation exposure 1, 2

2. Ultrasound

   • Alternative when CT is unavailable or contraindicated
   • First-line for pregnant patients and children
   • Lower sensitivity (45%) but good specificity (88%) 1

3. X-ray (KUB - Kidney, Ureter, Bladder)

   • Limited sensitivity (49%) but high specificity (99%)
   • May be useful for follow-up of radiopaque stones 1

Additional Testing for High-Risk or Recurrent Stone Formers

1. Stone analysis

   • Should be obtained at least once when a stone is available
   • Essential for directing preventive measures and treatment 2
   • Most common stone types:
     • Calcium oxalate (61%)
     • Calcium phosphate (15%)
     • Uric acid (12%) 3

2. 24-hour urine collection

   • Analyze for:
     • Total volume
     • pH
     • Calcium
     • Oxalate
     • Uric acid
     • Citrate
     • Sodium
     • Potassium
     • Creatinine 2
   • Additional measurements for suspected cystine stones or hyperoxaluria

3. Parathyroid hormone level

   • Obtain if primary hyperparathyroidism is suspected (high or high-normal serum calcium) 2

Special Considerations

Pregnant Patients

• Ultrasound is the imaging modality of choice
• MRI considered as second-line option if ultrasound is inconclusive 1

Children

• Ultrasound recommended as first-line imaging
• Low-dose CT if ultrasound is inconclusive 1

Interpretation of Results

• Stone size and location: Critical factors in determining treatment approach

  • Stones <5mm in the proximal ureter likely to pass spontaneously
  • Stones >10mm typically require intervention 1

• Hydronephrosis: Indicates obstruction requiring prompt attention

• Stone composition: Guides preventive strategies

  • Calcium oxalate stones (most common): Dietary modifications
  • Uric acid stones: Urinary alkalinization
  • Cystine stones: Specific medical therapy

Common Pitfalls to Avoid

1. Relying solely on urinalysis for diagnosis

   • While helpful, urinalysis alone cannot definitively diagnose kidney stones in all cases

2. Missing underlying metabolic disorders

   • Comprehensive metabolic evaluation needed for recurrent stone formers

3. Overlooking silent obstruction

   • Absence of symptoms does not necessarily mean absence of risk
   • Silent obstruction can lead to renal damage over time 1

4. Inadequate follow-up imaging

   • Periodic monitoring with ultrasound every 3-6 months recommended for known stones
   • Closer monitoring if symptoms develop or hydronephrosis worsens 1

By following this diagnostic approach, clinicians can accurately diagnose kidney stones, determine appropriate treatment, and identify patients at risk for recurrence who would benefit from preventive measures.