What labs should be ordered for a patient with a history of kidney stones, presenting with acute lower back pain and itching?

Laboratory Evaluation for a Patient with Kidney Stones and Lower Back Pain

For a patient with history of kidney stones presenting with lower back pain and itching, a comprehensive metabolic evaluation should be performed including serum chemistries, urinalysis, and urine culture to assess for stone recurrence and possible infection.

Initial Laboratory Evaluation

• Serum chemistries: Obtain electrolytes, calcium, creatinine, and uric acid to evaluate for underlying metabolic abnormalities and kidney function 1
• Urinalysis: Complete dipstick and microscopic evaluation to assess for:
  • Hematuria (indicating possible stone movement)
  • Urine pH (acidic urine pH associated with uric acid stones)
  • Crystalluria (can help identify stone type)
  • Signs of infection (leukocyte esterase, nitrites, WBCs) 1
• Urine culture: Indicated if urinalysis suggests infection or if patient has history of recurrent UTIs 1
• Serum intact parathyroid hormone: Should be obtained if primary hyperparathyroidism is suspected, particularly with high or high-normal serum calcium 1

Additional Targeted Testing

• 24-hour urine collection: Recommended for patients with recurrent stones to identify specific metabolic abnormalities:
  • Volume
  • Calcium
  • Oxalate
  • Citrate
  • Uric acid
  • Sodium
  • Creatinine 1

Imaging Considerations

• Ultrasound of kidneys and bladder: First-line imaging for kidney stones with sensitivity up to 100% for detecting obstruction 1
• Non-contrast CT scan: Consider if diagnosis is uncertain or to assess stone burden, as it has 95% or higher sensitivity and specificity 1

Special Considerations

• Stone analysis: If the patient passes a stone, it should be collected and analyzed to determine composition, which will guide preventive measures 1
• Pain assessment: Evaluate severity of pain to determine need for analgesics, preferably NSAIDs as first-line treatment 1
• Dermatologic evaluation: For the itching complaint, consider:
  • Urinalysis to rule out uremia (advanced kidney disease can cause itching)
  • Serum calcium (hypercalcemia can cause pruritus)
  • Liver function tests if systemic cause of pruritus is suspected 1

Clinical Pearls and Pitfalls

• Don't miss infection: Fever with flank pain could indicate pyelonephritis or infected stone (struvite), requiring urgent intervention 1
• Consider alternative diagnoses: Lower back pain may not always be due to kidney stones; consider musculoskeletal causes if urinalysis is negative for hematuria 1
• Avoid excessive imaging: Limit radiation exposure by using ultrasound as first-line imaging when appropriate, especially in younger patients 1
• Hydration status: Assess for dehydration, which increases risk of stone formation and may require IV fluids if severe 2

Follow-up Recommendations

• Repeat urinalysis: If initial results are negative but symptoms persist 1
• Metabolic evaluation: Complete within 6 weeks of stone event to guide prevention strategies 1
• Dietary counseling: Based on stone type and metabolic evaluation results:
  • Increased fluid intake (2.5-3.0 L/day)
  • Appropriate calcium intake (1000-1200 mg/day)
  • Limited sodium (2-5 g/day)
  • Limited animal protein (0.8-1.0 g/kg/day) 2