What laboratory tests should be ordered for a patient suspected of having kidney stones?

Laboratory Tests for Kidney Stone Evaluation

For patients suspected of having kidney stones, clinicians should obtain serum electrolytes, calcium, creatinine, and uric acid, along with urinalysis including dipstick and microscopic evaluation, and urine culture if infection is suspected. 1

Initial Laboratory Evaluation

Basic Laboratory Tests

• Serum chemistries:

  • Electrolytes
  • Calcium (to assess for hypercalcemia)
  • Creatinine (to assess kidney function)
  • Uric acid (may suggest uric acid stones)

• Urinalysis:

  • Dipstick evaluation (pH, blood, protein, leukocyte esterase, nitrites)
  • Microscopic evaluation (crystals, RBCs, WBCs)
  • Assessment of urine pH (critical for determining stone type)
  • Identification of crystals pathognomonic for specific stone types

• Urine culture:

  • Indicated when urinalysis suggests infection
  • Required for patients with recurrent UTIs
  • Essential before any surgical intervention 1

Additional Testing for High-Risk Patients

For high-risk or recurrent stone formers, more comprehensive testing is warranted:

24-hour Urine Collection

• Recommended parameters to measure:
  • Total volume
  • pH
  • Calcium
  • Oxalate
  • Uric acid
  • Citrate
  • Sodium
  • Potassium
  • Creatinine
  • Cystine (if cystinuria is suspected)

Specialized Testing

• Parathyroid hormone (PTH):

  • Obtain when primary hyperparathyroidism is suspected
  • Particularly when serum calcium is high or high-normal 1

• Stone analysis:

  • Should be performed at least once when a stone is available
  • Critical for guiding preventive measures based on stone composition 1

Risk Stratification for Testing

First-time Stone Formers

• Basic evaluation (serum chemistries, urinalysis)
• Consider 24-hour urine collection if:
  • Large stone burden
  • Comorbidities exist
  • Patient desires comprehensive prevention 2

High-Risk Stone Formers (requiring comprehensive metabolic evaluation)

• Recurrent stone formers
• Stone formers with family history
• Patients with solitary kidney
• Children with stones
• Patients with intestinal malabsorption
• Patients with large or multiple stones at presentation 1, 3

Timing of Follow-up Testing

• Obtain a 24-hour urine specimen within 6 months of initiating treatment to assess response
• Repeat 24-hour urine collection annually or more frequently depending on stone activity
• Periodic blood testing for patients on pharmacological therapy to monitor for adverse effects 1

Common Pitfalls to Avoid

1. Inadequate collection: Ensure proper 24-hour urine collection technique is explained to patients
2. Timing errors: Avoid collecting urine during acute stone episodes or within 4-6 weeks of stone passage
3. Medication interference: Document all medications that may affect results
4. Missing stone analysis: Always attempt to retrieve stones for analysis
5. Overlooking microscopic hematuria: Persistent microscopic hematuria requires thorough evaluation including imaging and possibly cystoscopy 1

Remember that kidney stones are associated with systemic conditions like hypertension, diabetes, and obesity, so laboratory evaluation should consider these comorbidities. The goal of comprehensive testing is to identify modifiable risk factors that can guide preventive strategies and reduce stone recurrence.