How do you determine kidney stone type?

Determining Kidney Stone Type

When a stone is available, clinicians should obtain a stone analysis at least once to determine the stone type, as this is essential for directing preventive measures and treatment. 1

Stone Analysis Methods

Direct Stone Analysis

1. Stone retrieval:

   • Collect passed stones using a strainer
   • Retrieve stones during surgical procedures (ureteroscopy, PCNL)

2. Laboratory analysis techniques:

   • Infrared spectroscopy (IR): Most commonly used method that identifies major components 2
   • X-ray diffraction: Considered the most accurate method for stone composition determination 3
   • Scanning electron microscopy (SEM) with energy dispersive X-ray microanalysis (EDAX): Provides detailed structural analysis 2

3. Comprehensive stone analysis should include:

   • Identification of all stone components
   • Determination of molecular structure and crystalline forms
   • Quantitative determination of each component 4

Indirect Methods When Stone Is Not Available

1. Urinalysis:

   • Dipstick and microscopic evaluation to assess urine pH
   • Identification of crystals pathognomonic of stone type 1
   • Crystal morphology can suggest stone type:
     • Hexagonal crystals: Cystine
     • Coffin-lid shaped: Struvite
     • Envelope-shaped: Calcium oxalate dihydrate

2. Imaging studies:

   • Non-contrast CT scan: First-line diagnostic test with >95% sensitivity and specificity 5
     • Can provide information about stone density (Hounsfield units)
     • Uric acid stones: 200-450 HU
     • Calcium oxalate: 600-1200 HU
     • Cystine: 600-800 HU
   • Low-dose CT protocol should be used to reduce radiation exposure 5

3. 24-hour urine collection:

   • Should be analyzed for:
     • Total volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium, and creatinine 1
     • For suspected cystine stones: Add urinary cystine measurement 1
     • For suspected hyperoxaluria: Note if urinary oxalate exceeds 75 mg/day in adults 1

Stone Types and Their Characteristics

1. Calcium oxalate stones (61% of all stones) 6:

   • Typically calcium oxalate monohydrate (whewellite)
   • Appearance: White or pale yellow with disorganized internal structure 1
   • Finding >200 pure whewellite crystals per cubic millimeter in urinary sediment is highly suggestive of Primary Hyperoxaluria type 1, especially in young children 1

2. Calcium phosphate stones (15% of stones) 6:

   • Often contain mixtures of calcium oxalate and calcium phosphate
   • Cannot be easily distinguished from idiopathic stone formers based on composition alone 1

3. Uric acid stones (12% of stones) 6:

   • Associated with persistently acidic urine (pH <5.5)
   • May indicate metabolic abnormalities

4. Struvite stones:

   • Associated with urinary tract infections with urease-producing organisms
   • Alkaline urine pH

5. Cystine stones:

   • Rare, genetic disorder (cystinuria)
   • Hexagonal crystals in urine

Clinical Approach to Stone Type Determination

1. First episode stone former:

   • Obtain stone for analysis if possible
   • Basic metabolic evaluation: serum electrolytes, calcium, creatinine, uric acid 1
   • Urinalysis with microscopic examination 1
   • Non-contrast CT scan for stone identification and characterization 5

2. High-risk or recurrent stone formers:

   • Comprehensive metabolic testing with 24-hour urine collections 1
   • Stone analysis whenever possible
   • Consider specialized testing based on suspected stone type:
     • Parathyroid hormone level if primary hyperparathyroidism suspected 1
     • Genetic testing for suspected hereditary stone diseases

Common Pitfalls to Avoid

1. Relying solely on urinalysis - While helpful, it cannot definitively determine stone type in all cases

2. Using contrast-enhanced CT as initial imaging - Can obscure stones and interfere with density measurements 5

3. Failing to collect passed stones - Patients should be instructed to strain urine during stone episodes

4. Inadequate stone analysis - Using only chemical methods rather than more accurate spectroscopic techniques can lead to incomplete characterization 4

5. Not considering rare stone types - Uncommon stones like cystine or medication-induced stones may be missed without specific testing

By following this systematic approach to determining kidney stone type, clinicians can provide targeted prevention strategies and treatment options to reduce morbidity and recurrence rates in patients with urolithiasis.