Renal Stones: NAC OSCE Guide
CAUSES
Kidney stones form when urine becomes supersaturated with minerals that crystallize and aggregate in the urinary tract 1, 2.
Primary Risk Factors:
- Dehydration/Low fluid intake - Most important modifiable risk factor
- Dietary factors:
- High sodium intake (increases urinary calcium)
- High animal protein/purines (increases uric acid)
- Low calcium intake (paradoxically increases oxalate absorption)
- High oxalate foods (spinach, nuts, chocolate)
- Low fruit/vegetable intake (reduces urinary citrate)
Medical Conditions:
- Metabolic syndrome - obesity, diabetes, hypertension 1
- Primary hyperparathyroidism - suspect when calcium is high/high-normal 1
- Renal tubular acidosis type 1
- Chronic UTIs (especially with urease-producing organisms)
- Inflammatory bowel disease/malabsorption (causes hyperoxaluria)
- Gout/hyperuricemia
Genetic Disorders:
- Cystinuria
- Primary hyperoxaluria
- Dent disease
- Medullary sponge kidney
TYPES OF STONES
Approximately 80% of stones are calcium-based 2:
1. Calcium Oxalate (Most Common - ~70%)
- Appearance: Brown/black, spiculated
- X-ray: Radiopaque
- Urine pH: Variable (5.5-7.0)
- Risk factors: Hypercalciuria, hyperoxaluria, hypocitraturia, low urine volume
2. Calcium Phosphate (~10-15%)
- Appearance: White/gray, smooth
- X-ray: Radiopaque
- Urine pH: Alkaline (>7.0)
- Associated with: RTA type 1, hyperparathyroidism, UTIs
3. Uric Acid (~5-10%)
- Appearance: Yellow/orange, smooth
- X-ray: Radiolucent (won't see on plain film!)
- Urine pH: Acidic (<5.5) - KEY FEATURE
- Risk factors: Gout, high purine diet, chronic diarrhea, metabolic syndrome
4. Struvite/Infection Stones (~5-10%)
- Appearance: Staghorn configuration (fills renal pelvis)
- X-ray: Radiopaque
- Urine pH: Alkaline (>7.0)
- Cause: UTI with urease-producing bacteria (Proteus, Klebsiella)
- Crystals: "Coffin-lid" shaped
5. Cystine (<1%)
- Appearance: Yellow, hexagonal crystals
- X-ray: Faintly radiopaque
- Urine pH: Variable
- Cause: Genetic defect in cystine transport (autosomal recessive)
SYMPTOMS (Clinical Presentation)
Classic Presentation - Renal Colic:
- Sudden onset severe colicky flank pain radiating to groin/testicle/labia
- Pain comes in waves (due to ureteral peristalsis)
- Patient cannot find comfortable position (writhes in pain - unlike peritonitis where patient stays still)
- Nausea and vomiting (common)
- Hematuria (microscopic or gross) - present in 85-90% of cases
Location-Specific Symptoms:
- Kidney stones: Dull flank pain, may be asymptomatic
- Upper ureter: Flank pain radiating to abdomen
- Mid ureter: Lower abdominal pain
- Distal ureter: Suprapubic pain, urinary frequency, urgency, dysuria (mimics UTI)
- Bladder: Suprapubic pain, dysuria, interrupted urinary stream
Red Flag Symptoms (Require Urgent Intervention):
- Fever + obstructing stone = infected obstructed kidney (EMERGENCY - sepsis risk)
- Anuria = bilateral obstruction or obstruction in solitary kidney
- Acute kidney injury
- Intractable pain/vomiting
- Solitary kidney with obstruction
DIAGNOSIS
Initial Evaluation 1:
Every newly diagnosed patient needs a screening evaluation consisting of:
1. Detailed History:
- Medical conditions: Hyperparathyroidism, gout, IBD, RTA, recurrent UTIs, diabetes
- Medications/supplements: Calcium, vitamin D, vitamin C (converts to oxalate), topiramate, protease inhibitors
- Dietary history:
- Fluid intake (amount and types)
- Sodium intake
- Protein intake (type and amount)
- Calcium intake
- High-oxalate foods
- Fruits/vegetables
- Family history of stones
- Stone history: Previous episodes, treatments, stone analysis results
2. Serum Chemistries:
- Electrolytes
- Calcium (if high/high-normal → check PTH for hyperparathyroidism) 1
- Creatinine (assess renal function)
- Uric acid
3. Urinalysis:
- Dipstick: pH, blood, leukocyte esterase, nitrites
- Microscopy:
- RBCs (hematuria)
- WBCs (infection)
- Crystals (can identify stone type):
- Calcium oxalate: envelope-shaped
- Uric acid: rhomboid/rosettes
- Cystine: hexagonal
- Struvite: coffin-lid
- Urine culture if infection suspected 1
4. Stone Analysis (if stone available):
Must obtain at least once - guides prevention strategy 1
5. Imaging:
Non-contrast CT abdomen/pelvis is the gold standard 3:
- Sensitivity/specificity ~100% for detecting stones
- Shows stone size, location, number, density
- Assesses hydronephrosis
- Identifies anatomic abnormalities
- Can detect radiolucent uric acid stones
Alternative imaging:
- Ultrasound: Good for hydronephrosis, pregnancy, children; misses small stones
- KUB (plain X-ray): Only shows radiopaque stones; misses uric acid stones
- Low-dose CT: Reduces radiation while maintaining diagnostic accuracy 3
Additional Metabolic Testing 1:
Indicated for:
- High-risk patients:
- Recurrent stone formers
- First-time formers with: solitary kidney, CKD, transplant, pediatric, strong family history, nephrocalcinosis, multiple/bilateral stones
- Interested first-time stone formers
24-hour urine collection (×2 collections):
- Volume (goal >2L/day)
- Calcium
- Oxalate
- Citrate
- Uric acid
- Sodium
- Creatinine (validates collection)
- pH
- Cystine (if suspected)
MANAGEMENT
Acute Management (Renal Colic):
1. Pain Control:
NSAIDs are first-line (superior to opioids):
- Ketorolac 30mg IV or 10mg PO
- Ibuprofen 600-800mg PO
- Diclofenac 50mg PO
2. Medical Expulsive Therapy (MET):
For distal ureteral stones ≤10mm, offer alpha-blockers for ~30 days 4:
- Tamsulosin 0.4mg daily (most commonly used)
- Alfuzosin, doxazosin, silodosin also effective
- Increases stone passage rate, reduces pain, decreases ED visits 4
For mid/proximal ureteral stones ≤10mm: May offer alpha-blockers but evidence weaker 4
3. Hydration:
- Encourage oral fluids
- IV fluids if vomiting
4. Follow-up Imaging:
- Within 14 days to monitor stone position and hydronephrosis 5
Indications for Urgent Urological Intervention:
- Infected obstructed kidney (fever + obstruction) - EMERGENCY
- Bilateral obstruction or obstruction in solitary kidney
- Acute kidney injury from obstruction
- Intractable pain/vomiting
- Stone >10mm (unlikely to pass spontaneously)
- Failure to pass after 4-6 weeks of conservative management
Surgical Options:
- Shock wave lithotripsy (SWL): For stones <20mm
- Ureteroscopy (URS): Most common, for ureteral stones
- Percutaneous nephrolithotomy (PCNL): For large kidney stones (>20mm), staghorn calculi 6
- Open surgery: Rarely needed 6
PREVENTION (Long-term Management)
Universal Dietary Recommendations 1, 2:
All stone formers should receive these recommendations:
1. Increase Fluid Intake (MOST IMPORTANT):
Target urine output ≥2L/day 2
- Drink 2.5-3L fluid daily
- Spread throughout day and night
- Water is best; avoid excessive sugary drinks
2. Moderate Sodium Restriction:
- Limit to <2300mg/day (ideally <1500mg)
- High sodium increases urinary calcium excretion
3. Normal Calcium Intake:
- Do NOT restrict calcium (paradoxically increases oxalate absorption)
- Aim for 1000-1200mg/day from food sources
- Take with meals to bind dietary oxalate
4. Moderate Protein Intake:
- Limit animal protein to 0.8-1.0 g/kg/day
- Excessive protein increases urinary calcium and uric acid
5. Increase Fruits/Vegetables:
- Increases urinary citrate (stone inhibitor)
- Provides alkaline load
6. Limit High-Oxalate Foods (for calcium oxalate stones):
- Spinach, rhubarb, beets, nuts, chocolate, tea
Pharmacologic Therapy 1, 2, 1:
Consider when dietary measures fail or for high-risk patients:
For Calcium Oxalate/Phosphate Stones:
1. Thiazide Diuretics (for hypercalciuria) 1:
- Hydrochlorothiazide 25mg PO twice daily or 50mg once daily
- Chlorthalidone 25mg daily
- Indapamide 2.5mg daily
- Reduces urinary calcium excretion
- Must continue sodium restriction for effectiveness
- May need potassium supplementation
2. Potassium Citrate (for hypocitraturia) 1:
- 10-20 mEq PO 2-3 times daily
- Increases urinary citrate (stone inhibitor), alkalinizes urine
- Preferred over sodium citrate (sodium increases calcium excretion)
- Also effective for calcium phosphate stones
3. Allopurinol (for hyperuricosuria with normocalciuria) 1:
- 100-300mg daily
- For calcium oxalate stones with uric acid >800mg/day
- Reduces uric acid excretion
For Uric Acid Stones 1:
Potassium Citrate (First-line):
- Target urine pH 6.0-6.5 (increases uric acid solubility)
- 10-20 mEq PO 2-3 times daily
- Monitor urine pH with home strips
Allopurinol:
- Second-line if hyperuricosuria present
- Not first-line (low pH is main problem, not hyperuricosuria) 1
For Cystine Stones 1:
1. Potassium Citrate:
- Target urine pH 7.0-7.5
- Higher pH increases cystine solubility
2. Thiol-containing drugs (if citrate insufficient):
- Tiopronin or D-penicillamine
- Bind cystine to increase solubility
3. High fluid intake:
- Target >3L urine output daily
For Struvite Stones:
Complete surgical removal + antibiotics 6
- Medical therapy alone insufficient
- Prevent recurrent UTIs
Common Pitfalls to Avoid:
- Don't restrict dietary calcium - worsens oxalate absorption
- Don't miss infected obstructed kidney - check for fever in all stone patients
- Don't forget uric acid stones are radiolucent - need CT, not just KUB
- Don't use thiazides without sodium restriction - won't work effectively
- Don't over-alkalize urine in calcium phosphate formers - promotes stone formation
- Don't forget to analyze stone composition - guides prevention 1
- Don't assume all flank pain is stones - consider AAA, pyelonephritis, other causes
- Don't discharge patients with fever + obstruction - requires urgent decompression