Struvite stone vs Cystine stone
How to tell Struvite stone and Cystine stone apart when a board question makes them look alike.
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Alkaline urine with urease-positive infection points to struvite, while hexagonal crystals point to cystine.
Side by side
Recurrent UTIs with alkaline urine and staghorn calculi suggest magnesium ammonium phosphate struvite stones.
Early-onset recurrent nephrolithiasis with hexagonal crystals and positive cyanide-nitroprusside test indicates cystinuria.
Urease-producing bacteria split urea to ammonia, increasing urine pH and precipitating struvite matrix.
Autosomal recessive proximal tubular transport defect causes excess urinary cystine that crystallizes in acidic urine.
Urine pH is typically above 7.2, culture often grows Proteus or Klebsiella, and CT may show branching staghorn calculus occupying the renal pelvis.
Urinalysis shows hexagonal crystals, urine cyanide-nitroprusside test is positive, and 24-hour urine cystine excretion exceeds 250 mg/day.
The trap
A large stone in a young patient can be mislabeled as metabolic calcium stone. The resolving clues are urine pH and microbiology for struvite versus crystal shape and cystine testing for cystinuria.
The repair move
Next best step: treat active infection with culture-directed antibiotics, then arrange complete stone removal by percutaneous nephrolithotomy because retained struvite nidus causes recurrence. For cystine stones, start high fluid intake targeting urine output above 3 L/day, urine alkalinization with potassium citrate, and add tiopronin if recurrent despite conservative therapy.
Try a free question
CoreA 24-year-old woman has fever, dysuria, and right flank pain. BP 118/70 mm Hg, HR 106/min, temp 38.5 C. Urinalysis shows pH 8.1 and pyuria, urine culture grows Proteus mirabilis, and CT reveals a staghorn calculus. What is the next best step in management?
Show answer and explanation
Answer: B. Culture-directed antibiotics followed by percutaneous nephrolithotomy. Struvite stones are infection stones requiring both antimicrobial treatment and definitive removal.
Why the other choices are tempting
- A. Hydration alone does not clear infected staghorn burden.
- C. Allopurinol is used for uric acid overproduction, not struvite infection stones.
- D. Acidification is not standard immediate management for infected staghorn stones.
- E. Large staghorn calculi rarely pass and should not be observed.
Flashcards
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Which urinalysis profile strongly favors struvite nephrolithiasis?
Urine pH above 7 with pyuria and culture growth of urease-producing organisms supports struvite stones.
What crystal shape identifies cystine stones on microscopy?
Hexagonal crystals are classic for cystinuria-related cystine stones.
Quick answers
What is the fastest way to tell Struvite stone from Cystine stone?
Alkaline urine with urease-positive infection points to struvite, while hexagonal crystals point to cystine.
What is the key clue for Struvite stone?
Recurrent UTIs with alkaline urine and staghorn calculi suggest magnesium ammonium phosphate struvite stones.
What is the key clue for Cystine stone?
Early-onset recurrent nephrolithiasis with hexagonal crystals and positive cyanide-nitroprusside test indicates cystinuria.
What is the common trap with Struvite stone vs Cystine stone?
A large stone in a young patient can be mislabeled as metabolic calcium stone. The resolving clues are urine pH and microbiology for struvite versus crystal shape and cystine testing for cystinuria.