Calcium oxalate / uric acid stones vs Cystine / struvite stones
How to tell Calcium oxalate / uric acid stones and Cystine / struvite stones apart when a board question makes them look alike.
Envelope/radiopaque is calcium oxalate; acidic/radiolucent is uric acid; hexagonal is cystine; infection/alkaline/staghorn is struvite.
Side by side
Calcium oxalate stones are radiopaque and envelope-shaped; uric acid stones are radiolucent and form in acidic urine.
Cystine stones are hexagonal from cystinuria; struvite stones are urease-positive infection stones that form in alkaline urine and can become staghorn calculi.
Calcium oxalate crystallizes with hypercalciuria, hyperoxaluria, low citrate, or dehydration; uric acid precipitates when urine is persistently acidic.
Cystinuria impairs dibasic amino acid reabsorption; urease-positive organisms split urea into ammonia, alkalinizing urine and precipitating magnesium ammonium phosphate.
Calcium oxalate stones (75 percent of all stones): radiopaque, envelope or dumbbell crystals, form in any urine pH, increased by hypercalciuria, hyperoxaluria, hypocitraturia; calcium PHOSPHATE stones form in alkaline urine and are associated with distal RTA. Uric acid stones: RADIOLUCENT (not visible on plain X-ray, see on CT), rhomboid crystals, form in ACIDIC urine, classic in gout and tumor lysis syndrome.
Cystine stones: faintly radiopaque (sulfur content), hexagonal crystals, autosomal recessive cystinuria, present in childhood with recurrent stones, positive urinary cyanide-nitroprusside test. Struvite stones: radiopaque, coffin-lid crystals, form in ALKALINE urine, always from urea-splitting organisms (Proteus is classic), form STAGHORN calculus, require surgical removal plus antibiotics.
The trap
Radiopacity helps, but urine pH, crystal shape, and infection history usually make the stone type.
Watch it explained
The repair move
For kidney stones, sort by X-ray visibility, urine pH, crystal shape, and infection: (1) CALCIUM OXALATE (75 percent of all stones): radiopaque, envelope or dumbbell crystals, forms in any urine pH; risk factors include hypercalciuria, hyperoxaluria (ethylene glycol, vitamin C excess, Crohn ileal disease, gastric bypass), hypocitraturia; prevent with hydration, thiazide (reduces urinary calcium), citrate, oxalate restriction; (2) URIC ACID: radiolucent (NOT visible on plain X-ray, requires CT or ultrasound), rhomboid or rosette crystals, forms in ACIDIC urine (pH under 5.5); risk factors include gout, tumor lysis syndrome, ileostomy with chronic acidosis; prevent with hydration, alkalinize urine to pH 6.5 with potassium citrate, allopurinol; (3) CYSTINE: faintly radiopaque (sulfur in cystine), hexagonal crystals, autosomal recessive cystinuria from defective dibasic amino acid transporter (COLA: cystine, ornithine, lysine, arginine); presents in childhood with recurrent stones; prevent with hydration, alkalinize urine, tiopronin or penicillamine; (4) STRUVITE (magnesium ammonium phosphate, 'infection stones'): radiopaque, coffin-lid crystals, forms in ALKALINE urine (pH above 7.5); always associated with urea-splitting organisms (Proteus, Klebsiella, Staphylococcus saprophyticus); classic STAGHORN calculus filling renal pelvis and calyces; treat with antibiotics plus surgical removal (percutaneous nephrolithotomy).
Try a free question
CoreA patient has radiopaque kidney stones shaped like envelopes on microscopy. Urinalysis shows calcium oxalate crystals. Which stone type is most likely?
Show answer and explanation
Answer: A. Calcium oxalate stone. Calcium oxalate stones are common, radiopaque, and classically envelope-shaped.
Why the other choices are tempting
- B. Uric acid stones are radiolucent and often form in acidic urine.
- C. Cystine stones are hexagonal and occur from cystinuria.
- D. Struvite stones are infection stones with urease-positive organisms and can form staghorn calculi.
- E. Cholesterol stones are gallstones, not kidney stones.
Flashcards
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What is the fastest separator for Calcium oxalate / uric acid stones vs Cystine / struvite stones?
Envelope/radiopaque is calcium oxalate; acidic/radiolucent is uric acid; hexagonal is cystine; infection/alkaline/staghorn is struvite.
What clue should make you choose Calcium oxalate / uric acid stones?
Calcium oxalate stones are radiopaque and envelope-shaped; uric acid stones are radiolucent and form in acidic urine.
Quick answers
What is the fastest way to tell Calcium oxalate / uric acid stones from Cystine / struvite stones?
Envelope/radiopaque is calcium oxalate; acidic/radiolucent is uric acid; hexagonal is cystine; infection/alkaline/staghorn is struvite.
What is the key clue for Calcium oxalate / uric acid stones?
Calcium oxalate stones are radiopaque and envelope-shaped; uric acid stones are radiolucent and form in acidic urine.
What is the key clue for Cystine / struvite stones?
Cystine stones are hexagonal from cystinuria; struvite stones are urease-positive infection stones that form in alkaline urine and can become staghorn calculi.
What is the common trap with Calcium oxalate / uric acid stones vs Cystine / struvite stones?
Radiopacity helps, but urine pH, crystal shape, and infection history usually make the stone type.