Calcium oxalate stone vs Uric acid stone
How to tell Calcium oxalate stone and Uric acid stone apart when a board question makes them look alike.
Step 2 CK pairs are in open beta while clinical review finishes. Spot an issue? Tell us.
Persistently acidic urine pH below 5.5 with rhomboid crystals points to uric acid stone.
Side by side
Acute colicky flank pain with radiopaque stone and envelope or dumbbell crystals suggests calcium oxalate nephrolithiasis.
Flank pain with persistently acidic urine and rhomboid crystals, often with radiolucent stone on plain film, suggests uric acid stone.
Supersaturation of urinary calcium and oxalate leads to crystal nucleation, especially with low urine volume and hypercalciuria.
Purine overproduction or high cell turnover raises uric acid load, and acidic urine drives uric acid precipitation.
Urinalysis may show envelope crystals, urine pH can vary, and noncontrast CT shows a hyperdense stone; 24-hour urine often reveals hypercalciuria above 200 mg/day.
Urine pH is usually below 5.5 with rhomboid or rosette crystals, serum uric acid may be elevated, and stones are often not seen on KUB radiograph despite CT detection.
The trap
A negative KUB film is misread as no stone, but uric acid calculi are often radiolucent; noncontrast CT and urine pH are the decisive tests, and alkalinization can dissolve uric acid stones without procedure when stable.
The repair move
Next best step: confirm stone burden with noncontrast CT if diagnosis is uncertain, control pain with ketorolac 15 to 30 mg IV, and hydrate. If urine is acidic with uric acid stone pattern, start potassium citrate 20 mEq orally two to three times daily to target urine pH 6.0 to 6.5; add allopurinol 300 mg daily when hyperuricemia or recurrent uric acid stones persist.
Try a free question
CoreA 49-year-old man with gout develops severe left flank pain. BP 150/92 mm Hg, HR 104/min, temp 37.0 C. Urinalysis shows hematuria, pH 5.1, and rhomboid crystals. KUB radiograph is negative. What is the next best step in management?
Show answer and explanation
Answer: A. Begin potassium citrate to alkalinize urine. The acidic urine and crystal morphology indicate uric acid stone physiology, so alkalinization is targeted treatment in a stable patient.
Why the other choices are tempting
- B. Wrong because thiazides are preventive for hypercalciuric calcium stones, not first acute treatment for uric acid crystallization.
- C. Wrong because loop diuretics increase calciuria and do not treat the uric acid mechanism.
- D. Wrong because emergent decompression is reserved for obstructed infected systems, sepsis, or solitary kidney compromise.
- E. Wrong because untreated pain and persistent acidic urine leave the stone process unaddressed.
Flashcards
Tap a card to flip it.
What urine pH threshold strongly favors uric acid nephrolithiasis over calcium oxalate stone?
A urine pH below 5.5 that remains acidic supports uric acid stone formation.
Which crystal shape in urine sediment supports calcium oxalate stones?
Envelope or dumbbell crystals are classic for calcium oxalate stones.
Quick answers
What is the fastest way to tell Calcium oxalate stone from Uric acid stone?
Persistently acidic urine pH below 5.5 with rhomboid crystals points to uric acid stone.
What is the key clue for Calcium oxalate stone?
Acute colicky flank pain with radiopaque stone and envelope or dumbbell crystals suggests calcium oxalate nephrolithiasis.
What is the key clue for Uric acid stone?
Flank pain with persistently acidic urine and rhomboid crystals, often with radiolucent stone on plain film, suggests uric acid stone.
What is the common trap with Calcium oxalate stone vs Uric acid stone?
A negative KUB film is misread as no stone, but uric acid calculi are often radiolucent; noncontrast CT and urine pH are the decisive tests, and alkalinization can dissolve uric acid stones without procedure when stable.