Uric Acid Kidney Stones
Uric acid kidney stones form mainly when urine is persistently acidic and low in volume, conditions common in people with gout and hyperuricemia. Because uric acid is a weak acid that comes out of solution at low pH, these stones can form even at normal uric acid levels and are often invisible on plain X-rays.
Uric acid kidney stones (uric acid nephrolithiasis) are a type of kidney stone that forms when uric acid crystallizes in the urinary tract. Uric acid is a weak acid with a pKa of about 5.3, so the single most important driver is persistently low urine pH (acidic urine): below that pH, uric acid stays in its poorly soluble undissociated form rather than the much more soluble urate ion, and crystals form once its concentration climbs high enough. Three urinary abnormalities predispose to these stones: persistently acidic urine (the dominant factor), low urine volume from dehydration, and high urinary uric acid excretion. Notably, because acidity matters so much, stones can form even when blood uric acid is normal, though hyperuricemia and gout markedly raise the risk; people with gout characteristically have acidic urine. Conditions causing fluid loss, such as diarrhea or inflammatory bowel disease, lower urine volume and pH and further increase risk. Symptoms overlap with other urinary problems: dysuria (painful urination), hematuria (blood in urine), flank pain, and crystalluria. A diagnostic quirk is that uric acid stones are typically radiolucent, meaning they do not show up on conventional X-rays (they lack calcium), so they are easily missed without ultrasound or CT. Because high uric acid can cause both Gout: Inflammatory Arthritis from Urate Crystals and these stones, hyperuricemia is worth flagging to a clinician investigating unexplained urinary symptoms. Prevention centers on generous water intake and, when indicated, raising urine pH. See Hyperuricemia: Elevated Uric Acid in the Blood. This is general medical information, not personalized advice.