The main cause of increases in uric acid supersaturation is a low urine pH (Fig. 69-3). The concentration of undissociated uric acid, which is the only form that crystallizes under most circumstances, is controlled by pH.2 Two other factors, low urine volume and hyperuricosuria, are less important. Reasons for a low urine pH include a hereditary predisposition (as in patients with clinical gout and familial uric acid nephrolithiasis),
ileostomy, small-bowel malabsorption, colitis, chronic renal insufficiency, and diets with a high methionine content (so-called acid ash diets).50 The latter is common in calcium stone formers who eat large amounts of meat. Dehydration, from any cause, increases the urine uric acid concentration and increases supersaturation at any given pH.