Uric acid stones account for 5 to 10 percent of urinary tract stones in the United States and Europe. In contrast, they comprise 40 percent or more of stones in areas with hot, arid climates in which the tendency to a low urine volume and acid urine pH promote uric acid precipitation. The risk of both uric acid and calcium stones is also increased in patients with gout. (See "Clinical manifestations and diagnosis of gout", section on 'Nephrolithiasis' and 'Gout' below and "Risk factors for calcium stones in adults", section on 'Hyperuricosuria' and "Risk factors for calcium stones in adults", section on 'Other factors'.)
Issues related to uric acid stone disease will be reviewed here. The general approach to the patient with nephrolithiasis and other management issues related to hyperuricemia and gout are discussed separately. (See "Diagnosis and acute management of suspected nephrolithiasis in adults" and "Evaluation of the adult patient with established nephrolithiasis and treatment if stone composition is unknown" and "Clinical manifestations and diagnosis of gout" and "Asymptomatic hyperuricemia".)
The two major factors that promote uric acid precipitation are a high urine uric acid concentration and an acid urine pH, which drives the following reaction toward the right, converting the relatively soluble urate salt into insoluble uric acid [1,2].
H+ + Urate- ↔ Uric acid
A low urine pH is the more significant of these two biochemical risk factors for the development of uric acid nephrolithiasis [3-9]. Uric acid solubility in the urine falls from approximately 200 mg/dL (1.2 mmol/L) at a urine pH of 7 (a setting in which 95 percent of uric acid is present as the more soluble urate anion) to 15 mg/dL (0.09 mmol/L) at a urine pH of 5 (a setting in which most of the uric acid is the less soluble, undissociated acid) [1,10].