Forms:
(1) uric acid
(2) uric acid dihydrate (requires very low urine pH)
The pK for uric acid and urate. If the urine pH is < 5.5 then more uric acid is formed. Uric acid is much less soluble than urate.
Risk factors:
(1) older age (often > 60 years)
(2) male gender
(3) obesity, often with glucose intolerance or diabetes
(4) diet rich in purines, proteins and alcohol
(5) gout with hyperuricemia
(6) low urine pH from a high acid load or decreased alkali
(7) dehydration
Imaging studies:
(1) X-ray: radiolucent
(2) ultrasonography: high density and typical echo
(3) spiral CT: visible
Urine findings in uric acid stones:
(1) acid pH (< 6.0, often < 5.5)
(2) usually an increased concentration of uric acid (hyperuricosuria), with uric acid >= 4.0 mmol per 24 hours
(3) uric acid crystals in the urine sediment
(4) stone analysis shows increased uric acid
