Depletion of Uric Acid by Continuous Kidney Replacement Therapy

Uric acid levels are commonly elevated in patients maintained on hemodialysis because hemodialysis provides a lower time-average uric acid clearance than the native kidney. However continuous kidney replacement therapy (CKRT) provides higher time-averaged clearances of small solutes than hemodialysis. We therefore examined whether the high small solute clearance provided by CKRT would reduce plasma uric acid levels below normal.

We measured plasma levels of uric acid and other selected solutes in 10 patients initiating CKRT and in 17 patients maintained on CKRT for 5 - 110 days. We estimated solute generation rates from their removal rates. The removal rates of uric acid and phosphate were estimated from their plasma levels while those of urea and creatinine were calculated directly from their effluent levels. CKRT prescription met current guidelines (24 ± 2 ml/kg/hr in patients initiating CKRT and 26 ± 3 ml/kg/hr in patients maintained longer on CKRT).

Plasma uric acid levels declined from 8.7 ± 3.0 mg/dL to 1.8 ± 0.5 mg/dL after 2 to 4 days of initiating CKRT (p < 0.001) and were similarly low at 1.7 ± 0.6 mg/dL during ongoing CKRT. Plasma uric acid levels at both intervals were below the normal range. The average uric acid removal rate was not different at 2-4 days post-initiation (922 ± 367 mg/day) and during ongoing CKRT (855 ± 346 mg/day).

CKRT prescribed according to current guidelines reduces uric acid levels below normal. The clinical consequences of low uric acid levels in this setting are unknown.