Uric acid is the major determinant of absorbance in spent dialysate allowing spectrophotometric evaluation of dialysis dose.

Background: Ultraviolet (UV) absorbance of spent dialysate has been proposed as a method for monitoring hemodialysis efficiency. The contribution of the various uremic toxins to UV absorption, however, needs clarifying.

Methods: Urea, creatinine and uric acid were measured in blood and dialysate before and during dialysis in 22 maintenance hemodialysis patients. Absorbance was measured in dialysate.

Results: High-performance liquid chromatography (HPLC) analyses of dialysate revealed uric acid as the predominant peak. Spent dialysate absorbance decreased, during dialysis, similarly to serum and dialysate urea, creatinine and uric acid. Dialysate urea correlated closely with absorbance, though urea did not contribute to absorbance, which was determined mostly by uric acid. Uric acid and urea removals were very similar. The spectrophotometric Kt/V correlated with spKt/V urea, with slight but significant differences between the two measurements.

Conclusions: UV absorbance is determined mostly by uric acid. Absorbance measurements seem suitable as a method for monitoring dialysis efficiency.