Monitoring urea in spent dialysate by ultraviolet (UV) spectroscopy is a useful noninvasive method for reagent-free, real-time assessment of the kinetics of uremic substances during hemodialysis. This study investigated the UV absorption properties of creatinine (Cr) and uric acid (UA), which are uremic nitrogen compounds like urea, and the application of UV detection of these compounds to the monitoring of spent dialysate. We found that the UV absorbance of urea from 230nm to 300nm was less than 0.001. On the other hand, the absorbance spectrum of Cr had a large peak at wavelength 236nm, while the spectrum of UA showed two peaks at 236nm and 290nm. There was a significant correlation between the actual concentration in spent dialysate and that predicted from the peak value at 236nm for Cr (r=0.96, n=27) and UA (r=0.88, n=27). In addition, there was a significant correlation between the actual concentration of UA in spent dialysate and that predicted from the peak value at 290nm (r=0.99, n=27), and the correlation was stronger at 290nm than at 236nm. Furthermore, at wavelength 290nm, there was a significant correlation between actual and predicted concentrations for both Cr (r=0.89) and urea (r=0.91, n=27), suggesting that low molecular weight uremic substances in spent dialysate can be monitored at this wavelength. These findings indicate that monitoring Cr at 236nm and UA at 290nm in spent dialysate is more suitable than monitoring urea.
