Background: The uraemic retention solutes p-cresylsulphate (pCS) and p-cresylglucuronide (pCG), two conjugates of p-cresol, were never determined simultaneously. In the present paper, a high-performance liquid chromatography (HPLC) method was developed and used to quantify both compounds in parallel in an in vivo observational study and their in vitro effect was evaluated by flow cytometry.

Methods: pCS and pCG were determined in serum. For the validation specificity, linearity, recovery, precision and the quantification limit were evaluated. In vivo, concentrations of both compounds were determined in 15 controls and 77 haemodialysis patients, as well as protein binding in the dialysed group and the reduction ratios during haemodiafiltration. In addition, the in vitro effect of the solutes on leucocyte free radical production at measured concentrations was assessed.

Results: A fast and accurate HPLC method was developed to simultaneously quantify pCS and pCG. Both conjugates are retained in uraemia with a substantially higher total serum pCS in comparison to pCG (31.4 ± 15.8 versus 7.3 ± 6.5 mg/L) but also a substantial difference in protein binding (92.4 ± 3.0 versus 8.3 ± 4.4%) and in reduction ratio during post-dilution haemodiafiltration (37.4 ± 7.1 versus 78.6 ± 6.4%). pCG per se has no effect on leucocyte oxidative burst activity, whereas in combination with pCS, a synergistic activating effect was observed.

Conclusions: Serum concentrations of pCS and pCG are elevated in uraemia. Both conjugates show a different protein binding, resulting in a different dialytic behaviour. Biologically, both conjugates are synergistic in activating leucocytes.