Serum total p-cresol and indoxyl sulfate (i.e. combined free and protein-bound fractions) were analyzed with High-performance liquid chromatography (HPLC). Briefly, serum samples were deproteinized for the determination of indoxyl sulfate by the addition of three parts methanol to one part serum. Total p-cresol (i.e. the combined free and protein-bound fraction) was analysed after deproteinization (acid and heat) and extraction (ethyl acetate) of serum samples.