The uremic syndrome can be defined as the deterioration of multiple biochemical and physiological functions in parallel with progressive renal failure, thereby resulting in complex but variable symptomatology [1-7]. Normally, healthy kidneys excrete a myriad of compounds. Uremic retention solutes accumulate in the patient with chronic kidney disease (CKD), including the patient with K/DOQI stage 5 disease or end-stage renal disease (ESRD) . The retention of these solutes is directly or indirectly attributable to deficient renal clearance.
These retained solutes are called uremic toxins when they contribute to the uremic syndrome. Only a few solutes have an established role. Apart from inorganic compounds, only a few compounds conform to the strictest definition of uremic toxins. However, this does not preclude a potential role for various other retention solutes. (See "Overview of the management of chronic kidney disease in adults".)
Uremic toxins can be subdivided into three major groups based upon their chemical and physical characteristics:
- Small, water-soluble, non-protein-bound compounds, such as urea
- Small, lipid-soluble and/or protein-bound compounds, such as the phenols
- Larger so-called middle-molecules, such as beta2-microglobulin
Views on the uremic syndrome and several uremic solutes have changed substantially during the last two decades [5,9]. The current state of knowledge about the biochemical, physiological, and/or clinical impact of those compounds is presented here.