Background. The choice of the correct concentration of potential uremic toxins for in vitro, ex vivo, and in vivo experiments remains a major area of concern; errors at this level might result in incorrect decisions regarding therpeutic correction of uremia and related clinical complications.

Methods. An encyclopedic list of uremic retention solutes was composed, containing their mean normal concentration (C-N), their highest mean/median uremic concentration (C-U), their highest concentration ever reported in uremia (C-MAX), and their molecular weight. A literature search of 857 publications on uremic toxicity resulted in the selection of data reported in 55 publications on 90 compounds, published between 1968 and 2002.

Results. For all compounds, C-U and/or C-MAX exceeded C-N. Molecular weight was lower than 500 D for 68 compounds; of the remaining 22 middle molecules, 12 exceeded 12,000 D. C-U ranged from 32.0 ng/L (methionine-enkephalin) up to 2.3 g/L (urea). C-U in the ng/L range was found especially for the middle molecules (10/22; 45.5%), compared with 2/68 (2.9%) for a molecular weight

Conclusion. Concentrations of retention solutes in uremia vary over a broad range, from nanograms per liter to grams per liter. Low concentrations are found especially for the middle molecules. A substantial number of molecules are protein bound and/or middle molecules, and many of these exert toxicity and are characterized by a high range of toxic over normal concentration (C-U/C-N ratio). Hence, uremic retention is a complex problem that concerns many more solutes than the current markers of urea and creatinine alone. This list provides a basis for systematic analytic approaches to map the relative importance of the enlisted families of toxins.