Uric acid is the major product of the catabolism of endogenous and exogenous (dietary) purine nucleosides (adenosine and guanosine). This transformation mainly occurs in the liver. Approximatively 75% of uric acid is eliminated by kidneys; the remainder is secreted into the gastrointestinal tract, where it is degraded by bacterial enzymes. Uric acid is not very soluble in water; urate crystals can occur in urines when the concentration is abnormally high. It can also happen in plasma, crystals then deposite essentially in joints, which induce intense inflammatory responses (gout). Some causes for increasing uric acid rate in serum are: increasing of purines synthesis, metabolic disorders (Lesch-Nyhan syndrome for example), nutritional troubles, increasing of nucleic acid turn-over in case of proliferation of tumor cells, leukaemia, psoriasis, cytotoxic drugs, renal failures… Decreasing of uric acid rate in serum is more uncommon. It can occur in different cases: failure in renal elimination of uric acid (Fanconi syndrome), Hodgkin’s disease for example.