URIC ACID Uric acid : Is the end product of purine metabolism. It is a wasteproduct derived from purines of the diet and those synthesized in the body. In human, uric acid arises from ingested nucleoproteins,degradation of nucleoproteins, in nuclear material and by synthesis from simple precursors Healthy adult human body contains about 1.1 g of uric acid. Normally about one half of uric acid is eliminated and replaced each day, partly by urinary excretion and partly through destruction in the intestinal tract by microorganisms.

Serum uric acid is freely filtered by the glomeruli and subsequently reabsorbed in the proximal tubules which is then followed by further secretion in the tubules. Uric acid concentration in serum is greatly affected by extrarenal as well as renal factors. Its concentration depends upon the net balance achieved between the rate of synthesis of purines or breakdown of nucleoproteins on one hand and the rate of elimination of uric acid on the other. :Clinical significance Normal levels: Males 0.21 – 0.42 mmol/L (3.5 – 7.0 mg/dl) Females 0.15 – 0.36 mmol/L (2.6 – 6.0 mg/dl) In the presence of normally functioning kidney, the ingestion ofnucleoproteins has little influence on serum uric acid unless very largeamounts are taken. Hyperuricaemia is most commonly defined by serum uric acid concentration over 0.42 mmol/l (7.0 mg/dl) in men, 0.35 mmol/l(6.0 mg/dl) in women.

The major causes of hyperuricaemia are:- 1. Essential hyperuricaemia (gout). 2. Impaired renal excretion: Renal failure, drug therapy e.g.diuretics, alcohol, ketoacidosis 3. Increased nucleic acids turnover: Myeloproliferative disorders e.g. leukaemia, polycythemia. 4. Specific enzyme defect: Deficiency of HGPRT enzyme in Lesch-Nyhan syndrome. Principle Serum proteins are first precipitated using sulfuric acid (0.66 N) and sodium tungstate (10%). Following centrifugation, uric acid in the protein-free filtrate is oxidized to allantoin and carbon dioxide by phosphotungstic acid reagent in alkaline solution supplied by (14%)sodium carbonate. Phosphotungstic acid is reduced in this reaction to tungsten blue which is measured colorimetrically. alkaline Uric acid + Phosphotungstic acid Allantoin + CO2 + Tungsten blue