INULIN CLEARANCE

procedure by which the filtering capacity of the glomeruli (the main filtering structures of the kidney) is determined by measuring the rate at which inulin, the test substance, is cleared from blood plasma. Inulin is the most suitable and accurate substance to measure because it is a small, inert polysaccharide molecule that readily passes through the glomeruli into the urine without being reabsorbed by the renal tubules. The steps involved in this measurement, however, are quite involved; consequently, inulin is seldom used in clinical testing, although it is used in research. Creatinine clearance (q.v.) is the more common procedure used to assess renal function. The average rate at which substances are filtered out of the plasma (the glomeruli filtration rate) is about 115 to 150 millilitres per minute for women and 65 ml per minute for men. The rate decreases with age. It is markedly reduced in such conditions as acute glomerulonephritis (also called Bright's disease), which is characterized by inflammation of the small blood vessels that loop through the glomeruli.