Serum and urine creatinine and its clearance

Creatinine is the internal anhydride of creatine ,methyl guanidoacetic acid produced by loss of water or phosphoric acid from creatine or phosohocreatine in an irreversible reaction.
This conversion takes place readily on warming in acid solution .In a neutral solution of creatinine there is a slow partial formation of creatine .
Creatine is synthesized in the liver from which it pass into the circulation to be taken up almost entirely by muscle in which it is converted into phosphocreatine .In muscle contraction energy is supplied by the conversion of ATP to ADP. Creatine phosphate is available as the source of a high energy phosphate bond for the immediate re-formation of ATP by the action of creatine kinase.re-synthesis of ATP largely occurs during aerobic glycolysis and requires oxygen .The creatine phosphate thus provides additional synthesis of ATP during muscular contraction under relatively anaerobic conditions.
Following aerobic glycolsis some of ATP is used to reconvert creatine to creatine phosphate .There is some spontaneous conversion of creatine and creatine phosphate into creatine, about 2% of the total each day .This is related to the total muscle mass and so roughly to the body wight and so remines roughly the same from day to day unless the muscle mass changes. Creatinine is not reutilized but rather is excreted from the body via the urine .It is formed at a nearly constant rate which is proportional to the body muscle mass .Because of the way in which creatinine is excreted by the kidney creatinine measurement is used almost exclusively I the assessment of kidney function .In fact creatinine is generally regarded as the most useful naturally occurring substance to measure in the diagnosis and following up of kidney disease .A single, random measurement of serum creatinine may be used as a qualitative and semi-quantitative indicator of impared kidney function ,more quantitative information regarding the extent of kidney damage, specifically damage to the filtration mechanism ,can be gained though the use of the (creatinine clearance test).Creatinine is preasent in the ultrafiltrate of plasma which is formed by the glomerulus of the kidney .The filtered creatinine is not reabsorbed by the kidney tubules. A small amount of creatinine is added to the urine by the process of tubular secretion ,thus the amount of creatinine excreted in the urine is primarily a function of glomerular filtration .Measurment of the amount of creatinine in the urine produced during a specified period of time (urine creatinine concentration x urine volume ) and the plasma creatinine level allows one to calculate a value called the creatinine clearance (clearance means the volume of plasma cleared of creatinine ).This is measured in milliliters of plasma per minute and represent the volume of plasma which must be filtered by the kidney to account for the amount of creatinine found in the urine .The value obtained for creatinine clearance correlates fairly well with more exact measures of glomerular filtration rate (GFR) .
Simultaneous urea nitrogen and creatinine determinations(normal ratio between 15/1 and24/1)appear to have some clinical significance ,creatinine determinations have one advantage over urea determination ,they are not affected by a high protein diet as in the case for urea level. The daily excretion of creatinine is between 9-18 m mol in a normal adult
Varying according to muscular weight .In a given individual on a creatinine-free diet, the out put varied little from day to day .It is not always constant enough ,however, to supply an accurate check on the completeness of a 24 hour urine collection .
Clinical Significance
Serum creatinine is increased in renal failure an elevation of serum creatinine level must represent under excretion .i.e ,kidney impairment Acromegaly, diabetes mellitus, infections .And hypothyroidism increases the urinary out put of creatinine, while anemia, advance renal disease ,leukemia, hyperthyroidism and muscular dystrophy decrease urinary out put of creatinine.