creatinine in 24 hour urine

Reference interval

• 0.5-2.0 g/24 hours

Principle

Creatinine is synthesized in a two-step process involving the initial synthesis of guanidoacetate (glycocyamine), which takes place in the kidneys, small intestinal mucosa, pancreas, & probably liver. This reaction between glycine & arginine is catalyzed by a transamidase, which is subject to feedback inhibition by increased creatine.. Guanidoacetate is transported to the liver, where it is methylated to creatine. Creatine then enters the blood to be widely distributed, chiefly to muscle cells, which contain about 98 per cent of the total body creatine pool. The body content of creatine is proportional to the muscle mass.

Creatinine is formed as a result of non-enzymatic dehydration of muscle creatine. Free creatinine is not reutilized in the body's metabolism & thus functions solely as a waste product of creatine. Formation of creatinine is reasonably constant, & 1.5 to 1.6 per cent of the creatine is so transformed every 24 hours. Consequently, creatinine formation also has a direct relationship to muscle mass.

On the ACA the creatinine method employs a modification of the kinetic Jaffe reaction reported by Larsen. In the presence of a strong base such as NaOH, picrate reacts with creatinine to form a red chromophore. The rate of increasing absorbance at 510 nm due to the formation of this chromophore during a 17.07-second measurement period is directly proportional to the creatinine concentration in the sample.

Clinical significance

Both serum creatinine & urine creatinine have been used as indices of renal function because of the constancy of creatinine formation. Creatinine is freely filtered at the glomerulus & is not reabsorbed by the tubules. A small amount of creatinine in the final urine is derived from tubular secretion. Because of these properties of creatinine, the creatinine clearance can be used to estimate the GFR (glomerular filtration rate). More than two thirds of the GFR may be lost in the course of chronic renal disease with few clinical symptoms & biochemical abnormalities.

Total creatinine measurement is commonly used as an index of the completeness of 24-hour urine collections. However, some investigators have concluded that this is an unreliable practice.

Increases

• glomerulonephritis
• high meat diet
• muscular dystrophy (late stage)
• myasthenia gravis
• pyelonephritis
• rhabdomyolysis

Decreases

• renal failure
• rerenal azotemia
• reduced kidney blood flow
  • shock
  • congestive heart failure
• urinary tract obstruction

Specimen

2 mL aliquot of a 24-hour urine collection. Proper 24 hour urine collection procedure should be followed, & collection container should be refrigerated at 2-6 C during collection. Upon receipt in the work area, it should be well-mixed & measured in a graduated cylinder. The total volume should be recorded.

Interferences

• pharmaceuticals
  • cephalosporins (cefoxitin)
  • cimetidine
  • cisplatin
  • gentamicin
  • trimethoprim

More general terms

• 24 hour urine
• creatinine in urine

Component of

• creatinine clearance
• 17-hydroxycorticosteroids/creatinine in 24 hour urine
• cystine/creatinine in 24 hour urine
• histamine/creatinine in 24 hour urine
• selenium/creatinine in 24 hour urine
• albumin/creatinine in 24 hour urine
• cadmium/creatinine in 24 hour urine
• renal function tests; renal function panel