Creatinine urine test
Creatinine — is the end product of metabolism of creatine. Creatinine formed in the body from creatine, which is found mainly in muscle tissue where it is derived — phosphocreatine, serves as a reserve expended in muscle contraction. Normally, there is no creatine in urine. By increasing its concentration in the blood over 120 µmol/l, it appears in the urine.
Creatinine refers to low molecular weight ("non-threshold") substances. Filtered by the glomerulus, a part of the primary urine, but not reabsorbed. Therefore its concentration in plasma and urine in the same primary. Creatinine, ringing in the filtrate is excreted in the final urine.
Urinary excretion of creatine was observed in the period of puberty (14-16 years) — physiological creatinuria, as a result of active synthesis of creatine, due to the necessity of it is muscle. Creatinuria in the elderly is a consequence of muscle atrophy and incomplete use of synthesized in the liver creatine.
Creatine may appear in the urine of healthy people with severe physical work, in children with carbohydrate starvation.
Disorder of creatine-creatinine metabolism
Disorder of creatine-creatinine metabolism can occur in case of:
- liver disease
- infectious diseases
- muscle diseases and other pathologies
Muscular dystrophy (myopathy of various origins) and radiation disease characterized by the appearance in the urine of creatine and creatinine.
Creatinine is removed from the body not only glomerular filtration, but also tubular secretion the. The increasing of creatinine in serum with the progression of renal failure can lead to the increase of its tubular secretion. The decrease of creatinine in urine is observed in various diseases, based on genesis which is a impaired conversion of creatine to creatinine.
By the clearance of endogenous creatinine measure the amount of glomerular filtration, which in healthy people varies from 80 to 160 ml/min.
The ratio of the concentration of creatinine in the urine and blood is the concentration index (CI), which characterizes the concentration renal function, its value is normal more than 60.
The calculation of the concentration index is performed by the formula:
U - the concentration of creatinine in final urine,
Р - the concentration of creatinine in plasma.
The difference between the volume of primary urine (glomerular filtration) and minute diuresis corresponds to the volume of tubular reabsorption, which is expressed by a relative value:
R% = (F-V)/Fx 100, where
F - the value of glomerular filtration,
V - minute volume of urine output.
It normally relative valu more than 97%.
The calculation of the endogenous creatinine clearance depends on the amount of urination. The amount of filtration endogenous creatinine may increases by water load, eating lots of meat, and reduces by the lack of sodium and potassium in food. A low urine output can underestimate the value of filtration as a result of the reabsorption of creatinine from the primary urine.
The value of glomerular filtration estimate the mass of existing nephrons in kidney diseases.