Concentration of creatinine

Serum creatinine (a blood measurement) is an important indicator of renal health because it is an easily measured by-product of muscle metabolism that is excreted unchanged by the kidneys. Creatinine itself is produced via a biological system involving creatine, phosphocreatine (also known as creatine phosphate), and adenosine triphosphate (ATP, the body's immediate energy supply).

Creatine is synthesized primarily in the liver from the methylation of glycocyamine (guanidino acetate, synthesized in the kidney from the amino acids arginine and glycine) by S-adenosyl methionine. It is then transported through blood to the other organs, muscle, and brain, where, through phosphorylation, it becomes the high-energy compound phosphocreatine. During the reaction, creatine and phosphocreatine are catalysed by creatine kinase, and a spontaneous conversion to creatinine may occur.

Creatinine is removed from the blood chiefly by the kidneys, primarily by glomerular filtration, but also by proximal tubular secretion. Little or no tubular reabsorption of creatinine occurs. If the filtration in the kidney is deficient, creatinine blood levels rise. Therefore, creatinine levels in blood and urine may be used to calculate the creatinine clearance (CrCl), which correlates with the glomerular filtration rate (GFR). Blood creatinine levels may also be used alone to calculate the estimated GFR (eGFR).

The GFR is clinically important because it is a measurement of renal function. However, in cases of severe renal dysfunction, the CrCl rate will overestimate the GFR because hypersecretion of creatinine by the proximal tubules will account for a larger fraction of the total creatinine cleared.[5] Ketoacids, cimetidine, and trimethoprim reduce creatinine tubular secretion and, therefore, increase the accuracy of the GFR estimate, in particular in severe renal dysfunction. (In the absence of secretion, creatinine behaves like inulin.)

An alternate estimation of renal function can be made when interpreting the blood (plasma) concentration of creatinine along with that of urea. BUN-to-creatinine ratio (the ratio of blood urea nitrogen to creatinine) can indicate other problems besides those intrinsic to the kidney; for example, a urea level raised out of proportion to the creatinine may indicate a prerenal problem such as volume depletion.

Each day, 1-2% of muscle creatine is converted to creatinine. Men tend to have higher levels of creatinine than women because, in general, they have a greater mass of skeletal muscle. Increased dietary intake of creatine or eating a lot of meat can increase daily creatinine excretion.

Serum creatinine level

Creatinine usually enters your bloodstream and is filtered from the bloodstream at a generally constant rate. The amount of creatinine in your blood should be relatively stable. An increased level of creatinine may be a sign of poor kidney function.

Serum creatinine is reported as milligrams of creatinine to a deciliter of blood (mg/dL) or micromoles of creatinine to a liter of blood (micromoles/L). The typical range for serum creatinine is:

For adult men, 0.74 to 1.35 mg/dL (65.4 to 119.3 micromoles/L)

For adult women, 0.59 to 1.04 mg/dL (52.2 to 91.9 micromoles/L)

What creatinine levels indicate kidney failure?

Creatine levels alone don’t determine if you have kidney disease. Your healthcare provider uses many tests to make a diagnosis of kidney failure. A serum creatinine test (uses blood only) can give clues to how your kidneys are working. These results typically come in milligrams per deciliter (mg/dL). Your age, muscle mass and assigned sex help determine your level. Generally, a normal creatinine level is:

Up to 1.3 mg/dL (milligrams per deciliter) for people assigned male at birth (AMAB).

Up to 1.1 mg/dL for people assigned female at birth (AFAB).

References:

http://en.wikipedia.org/wiki/Creatinine 

Merck Index, 11th Edition, 2571

http://www.medicinenet.com/creatinine_blood_test/article.htm

https://my.clevelandclinic.org/health/diagnostics/16380-creatinine-clearance-test