Creatinine: Definition, Biological Relevance, Clinical Significance, and Urine Levels

It is a waste product that comes from the normal wear and tear of the body’s muscles.

The more muscle a person has, the more creatinine they produce. Creatinine levels in the blood reflect both the amount of muscle a person has and their amount of kidney function.

Most men with normal kidney function have about 0.6 to 1.2 milligrams / deciliters (mg / dL) of creatinine. Most women with normal kidney function have between 0.5 and 1.1 mg / dl of creatinine.

Women generally have lower creatinine levels than men because women, on average, have less muscle than men.

Other factors that can affect blood creatinine level include body size, activity level, and medications.

Biological relevance

Serum creatinine (a measure of the blood) is an important indicator of kidney health , as it is a by-product of muscle metabolism that is easily excreted unchanged by the kidneys.

Creatinine itself is produced through a biological system involving creatine, phosphocreatine (also known as creatine phosphate), and adenosine triphosphate (ATP, the body’s immediate energy supply).

Creatine is mainly synthesized in the liver from the methylation of glucocyamine (guanidine acetate, synthesized in the kidney from the amino acids arginine and glycine) by S-adenosyl methionine.

It is then transported through the blood to other organs, muscles, and the brain, where, through phosphorylation, it is converted to the high-energy compound phosphocreatine.

Creatine kinase catalyzes the conversion of creatine to phosphocreatine; spontaneous creatinine formation occurs during the reaction.

Creatinine is removed from the blood primarily by the kidneys, primarily by glomerular filtration, but also by proximal tubular secretion. There is little or no tubular reabsorption of creatinine.

If the filtration in the kidney is poor, creatinine levels in the blood increase.

Therefore, blood and urine creatinine levels can be used to calculate creatinine clearance (CrCl), which roughly correlates with glomerular filtration rate (GFR).

Blood creatinine levels can also be used alone to calculate estimated GFR (eGFR).

Diagnostic use

Serum creatinine is the most widely used indicator (but it is not a direct measure) of kidney function.

Elevated creatinine is not always representative of a true reduction in glomerular filtration rate.

A high reading may be due to increased creatinine production not due to decreased kidney function, interference with the assay, or decreased tubular creatinine secretion.

An increase in serum creatinine may be due to a higher intake of cooked meat (which contains creatinine converted from creatine by the heat of cooking) or an excessive intake of protein and creatine supplements, taken to enhance athletic performance.

Intense exercise can increase creatinine by increasing muscle breakdown. Dehydration secondary to an inflammatory process with fever can cause a false increase in creatinine levels unrelated to actual kidney injury, as in some cases with cholecystitis .

Various drugs and chromogens can interfere with the assay. The secretion of creatinine by the tubules can be blocked by some medications, again increasing the measured creatinine.

Creatinine in urine

Males produce approximately 150-200 micromoles of creatinine per kilogram of body weight per 24 hours, while females produce approximately 100-150 mcmols / kg / 24h. Under normal circumstances, all of this daily creatinine production is excreted in the urine.

Creatinine concentration is checked during standard urine drug tests.

An expected creatinine level indicates that the test sample is not being diluted, while low amounts of creatinine in the urine indicate a tampered test or initial low creatinine levels.

Test samples considered tampered with due to low creatinine are not tested, and the test is sometimes considered unsuccessful.