What is creatinine?

Creatinine is a compound that is generated from the degradation or metabolism of creatine.Creatinine is the result of the normal metabolism of creatine. It is considered a metabolic waste product. Its production is constant and depends on the muscle mass present. Normally, it is filtered by the kidneys and excreted in urine.

Metabolism of creatine.

Creatine is primarily produced in the liver and kidneys, with a portion produced in the pancreas. Its synthesis uses the amino acids arginine, glycine, and methionine as precursors. Creatine is mostly stored in the body's muscles. Its storage, along with phosphorus, allows for the formation of phosphocreatine. This process is mediated by creatine kinase.

During intense anaerobic exercise, phosphocreatine represents the first source of energy for the muscles.Creatine can donate and receive its phosphate group to ATP. The enzyme creatine phosphokinase is responsible for this process. During physical exertion, phosphocreatine donates its phosphate group to ADP molecules to form ATP. Once used, creatine is converted to its metabolic waste state, creatinine.

Diagnostic utility.

Blood sample.

Creatinine present in serum is used as an indicator of kidney function. It is filtered by the nephrons and excreted by the kidneys. Therefore, an elevation of creatinine indicates significant damage to the nephrons. It is important to note that creatinine levels rise with marked damage at the nephron level. Thus, it is not useful for detecting early stages of kidney disease.

Reference values for women are 0.6 to 1.1 mg/dL. Typical reference values for men are 0.8 to 1.3 mg/dL. It is important to consider that these values depend not only on kidney function but also on muscle mass. As mentioned earlier, creatinine is stored in muscles. Individuals with greater muscle mass will have higher physiological values. Bodybuilders, for example, may have a concentration of 2.0 mg/dL.

Conversely, the lower the muscle mass, the lower the creatinine values should be. An elderly woman with a value greater than 0.7 mg/dL may indicate kidney disease.

Urine sample

Creatinine in urine is the counterpart of its presence in serum. A healthy nephron filters creatinine from the blood into the urine. A low concentration of creatinine in urine may indicate advanced kidney disease.

For the test, a 24-hour urine sample is used. This means that urine collected over a 24-hour period is analyzed. Creatinine values can range from 500 to 2,000 mg/day. This depends on age and the amount of muscle mass.

The reference values for men are 14 to 26 mg per kg of body mass per day, while for women the reference value is 11 to 20 mg per kg of body mass per day.

Creatinine clearance

Urine and blood samples have diagnostic utility separately and when combined. Blood and urine samples are compared to obtain the creatinine clearance.This indicates the state of glomerular filtration. Clearance is used to assess the degree of renal insufficiency present in the patient.

To obtain the glomerular filtration rate, it is necessary to compare the concentration of creatinine in urine with the concentration in blood. It should be divided by the body surface area of the person to obtain the filtration result in milliliters/minute.

There are several empirical formulas that allow for a quicker approximation of the glomerular filtration rate. One of the most well-known approximate formulas is the Cockcroft-Gault formula. However, it should be noted that these are less precise.

The levels of clearance or glomerular filtration vary with age and muscle mass. The reference values for men are 97 to 137 mL/min. The reference levels for women are 88 to 128 mL/min. These values may vary slightly between different laboratories.