creatinín

Creatinine is a breakdown product of creatine phosphate in muscle, produced at a relatively constant rate that roughly reflects an individual’s muscle mass. It is transported in the blood to the kidneys for excretion in urine.

In the kidneys, creatinine is freely filtered by the glomeruli and is also secreted slightly by the proximal tubules. As a result, its renal clearance is close to, but typically slightly greater than, the glomerular filtration rate (GFR). Because of this, serum creatinine and urine creatinine measurements are used to estimate kidney function, and equations such as CKD-EPI and MDRD are commonly employed to calculate estimated GFR (eGFR). In some cases, creatinine clearance is determined from a 24-hour urine collection.

Clinically, serum creatinine is used to screen for and monitor kidney disease, to stage chronic kidney disease, and to guide dosing of medications eliminated by the kidneys. It is also used to detect acute changes in kidney function, although serum creatinine can be influenced by factors other than GFR, particularly muscle mass and dietary intake.

Reference ranges for serum creatinine vary by age, sex, and body size, and lab methods differ. Typical adult ranges are approximately 0.7–1.3 mg/dL (65–119 μmol/L) in men and 0.5–1.0 mg/dL (46–88 μmol/L) in women, but values must be interpreted in context. Factors that can raise or lower creatinine independent of kidney function include high muscle mass, malnutrition, age, race, and certain drugs that affect tubular secretion or creatinine production.

Limitations include that creatinine is an imperfect proxy for GFR, especially in early kidney disease or atypical muscle mass. In such cases cystatin C or alternative markers may be used to improve assessment.