Creatinine

Creatinine is a waste product produced during the normal breakdown of creatine phosphate in skeletal muscle. Its production is relatively constant and largely depends on an individual’s muscle mass. Creatinine is cleared from the blood mainly by the kidneys through glomerular filtration, with a small amount secreted by the renal tubules.

Serum creatinine concentration is a widely used indicator of kidney function. Because production varies with muscle mass, levels are interpreted in the context of age, sex, and body size. Kidney function is often estimated from serum creatinine to derive the estimated glomerular filtration rate (eGFR) using equations such as CKD-EPI or MDRD. In some cases, creatinine clearance obtained from a 24-hour urine collection is used. Laboratory methods include enzymatic assays and the traditional Jaffe reaction, with results reported in mg/dL or μmol/L and standardized to IDMS.

Normal reference ranges vary by laboratory, but typical adult serum creatinine is about 0.6–1.2 mg/dL (60–110 μmol/L) in men and 0.5–1.1 mg/dL (45–100 μmol/L) in women. Factors that raise creatinine without kidney disease include high muscle mass, older age, diet (notably red meat), vigorous exercise, and dehydration. Conversely, low muscle mass, malnutrition, or certain diseases can yield lower levels.

Elevated serum creatinine or reduced eGFR indicates decreased renal function and may reflect acute kidney injury or chronic kidney disease. eGFR is used for CKD staging, though creatinine-based measures have limitations due to nonrenal influences on creatinine production and the renal tubular secretion that can affect accuracy.