Creatinine and GFR (Kidney Function Test)
Definition
Creatinine is a normal waste product formed when muscles are used and a small part of the muscle tissue breaks down. Healthy kidneys filter this substance out of the blood and excrete it in the urine.
Purpose
Evaluates kidney function. Used in the diagnosis of kidney disease, in monitoring existing kidney problems and in detecting side effects of medications that affect the kidneys.
Results
The creatinine level in the blood is used to calculate the estimated glomerular filtration rate (eGFR), which shows how efficiently the kidneys filter waste. Because the eGFR calculation also includes age, weight, height and sex, it is more accurate than a creatinine measurement alone. High values may suggest kidney disease or damage; low values may suggest malnutrition or muscle loss.
Normal Values
Laboratory-specific. Values vary with muscle mass, diet, age and physical activity, so no single universal range can be given.
Risks and Limitations
A normal creatinine result does not necessarily mean the kidneys are healthy; values can be normal in early-stage kidney disease. Muscle mass, meat consumption, age, intense exercise, dehydration and pregnancy affect the results. The current KDIGO 2024 guideline recommends using creatinine together with cystatin C (eGFRcr-cys) to improve accuracy; for European populations, the EKFC equation is preferred over the race-free CKD-EPI 2021 equation.