Creatinine is a byproduct produced from the metabolism of creatine in muscle. Its plasma concentration is relatively constant and freely filtered at the glomerulus.
Its concentration is inverse to the GFR, i.e., higher plasma creatinine concentrations are present with lower GFR.
Creatinine is an imperfect biomarker; it exhibits great interindividual variability and is affected by numerous non-renal physiologic factors, including:
- Muscle mass (more muscle, higher creatinine)
- Diet (e.g., vegans have lower levels than those on a diet high in animal meat)
- Non-renal disease (e.g., sickle cell)
- Medications (may alter secretion and/or resorption)
- Some extrarenal clearance
