Expansion
Creatinine rises only after substantial loss of filtration, and depends on muscle mass
Expansion
Creatinine is produced at a constant rate from muscle, freely filtered, and only slightly secreted, which makes it a convenient marker. But it has important limitations:
- Hyperbolic relationship with GFR: a fall from 120 to 60 ml/min may leave creatinine within the normal range, the so-called creatinine blind range
- Depends on muscle mass: low in the elderly, women, amputees, and in cirrhosis and malnutrition, so a “normal” value may conceal poor function
- Rises with high protein or cooked meat intake
- Drugs such as trimethoprim, cimetidine and dolutegravir block tubular secretion and raise creatinine without changing GFR
- Rises slowly in acute kidney injury, lagging the true fall in filtration by 24 to 48 hours
eGFR equations adjust for age, sex and race, but assume a steady state and are unreliable in acute injury and at extremes of body habitus.
Cystatin C is independent of muscle mass and is an alternative where creatinine is misleading.