What it measures.
Creatinine is a waste product from muscle metabolism, filtered by the kidneys and excreted in urine. It's the standard marker for kidney function (eGFR), but levels also reflect muscle mass. Very low creatinine may indicate muscle wasting; high levels suggest kidney impairment.
The concentration of creatinine in blood. Creatinine is produced at a fairly constant rate from muscle creatine and cleared entirely by the kidneys.
Why it matters.
Creatinine is used to calculate eGFR (estimated glomerular filtration rate), the primary measure of kidney function. Declining kidney function is a major health risk and marker of biological aging. Muscle mass (reflected in creatinine production) is also a longevity factor.
Physiology.
Creatine in muscle is converted to creatinine at a steady rate (~1-2%/day). Creatinine is freely filtered by kidneys and not significantly reabsorbed or secreted. This makes it a reliable marker of filtration rate when muscle mass is stable. Production is proportional to muscle mass.
Testing & preparation.
How to prepare
- No special preparation needed
- Part of basic or comprehensive metabolic panel
- Avoid creatine supplements 24-48 hours before
- Strenuous exercise can transiently elevate
When to test
Routine health screening, monitoring kidney function, medication monitoring (especially NSAIDs, ACE inhibitors), or evaluating dehydration.
How often
Annually for most adults; more frequently with kidney disease or relevant medications.
Interpretation.
High creatinine
Common causes:
- Reduced kidney function (most common)
- Dehydration
- High protein/creatine intake
- Increased muscle mass (bodybuilders)
- Medications (NSAIDs, ACE inhibitors, trimethoprim)
- Acute kidney injury
- Muscle damage (rhabdomyolysis)
Implications:
- Calculate eGFR for accurate kidney function assessment
- High creatinine with normal eGFR may reflect high muscle mass
- Acute rise warrants urgent evaluation
- Chronic elevation requires monitoring and nephrology referral
Low creatinine
Common causes:
- Low muscle mass (sarcopenia)
- Malnutrition
- Liver disease (reduced creatine synthesis)
- Pregnancy (increased filtration)
- Advanced age with muscle wasting
Implications:
- May indicate sarcopenia/muscle loss
- eGFR may overestimate kidney function in low-muscle states
- Consider cystatin C for kidney function if very low creatinine
- Assess nutritional and muscle status
Optimization.
Diet
- Adequate protein for muscle maintenance
- Avoid excessive protein if kidney impairment
- Stay well-hydrated
Lifestyle
- Resistance training to preserve muscle mass
- Avoid nephrotoxic substances (NSAIDs excess, contrast dye)
- Manage blood pressure and blood sugar for kidney protection
Supplements
- Creatine supplementation is safe for healthy kidneys
- May modestly raise creatinine (not indicating kidney damage)
FAQs.
What is eGFR and how is it calculated?
eGFR (estimated Glomerular Filtration Rate) estimates how well your kidneys filter blood, measured in mL/min/1.73m². It's calculated from creatinine plus age, sex, and race using equations like CKD-EPI. Normal eGFR >90 mL/min; CKD stages are defined by eGFR levels. eGFR is more clinically useful than creatinine alone because it accounts for factors affecting creatinine production.
Will creatine supplements harm my kidneys?
For healthy individuals with normal kidney function, creatine supplementation is safe. However, it will modestly raise serum creatinine because you're producing more from the additional creatine. This doesn't indicate kidney damage—eGFR calculated from this elevated creatinine may underestimate your actual kidney function. If concerned, cystatin C provides an alternative kidney function marker unaffected by creatine supplementation.