Creatinine comes from the breakdown of creatine in your muscle. It is produced by the body at a constant rate and eliminated through urine. This makes it a good a marker to assess your kidney function. The higher the blood creatinine, the worse the kidney function. A blood test will tell you both numbers.
Remember: GFR is only an estimate..its not an exact number! GFR is even more inaccurate in extremes of body habitus such as in obese or malnourished people, pregnant women, and amputees.
Sometimes, blood creatinine varies without a change in the GFR or kidney function.
1. Muscle Mass Change.
Low muscle mass caused by: Amputation, obesity, liver cirrhosis, malnourishment
High muscle mass caused by: Athletic gains, muscle injury (rhabdomyolysis) causes a release of creatine phoshate from the muscle.
2. Heavy Meat Diet. Creatine from animal meat is converted to creatinine in your body. The conversion can also occur during cooking.
3. Medications. Examples that interfere with creatinine levels: cimetidine, amiloride, spironolactone, cephalosporins
4. Creatine supplements are used in health and fitness; however, for most people this will not increase creatinine levels.