It's been proven that using creatine long-term as a supplement causes creatinine levels to go above desirable thresholds into the unhealthy range, affecting kidney function
Blood creatinine levels are dependant on the amount of muscle mass one carries, diet, exercise, underlying kidney problems and many, many other factors.
I have never heard of 1 serious study proving that 5g of supplemental creatine per day in a healthy individual has ever caused any health problems whatsoever.
Here ya go sport:
Causes of high creatinine levels
1. Dehydration or inadequate water intake such as high fever, sweating, diuresis can cause blood concentration and reduced blood flow in the kidneys. Creatinine will increase.
2. Over-tiredness, lack of rest can cause elevation of creatinine level.
3. Urine abnormalities such as long term hematuria, proteinuria can be accompanied by high creatinine.
4. Intake of drugs that have renal toxicity can cause high creatinine which can even be irreversible.
5. If renal dysfunction patients have infections such as cold, pneumonia, intestinal infection, urinary tract infections can increase creatinine level in short time.
http://www.kidneyfailureweb.com/creatinine/251.html