After having a billion blood tests and kidney tests, I can answer this. First, I believe you mean creatinine, not creatine. Thats what is checked during a kidney function test.
Creatinine is a chemical waste molecule made from muscle metabolism. Creatinine is produced FROM creatine, a molecule of for energy production in muscles.
The kidneys are supposed to keep creatinine in a normal range. Creatinine is a reliable indicator of kidney function. Elevated creatinine level signifies impaired kidney function or kidney disease