Blood creatinine is the most widely used clinical indicator for kidney function evaluation, elevated blood creatinine probably means impaired kidney function. The normal upper limit of blood creatinine is 84umol/L for female patients and 106umol/L for male patients, but the normal range of blood creatinine test laboratory varies in different hospitals. Creatinine is generally affected by muscle mass and dietary habits. Some elderly patients with wasting body mass and muscle atrophy may have decreased renal function although their blood creatinine level is in the normal range and it does not reflect it. Only when the glomerular filtration rate is less than 50% of normal, the blood creatinine starts to rise, so for the early decline of renal function, the change of blood creatinine is also insensitive.