Our body produce creatinine at a fairly constant rate related to muscle mass. Serum creatinine in the body of someone who is tall and young and with larger muscle tissue volume is often supposed to be higher than the small and old one. Serum creatinine is filtered through glomerulus and only little is absorbed by kidney tubules which can also excrete. When the rate of glomerular filtration decline, the output of creatinine discharged by renal tubular plays a main part of the total discharged creatinine, leading to high evaluation level of GFR.
Since there are correlations between creatinine clearance rate and serum creatinine, the level of serum creatinine will not beyond normal range, especially among patients with smaller muscle mass, unless the glomerulus filtration rate decline substantially. When renal function has been impaired by 50%-70%(compensation stage),creatinine clearance also decrease, while the serum creatinine keep the normal level.
When GFR is reduced to more than 50% of the normal rate, creatinine begins to rise rapidly, so it is not sensitive to reflect the renal function. In addition, serum creatinine will lead to pseudo rise, if you eat meat, coffee, tea, exogenous sources of creatinine, take vigorous exercise which can increases muscle metabolism, and take drugs which are discharged by the channels of glomerular ion .The level of serum creatinine will have bigger changes with regards to different muscle mass, different gender, age, etc. Therefore, serum creatinine cannot often reflect the renal function accurately.
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