Serum creatinine level is probably the most widely used indirect measure of glomerular filtration rate. Unfortunately, correct interpretation in the critical care setting is a problem. It is important to consider variation in creatinine production from differences in muscle mass. Muscle mass may decline rapidly in a catabolic state, resulting in an underestimation of the amount of deteroration of renal function. Another confusing factor is the use of standard normal ranges for creatinine levels. In an elderly individual a normal serum creatinine concentration can indicate a twofold reduction in glomerular filtration rate as a result of a comparable reduction in muscle mass (Rpwe eLa/.; 1.9Z.6). Variations in creatinine production due to age- and sex-related differences in muscle mass have been measured and incorporated in formulas to improve the ability of serum creatinine to estimate glomerular filtration rate. The most widely used equation is where age is in years, weight is in kilograms, and Scr is the serum creatinine level in micromoles per liter.
Furthermore, failure to remember the potential effects of tubule secretion on serum creatinine, particularly in patients with reduced renal function, may lead to overestimation of the glomerular filtration rate.
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