Final answer:
Creatinine can overestimate GFR because it is not only filtered by the glomerulus but also secreted by the renal tubules. Accurate measurement of GFR is crucial for proper drug dosing, hence the discrepancy introduced by creatinine secretion can be significant. The creatinine clearance test for estimating GFR is convenient but less accurate compared to the inulin clearance method.
Step-by-step explanation:
Creatinine can overestimate the glomerular filtration rate (GFR) because, while it is freely filtered by the glomerulus, it is also secreted by the renal tubules in small amounts. The commonly used creatinine clearance test to estimate GFR involves collecting urine to measure creatinine's concentration in urine (U), collecting blood to measure plasma creatinine concentration (B), and the urine flow rate (V). The formulae is Ccr = (UxV)/B. This method is quick and easy but less accurate compared to the more cumbersome inulin clearance method, which measures GFR using a substance that is neither reabsorbed nor secreted by the kidney. In measuring GFR using creatinine, the slight secretion of creatinine by the nephron is not accounted for, which can lead to an overestimation of the actual filtration rate. This is of particular concern when accurate GFR measurements are needed for appropriate drug dosing in patients with reduced renal function.
Moreover, conditions that affect muscle mass can also affect blood creatinine levels, which in turn affects the GFR estimation. For instance, a person with greater muscle mass may have higher creatinine levels independent of renal function. Conversely, conditions such as chronic kidney disease (CKD) or acute renal failure (ARF) will cause alterations in creatinine levels due to changes in kidney function itself.