A study is conducted to standardize laboratory equipment in a hospital network. In one hospital, 2 healthy volunteers undergo testing. Both are found to have a serum creatinine level of 1.1 mg/dL. Glomerular filtration rate is estimated using the same equation and reveals values of 118 mL/min in one volunteer and 70 mL/min in the other. A difference in which of the following parameters best explains the observed laboratory findings in these volunteers?

Question

A study is conducted to standardize laboratory equipment in a hospital network.  In one hospital, 2 healthy volunteers undergo testing.  Both are found to have a serum creatinine level of 1.1 mg/dL.  Glomerular filtration rate is estimated using the same equation and reveals values of 118 mL/min in one volunteer and 70 mL/min in the other.  A difference in which of the following parameters best explains the observed laboratory findings in these volunteers?

Quizplus · Accepted Answer

Creatinine is a waste product generated by the breakdown of creatine in the muscles. It is released from muscle at a relatively constant rate, is neither metabolized nor reabsorbed by the kidneys, and is easily measured; therefore, it can be used to estimate the glomerular filtration rate (GFR). However, creatinine has several limitations in the estimation of GFR. Because its formation is due largely to muscle metabolism, differences in skeletal muscle mass (eg, higher in body builders and lower in elderly patients, those with amputations) affect the amount of creatinine synthesized. Alterations in dietary intake can also raise (eg, creatine supplements, high-meat diet) or lower (eg, low-protein vegetarian diet) creatinine levels. Therefore, patients with lower muscle mass or reduced dietary meat intake may have significantly lower GFRs for any given creatinine level. Another potential source of error is the active secretion of creatinine by the proximal tubules; if uncorrected, this results in a slight overestimation of GFR (~10%-20%). (Choice A) Alterations in basal metabolic rate can explain why individuals can have the same caloric intake but different body weight. However, they do not affect creatinine levels, which is dependent on muscle mass and dietary meat intake. (Choice B) Increased levels of dietary purines can result in gout formation in predisposed individuals but are not associated with altered creatinine formation. (Choice C) Creatine (creatinine precursor) is a nonessential nutrient synthesized in the liver and kidney and also obtained from meat consumption; although a disruption in hepatic synthetic function could reduce endogenous creatine production by the liver, continued production by the kidney along with creatine obtained from the diet make hepatic dysfunction alone an unlikely source of altered creatine/creatinine levels. (Choice D) Creatinine is freely filtered and neither metabolized nor reabsorbed by the kidneys; therefore, tubular reabsorptive function would not affect the GFR. Educational objective: Creatinine, a waste product generated by the breakdown of creatine in the muscles, is used to estimate the glomerular filtration rate (GFR). Creatinine formation is dependent on muscle mass and meat intake; therefore, patients with low muscle mass (eg, elderly patient, those with amputations) or low intake (eg, low-protein vegetarian diet) can have significantly lower GFRs for any given creatinine level.

A Study Is Conducted to Standardize Laboratory Equipment in a Hospital