13.19: Drug Dosing in Renal Diseases: Measurement of Serum Creatinine Concentration and Clearance

In healthy individuals, serum creatinine levels remain stable due to a balance between its constant production—primarily from muscle metabolism—and renal excretion. Creatinine is freely filtered by the glomeruli, making it a valuable marker for estimating renal function. When the glomerular filtration rate (GFR) decreases, the kidneys can only eliminate less creatinine, causing serum levels to rise.

Serum creatinine concentration is widely used to estimate creatinine clearance (Cl_cr), a practical and quick method for assessing kidney function. Ideally, creatinine clearance is calculated using the ratio of the urinary creatinine excretion rate to the serum creatinine concentration. This calculation reflects the kidney’s ability to clear creatinine from the bloodstream.

However, creatinine is also eliminated by other minor routes, such as tubular secretion, which causes creatinine clearance values to overestimate the actual GFR. Cl_cr values are normalized to body surface area (BSA), typically 1.73 m², and adjusted for body weight to improve accuracy. This normalization accounts for individual differences in body composition and helps compare kidney function across populations.

The average Cl_cr for healthy adults ranges between 100 and 125 mL/min per 1.73 m². Several methods are used to estimate Cl_cr without timed urine collection. The Cockcroft-Gault equation, based on age, weight, gender, and serum creatinine, is commonly used in clinical practice. Nomograms also provide a graphical method for quick estimation.

Special populations—such as children, the elderly, obese, or emaciated patients—require tailored approaches for accurate Cl_cr estimation due to variations in muscle mass, metabolic rate, and body composition. In these cases, pediatric-specific formulas or correction factors may be applied to avoid under- or overestimating renal function, which is critical for proper dosing and monitoring in clinical settings.

In healthy individuals, balanced excretion and creatinine production maintain constant serum creatinine levels.

However, in patients with a reduced glomerular filtration rate, the serum creatinine accumulates.

The serum creatinine concentration can be used to measure creatinine clearance, a rapid way to monitor kidney function.

Ideally, it is calculated as the ratio of the urinary creatinine excretion rate and serum creatinine concentration.

Creatinine is eliminated through various methods and routes, causing Cl_cr values to overestimate the actual GFR.

So, body surface area and weight adjustments are made to normalize Cl_cr values, accounting for body composition differences.

Average Cl_cr values are generally assumed to be between 100 and 125 mL/min per 1.73 m².

Various methods like the Cockcroft and Gault, and nomograms can also be used to estimate Cl_cr.

Special consideration is needed for Cl_cr calculation in children, women, the elderly, the obese, and emaciated patients.