What should creatinine (Cr) clearance be for an 86-year-old female with impaired renal function?

From the Guidelines

For an 86-year-old female with impaired renal function, a creatinine clearance of 30-45 mL/min is a reasonable estimate, considering the natural decline in kidney function with aging. This estimate is based on the understanding that creatinine clearance decreases by approximately 8-10 mL/min per decade after age 40 1. The Cockcroft-Gault equation, which takes into account age, weight, serum creatinine, and sex, is commonly used to estimate creatinine clearance. However, it is essential to note that individual values may vary based on overall health status, presence of kidney disease, hydration status, muscle mass, and other medical conditions.

Key Considerations

• The natural decline in kidney function with aging affects creatinine clearance, making it lower in elderly patients compared to younger adults.
• Medication dosing often needs adjustment based on reduced kidney function, particularly for drugs that are primarily eliminated by the kidneys.
• Regular monitoring of kidney function is recommended in elderly patients, especially when prescribing medications with potential kidney toxicity.
• The serum creatinine concentration alone is not a reliable indicator of kidney function, as it is affected by factors such as creatinine secretion and generation, and extrarenal excretion 1.

Estimating Creatinine Clearance

• The Cockcroft-Gault equation is a useful tool for estimating creatinine clearance, but it may not be accurate in all cases, particularly in patients with severe malnutrition, obesity, or rapidly changing kidney function.
• Clinical laboratories should report an estimate of GFR using a prediction equation, in addition to reporting the serum creatinine measurement, to provide a more accurate assessment of kidney function 1.

From the Research

Creatinine Clearance in Elderly Patients

The creatinine clearance (CrCl) for an 86-year-old female with impaired renal function can be estimated using various formulas, including the Cockcroft-Gault (CG) and Modification of Diet in Renal Disease (MDRD) formulas.

• The CG formula is considered more accurate in estimating CrCl in elderly patients, especially those with impaired renal function 2.
• A study published in 2009 found that the CG formula slightly underestimates CrCl, while the MDRD formula strongly overestimates it in elderly hospitalized patients 2.
• Another study published in 2008 found that the CG and MDRD1 formulas showed good average agreement with measured CrCl, but the MDRD2 formula was more biased 3.
• The estimated CrCl can vary significantly from the measured CrCl, with a bias of -8.9 and precision (standard deviation of bias) of 34 mL/min 4.

Factors Affecting Creatinine Clearance

Several factors can affect CrCl in elderly patients, including:

• Age: Advanced age is a significant predictor of dosing errors due to decreased renal function 5.
• Sex: Women are more likely to be classified as normal for serum creatinine but have abnormal creatinine clearance 6.
• Body weight: Low body weight is a predictor of dosing errors due to decreased renal function 5.
• Serum creatinine: Serum creatinine is not an adequate measure of renal function, especially in advanced age or in physically debilitated patients 6.

Clinical Implications

The accurate estimation of CrCl is crucial in elderly patients to avoid drug dosing errors and ensure safe medication use.

• The CG formula is recommended for estimating CrCl in elderly patients, especially those with impaired renal function 2.
• Measured CrCl should be used whenever possible, especially in patients with unstable renal function or those who are critically ill 4.
• Clinicians should be aware of the limitations of estimated CrCl and use it in conjunction with other clinical factors to make informed decisions about medication dosing 3, 4, 6, 5, 2.