Introduction
The glomerular filtration rate (GFR) is an estimate of the filtering capacity of the kidneys. It is usually expressed as milliliters (mL) per minute (min) and adjusted to a “standard” body size with a surface area of 1.73 meters
2. The normal GFR ranges between 95 -120 mL/min/1.73m
2 but it varies
depending on age, gender and body size.
The most commonly used method to estimate if GFR is grossly "normal" or "abnormal." It is not used to quantify GFR because it does not account for different body sizes and can be affected by a number of medications. For example, a creatinine of 1.5 mg/dL could be normal for a young body builder but very abnormal for a frail elderly lady.
Creatinine Clearance
This test involves collecting urine for 24 hours and taking blood to measure the serum creatinine. By looking at how well creatinine is "cleared" from the body and eliminated in the urine, it is possible to estimate the kidney function. Because it is hard for people collect all the urine accurately and there are easier ways to estimate GFR, it is not used as often. However, it is still used sometimes in pregnancy and with people who have extreme body types, malnutrition or muscle disorders.
A simple equation that tries to correct for different body types based on a person’s age, weight, serum creatinine and gender. Although simple to use, it is generally not as accurate as the MDRD equation.
A more accurate but complex equation based on age, serum creatinine, gender and race. It was derived from a large clinical trial and requires the use of a calculator. It is now the recommended method for estimating GFR in most cases.
