Estimating glomerular filtration rate (GFR)

Knowing your patient's glomerular filtration rate (GFR), and what can affect it, is critical to being able to assess overall kidney function. In this video, from our Kidney Function Test Essentials course, we'll take a deep dive into how GFR is calculated, the factors affecting GFR, and when you shouldn't use the Cockcroft-Gault equation.

Taking a shotgun approach to kidney function testing can be tempting if nephrology isn't your forte. By the end of this course, however, you’ll be comfortable ordering and interpreting all of the common kidney function tests. You’ll understand the clinical implications of abnormalities seen on these tests and be able to form a practical plan of action.

Glomerular filtration is one of the major functions of the kidney. The glomerular filtration rate, or GFR, is the sum of the filtration rates in all the functioning nephrons. Therefore, the GFR gives an estimate of the number of functioning nephrons and it is important in assessing overall kidney function. So, what factors affect the glomerular filtration rate?

The normal value for GFR depends upon age, sex and body size. The GFR is approximately 130 ml per minute per 1.73 meter square for men, and 120 ml per minute per 1.73 meter square for women. However, there is considerable variation even among normal individuals. GFR can be estimated by using the formula, GFR is equal to UA multiplied by V, divided by PA, where UA is the concentration of a solute in urine, V is the volume of urine in ml per minute, and PA is the concentration of the same solute in plasma.

The glomerular filtration rate formula is more accurate when taking into account the body surface area as follows. GFR is equal to UA times V, divided by PA, times 1.73, divided by A, where A is the body surface area in meters squared. Standard body surface is 1.73 meters squared. The body surface area of most adults is between 1.6 and 1.9 meters squared.

An ideal solute to be used for calculation of GFR is one which is freely filtered by the glomerulus, and is neither secreted nor reabsorbed by the tubules. Inulin is an inert chemical, which meets this criteria and it is the gold standard. However, clinical labs do not usually have the ability to measure inulin. Creatinine is the next best alternative for estimating the GFR.

Creatinine is produced in the body at a constant rate and is freely filtered and not reabsorbed, although a small amount of creatinine is secreted by the tubules. In order to calculate GFR based on creatinine clearance, a 24 hour urine collection is recommended, which should be from one morning void to the next morning void.

However, accurately collecting all urine produced in a 24 hour period is difficult. So, how do we estimate GFR? Estimation of GFR can be done using the plasma creatinine concentration, and relevant formulas. However, it is also important to consider the age, sex and race of the patient when performing such calculations.

This calculated value is known as the estimated glomerular filtration rate, or eGFR, and the objective is to make the eGFR as accurate as possible. There are many calculations that can be used to estimate GFR, and each has its own advantages and disadvantages. In the United States, one of the most common formulas used for calculating eGFR is the Cockcroft-Gault formula.

Creatinine clearance is equal to 140 minus age, times weight, divided by serum creatinine. Where the age is in years, the weight in kilograms and the serum creatine in in millimoles per liter. Don't forget to multiply by 0.85 if the patient is female. Another formula called the modification of diet in renal disease, or MDRD formula, is a modified version of the Cockcroft-Gault formula.

This equation is reasonably accurate in non hospitalized patients known to have CKD regardless of diagnosis. GFR in ml per minute per 1.73 meter squared is equal to 175 multiplied by serum creatinine, minus 1.154 times age, minus 0.203. Here serum creatinine is in milligrams per deciliter and the age is in years. Multiplied by 0.742 if the patient is female, and multiply by 1.212 if the patient is African American.

The modification of diet in renal disease study equation cannot be used for acute renal failure. It is only useful in estimating glomerular filtration rate in stable chronic kidney disease. The MDRD study equation and Cockcroft-Gault equation appear to be somewhat less accurate in obese individuals and in individuals with normal or near normal GFR.

There are also several other equations that can can be used to calculate eGFR, including one specific for children. We need to be aware of these formulas, but don't worry about memorizing them. Most of the time software will perform the calculations for you. But it is important that you are aware that different formulas exist, each with distinct advantages and disadvantages.