The glomerular filtration rate, or GFR, is a laboratory measurement that is ordered to evaluate kidney functioning. This measurement is important for diagnosing and assessing the stage of chronic kidney disease (CKD), and is often used as a guideline for the best treatment.

To calculate the GFR, it is necessary to take into account the person's sex, weight and age. It is normal for GFR to decrease as the person ages, and this decrease is not necessarily indicative of kidney damage.

There are several formulas that can be used to calculate the glomerular filtration rate, however the most used formulas are those that take either creatinine levels or cystatin-C levels. The use of cystatin-C levels is under ongoing review, because creatinine levels can be influenced by other factors outside of the kidneys (like your diet) and may not be the most accurate.

Normal GFR levels

The glomerular filtration rate can vary depending on the person's gender and age. General GFR levels calculated with creatinine levels are typically:

Normal: greater than or equal to 60 mL/min/1.73m²
Renal failure: less than 60 mL/min/1.73m²
Severe renal insufficiency or renal failure: when less than 15 mL/min/1.73m²

Normal levels for each age group are as follows:

Between 20 and 29 years old: 116 mL/min/1.73m²
Between 30 and 39 years old: 107 mL/min/1.73m²
Between 40 and 49 years old: 99 mL/min/1.73m²
Between 50 and 59 years old: 93 mL/min/1.73m²
Between 60 and 69 years old: 85 mL/min/1.73m²
From 70 years of age: 75 mL/min/1.73m²

Furthermore, GFR values may vary depending on whether or not the person is of African descent, with values considered normal in both cases exceeding 90 mL/min/1.73m².

GFR reference values may also vary from lab to lab. However, GFRs that are lower than the normal reference value for the patient's age, kidney disease may be suspected. In these cases, further testing may be recommended, such as imaging tests or a biopsy. GFR values can also be used for monitoring purposes and to guide the most appropriate treatment approach.

What it's for

The glomerular filtration rate is ordered to check kidney function. It works by assessing the levels of substances that are usually filtered in the kidneys for elimination through the urine, and are not reabsorbed into the blood. In the case of creatinine, for example, this protein is filtered by the kidneys and a small amount is reabsorbed into the blood, so that under normal conditions, creatinine levels are usually higher in the urine than in the blood.

With kidney injury, the filtration process can be altered, meaning less creatinine is filtered by the kidneys. This leads to higher levels in the blood, and a lower GFR. Read more about what causes high creatinine levels and how this is treated.

How GFR is calculated

The glomerular filtration rate is determined through a formula that takes several factors into account, such as the person's age and gender. For GFR to be calculated, a blood sample is collected to measure creatinine or cystatin-C levels, depending on the doctor's recommendation.

While GFR can be calculated using creatinine and cystatin-C levels, creatinine is not the most reliable marker. Creatinine levels can be altered for many reasons, such as diet, physical activity, inflammatory diseases and muscle mass composition. Read more about creatinine and how it is tested.

Alternatively, cystatin-C is produced by nucleated cells and is regularly filtered in the kidneys. The concentration of this substance in the blood is directly related to GFR, which is why it is a more precise marker of kidney function.