Glomerular Filtration Rate (GFR): what it is, how to determine it and when it may be altered
The glomerular filtration rate, or simply GFR, is a laboratory measure that allows the general practitioner and the nephrologist to assess the functioning of the person's kidneys, which is an important measure for the diagnosis and verification of the stage of chronic kidney disease (CKD), which makes the GFR also essential for establishing the best treatment, if necessary.
In order to calculate the glomerular filtration rate, it is necessary to take into account the person's sex, weight and age, as it is normal for GFR to decrease as the person ages, not necessarily indicating kidney damage or changes.
There are several calculations proposed to determine the glomerular filtration rate, however the most used in clinical practice are those that take into account the amount of creatinine in the blood or the amount of cystatin C, which is the most studied today, since the The amount of creatinine may be interfered with by other factors, including diet, thus not becoming an appropriate marker for the diagnosis and monitoring of CKD.
How GFR is determined
The glomerular filtration rate is determined in the laboratory using calculations that should take into account mainly the person's age and gender, as these factors interfere with the result. However, in order for the GFR to be calculated, a blood sample must be collected to measure creatinine or cystatin C, according to the doctor's recommendation.
The glomerular filtration rate can be calculated both taking into account the concentration of creatinine and the concentration of cystatin C. Although creatinine is the most used, it is not the most suitable, as its concentration may suffer interference from other factors, such as food, physical activity, inflammatory diseases and amount of muscle mass and thus does not necessarily represent renal function.
On the other hand, cystatin C is produced by the nucleated cells and is regularly filtered in the kidneys, so that the concentration of this substance in the blood is directly related to GFR, thus being a better marker of kidney function.
Normal GFR values
The glomerular filtration rate aims to verify the functioning of the kidneys, as it takes into account the dosage of substances that are filtered in the kidneys and are not reabsorbed into the blood, being essentially eliminated in the urine. 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, concentrations of creatinine in the urine much higher than that of the blood can be verified.
However, when there are changes in the kidneys, the filtration process can be altered, so that there is less creatinine being filtered by the kidneys, resulting in a higher concentration of creatinine in the blood and decreased glomerular filtration rate.
As the glomerular filtration rate can vary according to the person's gender and age, the GFR values when the calculation is made with creatinine are:
- Normal: greater than or equal to 60 mL / min / 1.73m²;
- Renal failure: less than 60 mL / min / 1.73m²;
- Severe renal failure or renal failure: when less than 15 mL / min / 1.73m².
According to age, normal GFR values are usually:
- Between 20 and 29 years: 116 mL / min / 1.73m²;
- Between 30 and 39 years: 107 mL / min / 1.73m²;
- Between 40 and 49 years: 99 mL / min / 1.73m²;
- Between 50 and 59 years: 93 mL / min / 1.73m²;
- Between 60 and 69 years: 85 mL / min / 1.73m²;
- From 70 years old: 75 mL / min / 1.73m².
The values may vary according to the laboratory, however when the GFR is lower than the normal reference value for age, the possibility of kidney disease is considered, being recommended by the performance of other tests to conclude the diagnosis. , such as imaging exams and biopsy. In addition, based on the values obtained for GFR, the doctor can verify the stage of the disease and, thus, indicate the most appropriate treatment.