What is GFR and why is it important?
The Glomerular Filtration Rate (GFR) is a critical indicator that reflects the efficiency of kidney function. It measures the volume of blood that the renal glomeruli (tiny filters in the kidneys) clear of waste products every minute. The kidneys play a vital role in maintaining the body's health by removing toxins, regulating fluid and electrolyte balance, and participating in hormone production.
A decrease in GFR is a key indicator of impaired kidney function and may signal the development of chronic kidney disease (CKD). Early detection of CKD allows for timely treatment, slowing the progression of the disease and preventing serious complications such as kidney failure, which requires dialysis or transplantation.
How GFR is measured: Key blood markers
Direct measurement of GFR is a complex procedure, so clinical practice most often uses the estimated Glomerular Filtration Rate (eGFR), which is determined based on a blood test. The main markers used to calculate eGFR are:
- Creatinine: This is a breakdown product of creatine phosphate found in muscles. Creatinine is constantly produced in the body and excreted by the kidneys. Its level in the blood depends directly on muscle mass, age, gender, and, most importantly, kidney function. Elevated blood creatinine levels are often observed in patients with chronic kidney disease (CKD) [7].
- Cystatin C: This is a protein produced by all nucleated cells in the body and is also filtered by the kidneys. Its blood level is less dependent on muscle mass than creatinine, making it a more accurate marker of GFR in certain cases, such as in people with atypical muscle mass (very developed or, conversely, significantly reduced).
A doctor chooses the most appropriate marker or combination of markers to assess GFR based on the patient's individual characteristics.
Formulas for calculating GFR
Since GFR is rarely measured directly, special mathematical formulas are used to estimate it, which take into account the level of creatinine and/or cystatin C in the blood, as well as age, gender, and sometimes other parameters (e.g., race, although this parameter is being revised in modern guidelines). The most common formulas include:
- CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) formula: Considered one of the most accurate formulas for estimating GFR and is widely used in clinical practice.
- MDRD (Modification of Diet in Renal Disease) formula: Also frequently used, but may be less accurate at high GFR values.
These formulas allow doctors to obtain a fairly accurate picture of a patient's kidney function without the need for invasive procedures.
What do GFR test results mean?
GFR test results are expressed in milliliters per minute per 1.73 square meters of body surface area (mL/min/1.73 m²). This allows for the standardization of indicators for people of different builds. Interpretation of GFR results helps determine the stage of CKD:
- GFR ≥ 90 mL/min/1.73 m²: Normal or high GFR. Kidney function is normal, but there may be other signs of kidney damage (e.g., protein in the urine).
- GFR 60-89 mL/min/1.73 m²: Mildly decreased GFR. Kidney function is slightly reduced, but it may be a sign of early CKD, especially if other markers of kidney damage are present.
- GFR 45-59 mL/min/1.73 m² (stage 3a): Moderately decreased GFR. Requires attention and regular monitoring.
- GFR 30-44 mL/min/1.73 m² (stage 3b): Moderately to severely decreased GFR. The risk of complications increases.
- GFR 15-29 mL/min/1.73 m² (stage 4): Severely decreased GFR. High risk of developing kidney failure.
- GFR < 15 mL/min/1.73 m² (stage 5): Kidney failure. Renal replacement therapy (dialysis or transplantation) is required.
Who should get a GFR test?
Regular GFR determination is especially important for people at risk of developing kidney disease. These groups include:
- Patients with diabetes.
- People with high blood pressure (hypertension).
- Patients with cardiovascular diseases.
- Individuals with a family history of kidney disease.
- Patients taking medications that are potentially toxic to the kidneys (nephrotoxic drugs).
- Older adults, as kidney function naturally declines with age.
- People with obesity.
A GFR test is also indicated for monitoring the condition of patients already diagnosed with CKD and for assessing the effectiveness of treatment.
Preparation for the test and factors affecting the result
Special preparation for a creatinine or cystatin C blood test is usually not required. However, to obtain the most accurate results, it is recommended to:
- Avoid intense physical exertion for 24 hours before the test.
- Inform your doctor about all medications you are taking, as some of them may affect creatinine levels.
- Avoid eating large amounts of meat the day before the test, as this can temporarily increase creatinine levels.
- Maintain a normal fluid intake to avoid dehydration, which can also affect the results.
It is important to remember that GFR test results should only be interpreted by a doctor, taking into account the full clinical picture, comorbidities, and the patient's individual characteristics.
Conclusion
A blood test for the glomerular filtration rate is a simple yet extremely informative tool for assessing kidney health. Regular monitoring of GFR allows for the timely detection of abnormalities, control of the progression of chronic kidney disease, and the implementation of necessary measures to maintain health. If you are in a high-risk group or have symptoms indicating kidney problems, be sure to consult a doctor to schedule the appropriate examination.
Sources
- [7] Prevalence of high on-treatment platelet reactivity in patients with chronic kidney disease receiving acetylsalicylic acid for stroke prevention. https://pubmed.ncbi.nlm.nih.gov/30335048/
Disclaimer: The information provided in this article is for reference purposes only and should not be used for self-diagnosis or self-treatment. Always consult a qualified medical professional for diagnosis and treatment planning.