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Estimated Glomerular Filtration Rate (eGFR) is the most fundamental kidney function test that indicates the kidneys’ capacity to filter the blood and remove toxins, expressed in milliliters per minute. In blood tests, an elevated eGFR or an eGFR within the expected reference range demonstrates that the kidneys are functioning at full capacity and are extremely healthy. In contrast, a low eGFR indicates that the filtering ability is compromised, harmful metabolic waste products are beginning to accumulate in the bloodstream, and significant damage has occurred to the organ. This decline is one of the strongest laboratory indicators of structural urinary tract obstructions, insidiously progressing kidney failure, or a chronic disease that disrupts the body’s internal balance.
What Functions Do Our Kidneys Perform in the Body and Why Is eGFR So Important?
The kidneys are not merely simple organs that produce urine; they are also highly complex biological factories that maintain our internal balance. They perform many vital functions, including regulating blood pressure, maintaining the body’s water and salt balance, controlling mineral levels essential for bone health, and secreting hormones that stimulate blood cell production. The blood in our body passes through the kidneys many times each day, and during this process harmful or excess substances are carefully filtered out.
If any obstruction or structural problem develops in the outflow tract of the kidneys—that is, in the channels carrying urine—preventing urine from flowing freely, this can create intense backward pressure on the kidney tissue. This backward pressure begins to strain the delicate filtering structures responsible for the kidneys’ filtration function and is typically reflected in clinical evaluations as a decrease in the eGFR value. Understanding how much this filtration capacity has been affected is extremely important for determining the severity of the problem.
What Does the Concept of eGFR (Estimated Glomerular Filtration Rate) Actually Mean?
Inside our kidneys are millions of tiny filtering units called glomeruli. The glomerular filtration rate is a universal measurement indicating how much blood plasma these microscopic filters cleanse of waste every minute. However, directly and easily measuring this filtration process in the human body is quite difficult. Some direct measurement methods used in the past required invasive procedures that were exhausting for patients and lasted for hours.
Therefore, instead of performing direct measurements today, the kidney’s filtration capacity is mathematically estimated by evaluating the levels of certain substances in the blood. In fact, the first letter of the abbreviation comes from the English word “estimated.” Thanks to this calculation system, it is possible to predict how well the kidneys are functioning with a high degree of accuracy using only a simple blood test, without burdening the patient, and to monitor changes over the years.
What Is Creatinine and Why Does It Play a Fundamental Role in eGFR Measurement?
The most commonly used and best-known substance in this calculation is creatinine. Creatinine is a waste product naturally produced during the daily energy metabolism of skeletal muscles and released into the bloodstream. Healthy kidneys filter creatinine from the bloodstream and regularly eliminate it through urine.
If the kidneys’ filtering function begins to decline, creatinine can no longer be eliminated efficiently and gradually accumulates in the bloodstream. Consequently, as the blood creatinine level rises, the kidney’s filtration rate, represented by the eGFR value, automatically decreases. However, the creatinine level does not depend solely on kidney function. A person’s muscle mass, age, sex, and even the amount of meat consumed in the diet may also influence this value. Therefore, modern laboratory formulas provide a much more accurate estimate by taking personal factors such as age and sex into account in addition to creatinine.
Is a Decline in eGFR an Expected Part of the Aging Process?
Just as the human body ages over time and the performance of certain organs changes, the kidneys also undergo a physiological aging process. With aging, there is generally a natural tendency for the number of actively functioning filtering units in the kidneys to decline. After people reach their thirties or forties, a gradual decrease in eGFR every decade is considered a normal part of aging in medicine.
This physiological process does not mean that the kidneys have suddenly become diseased or failed. Just as the skin loses some of its elasticity with age, the kidneys may also slow down somewhat due to years of wear. Therefore, mild declines observed in blood tests of older adults should not immediately be interpreted as alarming kidney disease. If there are no accompanying health problems, this condition is generally regarded as a natural consequence of aging, and patients are monitored without unnecessary concern.
What Problems Can a Low eGFR Indicate in the Body?
A reduction in filtration capacity may gradually alter the body’s overall balance. Kidney function is classified according to the range in which this value falls, and the effects on the body differ at each stage.
Some common complaints patients may experience when the value declines to certain levels include:
- Fatigue
- Loss of appetite
- Edema
- Nausea
- Sleep disturbances
- Blood pressure irregularities
- Difficulty concentrating
A value of 90 or above indicates that filtration function is excellent and, provided there are no additional problems such as protein leakage in the urine, the kidneys are considered healthy. When the value falls between 60 and 89, mild slowing of kidney function may be present. Although this may not be concerning in older adults, it should be evaluated more carefully in younger individuals. A value between 45 and 59 indicates mild to moderate loss of kidney function. At this stage, it becomes important to avoid strong painkillers that may place additional strain on the kidneys and to pay close attention to cardiovascular health. When the value declines to between 30 and 44, waste products may gradually accumulate in the bloodstream, requiring closer monitoring. A value between 15 and 29 indicates severe impairment, and these patients generally require close follow-up with nephrology specialists. A value below 15 indicates that the kidneys can no longer adequately support the body’s needs, and supportive treatments such as dialysis or kidney transplantation should be planned.
How Can Urological Diseases Cause a Low eGFR by Obstructing Urine Flow?
After the kidneys filter the blood and produce urine, they send this fluid to the bladder through long, narrow tubes called ureters. Partial or complete obstruction anywhere along the urinary tract disrupts the normal downward flow of urine. This can be compared to a blockage in a household plumbing pipe causing water to back up.
The accumulated urine creates backward pressure, causing the collecting system inside the kidney to become swollen. As long as the urinary tract remains obstructed, the internal pressure exerted on the kidney tissue continues to increase. The delicate filtering structures in the kidney, constantly exposed to this pressure, gradually lose their ability to cleanse the blood effectively. If the obstruction is not relieved in time, permanent damage may occur, and the person’s filtration capacity may be irreversibly impaired over the long term.
How Do the Kidneys and eGFR Respond to Obstructions in the Urinary Tract?
When faced with a sudden obstruction, the kidneys do not immediately give up; instead, they make an intensive effort to compensate. During the first hours after the obstruction begins, the kidneys attempt to preserve filtration pressure by dilating the blood vessels supplying them, thereby increasing blood flow. However, if the obstruction persists and pressure within the urinary tract reaches dangerous levels, the kidneys protect themselves by constricting these blood vessels and reducing blood flow. This defense mechanism causes the eGFR value to decline rapidly.
The structures of the kidneys and urinary tract that become strained under increasing pressure include:
- Glomeruli
- Nephron tubules
- Renal pelvis
- Ureters
- Smooth muscle cells
The ureters are composed of smooth muscle cells that contract and relax to propel urine toward the bladder. During an obstruction, these muscles initially stretch and dilate in an attempt to compensate for the increased pressure. However, if the obstruction persists, the tissues may gradually lose their structure due to excessive strain. Over time, flexible muscle tissue may be replaced by rigid, nonfunctional connective tissue. During this process, the kidney’s filtration rate may gradually fall to its lowest levels, bringing urine production close to a halt. Once recovery begins and the obstruction is relieved, the kidneys may temporarily produce excessive amounts of urine to eliminate the accumulated fluid.
How Do Congenital Urological Disorders Affect eGFR in Children?
Reduced filtration capacity and its associated complications are not limited to adults. A significant proportion of kidney problems during childhood result from congenital anatomical abnormalities. Certain developmental defects affecting the kidneys or urinary tract while the baby is still in the womb may disrupt urine flow after birth.
For example, narrowing between the renal pelvis and the ureter may cause urine to accumulate in the kidney. Valve abnormalities, particularly seen in male infants, that obstruct the bladder outlet can lead to excessive strain on the bladder and create severe backward pressure on both kidneys. In some cases, babies are born with one kidney covered in cysts and unable to function, or with kidneys located outside their normal anatomical position. Such structural abnormalities may force the healthy kidney to work excessively while also increasing the risk of urine reflux into the kidneys. Early diagnosis and regular ultrasound monitoring play a crucial role in preserving kidney function as children grow older.
Can Benign Prostatic Hyperplasia (BPH) Cause a Low eGFR in Adults?
One of the most common conditions affecting older men is enlargement of the prostate gland. The prostate surrounds the urethra like a ring just below the bladder. As prostate tissue enlarges or becomes tense over the years, it may compress the urethra, making it difficult for the bladder to empty completely.
When the bladder struggles during each urination and retains some urine, this residual urine may gradually create backward pressure toward the kidneys. Most patients do not perceive this as pain, but the kidneys may slowly begin to swell. Prolonged exposure of both kidneys to this backward pressure may gradually reduce the filtration rate and lead to chronic kidney problems. During routine health check-ups, evaluating not only prostate symptoms but also kidney function tests in the blood is highly beneficial for detecting a possible decline in filtration capacity at an early stage.
Why Do Kidney Stones and Especially Having Only One Kidney Pose a Major Risk for eGFR?
The best-known and most common cause of sudden urinary tract obstruction is kidney stones. In a person with two healthy kidneys, if a stone blocks one side, the other kidney usually compensates by temporarily increasing its filtration capacity. As a result, significant deterioration in blood test values may not be immediately apparent.
However, in individuals born with only one kidney, those who have had one kidney removed due to disease, or those with only one functioning kidney, the situation is much more critical. If a stone completely blocks the ureter of the only functioning kidney, there is no backup filtration system. Urine production rapidly ceases, waste products begin accumulating in the blood within hours, and the patient’s filtration rate may approach zero. In such scenarios, relieving the pressure on the kidney as quickly as possible is the highest priority to prevent permanent damage. During stone treatment, preserving healthy and substantial kidney tissue also helps facilitate a smoother recovery.
Which Methods Are Used to Detect Kidney Obstructions and eGFR Loss?
Various tools are used to determine the severity of kidney problems, evaluate the extent of anatomical enlargement, and assess how much the filtration function has been affected. These methods help identify not only the location of the problem but also the degree of stress placed on the kidneys.
The main diagnostic methods used to detect obstructions and filtration problems include:
- Ultrasonography
- Computed tomography
- Magnetic resonance imaging
- Dynamic renal scintigraphy
- Blood tests
- Urinalysis
Because ultrasonography does not expose the patient to radiation, it is often the first practical method used and allows swelling within the kidney to be visualized easily. However, if it is suspected that the kidney is being compressed externally by a stone or another structure, non-contrast computed tomography, which provides more detailed cross-sectional images, may be preferred. Dynamic renal scintigraphy, a highly sensitive nuclear medicine test, demonstrates how well the kidneys function. During this test, very small amounts of radioactive tracer are injected into a vein, allowing physicians to determine what percentage of the overall filtration work is performed by the right and left kidneys. This method is particularly valuable for distinguishing between simple urinary tract dilation and a true mechanical obstruction by observing how a diuretic administered during the test affects urine flow within the kidneys.
Which Interventions Can Be Performed to Relieve Obstructions and Preserve the eGFR Value?
When the filtration rate decreases because of an obstruction, the primary approach is to bypass the obstruction in the safest possible way and relieve pressure on the kidney tissue. Once the pressure is eliminated, the kidneys have a greater chance of recovering, allowing the patient’s health values to return closer to normal.
The methods most commonly used to relieve obstructions and preserve kidney function include:
- Double-J stent placement
- Percutaneous nephrostomy
- Robotic laparoscopic pyeloplasty
- Nephron-sparing surgery
- Laser stone fragmentation
Especially when stones or strictures block the urinary tract, a thin, flexible tube with curled ends may be inserted into the ureter. This device is called a Double-J stent. The stent bridges the site of obstruction, allowing urine to drain into the bladder and rapidly reducing pressure inside the kidney. However, it is extremely important that these stents are not forgotten inside the body, as this may lead to complications such as stone formation or infection. In more difficult cases where the stent cannot be inserted through the urinary tract, or when severe infection is present, a thin tube may be placed directly into the kidney through the patient’s back under ultrasound guidance to drain urine externally. In congenital ureteral narrowing, robotic surgical techniques may be used to reconstruct the affected segment and reconnect healthy tissue. During tumor surgery, removing only the diseased portion of the kidney instead of the entire organ may help the patient maintain a higher long-term filtration capacity.
When Does eGFR Monitoring and Nephrology Follow-Up Become Necessary After Urological Procedures?
Relieving a urinary tract obstruction through stone fragmentation, stent placement, or surgery resolves the mechanical problem affecting the kidney, but the patient’s overall medical follow-up does not end there. Once the obstruction has been removed, a monitoring period begins to determine how much damage the kidney tissue has sustained. If, months after the obstruction has been relieved, the filtration rate still does not return to the expected normal level and remains below 60, permanent impairment of the kidney tissue may be suspected.
From this stage onward, it becomes especially important for nephrology specialists to participate in the follow-up of patients with significant protein leakage in the urine, persistently elevated blood pressure, or substantially deteriorated blood test results. In nephrology clinics, medication dosages are adjusted according to the patient’s remaining kidney function, while treatments supporting bone health and protecting the cardiovascular system are planned. Preserving filtration capacity for as many years as possible and maintaining the patient’s quality of life can largely be achieved through these structured follow-up programs involving collaboration among multiple medical specialties.
Frequently Asked Questions
eGFR (Estimated Glomerular Filtration Rate) is a calculated value that reflects the kidneys’ ability to filter blood. It is determined using creatinine, age, sex, and certain calculation methods, and plays an important role in evaluating kidney function.
A low eGFR may result from chronic kidney disease, diabetes, hypertension, kidney infections, kidney stones, dehydration, or the effects of certain medications. Results should always be interpreted together with the patient’s clinical evaluation.
A high eGFR usually does not indicate a serious disease and may occur in young individuals, during pregnancy, or under certain physiological conditions. However, it should be evaluated together with other kidney function tests.
eGFR is calculated using special formulas based on blood creatinine levels, age, sex, and other factors. This calculation estimates the kidneys’ filtration capacity and is often reported automatically as part of laboratory test results.
It may not cause symptoms in the early stages. In advanced stages, fatigue, edema, loss of appetite, nausea, changes in urine output, shortness of breath, and high blood pressure may develop.
Age, muscle mass, fluid balance, pregnancy, certain medications, intense exercise, and health conditions affecting creatinine levels may influence the eGFR result. Therefore, clinical decisions should not be based on a single test result alone.
Treatment depends on the underlying cause. Controlling diabetes and hypertension, avoiding medications that may harm the kidneys, maintaining a healthy diet, and regular follow-up may help preserve kidney function.
No. eGFR is an important indicator, but a definitive diagnosis requires evaluation together with creatinine, urinalysis, urine albumin levels, kidney imaging studies, and the patient’s clinical findings.
The frequency of monitoring depends on the individual’s health status. People with diabetes, hypertension, chronic kidney disease, or an increased risk of kidney disease should have regular follow-up according to their physician’s recommendations.
Individuals with a low eGFR, suspected kidney disease, or those requiring evaluation of kidney function should consult a family physician, an internal medicine specialist, a nephrologist, or a urologist.
Prof. Dr. Berkan Reşorlu, born in Ankara in 1979, completed his medical education at Ankara University between 1997 and 2003, and his specialization in urology at Ankara University Faculty of Medicine between 2003 and 2008. Academically, he received the title of associate professor in 2013 and professor in 2022.
He has conducted extensive case series studies in the field of endoscopic stone surgery, contributed surgeries performed on children under the age of 1 to the literature, and has gained international recognition with his stone scoring system and experimental surgical models.
