Glomerular Filtration Rate (GFR) – Kidney Function
The glomerular filtration rate (GFR) measures how well the kidneys filter the blood. It is the key indicator for assessing kidney function and diagnosing kidney disease.
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The glomerular filtration rate (GFR) measures how well the kidneys filter the blood. It is the key indicator for assessing kidney function and diagnosing kidney disease.
What is the Glomerular Filtration Rate?
The glomerular filtration rate (GFR) is a measure of how efficiently the kidneys filter waste products from the blood. It indicates the volume of blood plasma filtered per minute through tiny filtering units in the kidneys called glomeruli. GFR is expressed in milliliters per minute (ml/min) and is considered the most important laboratory marker for evaluating kidney function.
In clinical practice, the estimated glomerular filtration rate (eGFR) is most commonly used. It is calculated from blood serum values – primarily serum creatinine – along with age, sex, and body size. Direct measurement of GFR is more complex and is typically reserved for specialized clinical or research settings.
Normal Values and Interpretation
In healthy adults, the GFR is typically between 90 and 120 ml/min/1.73 m². GFR naturally declines with age. A persistently reduced GFR below 60 ml/min/1.73 m² for more than three months is considered indicative of chronic kidney disease (CKD).
Kidney function is classified according to the KDIGO guidelines (Kidney Disease: Improving Global Outcomes):
- Stage G1: GFR ≥ 90 ml/min/1.73 m² – normal or high kidney function
- Stage G2: GFR 60–89 ml/min/1.73 m² – mildly decreased
- Stage G3a: GFR 45–59 ml/min/1.73 m² – mildly to moderately decreased
- Stage G3b: GFR 30–44 ml/min/1.73 m² – moderately to severely decreased
- Stage G4: GFR 15–29 ml/min/1.73 m² – severely decreased
- Stage G5: GFR below 15 ml/min/1.73 m² – kidney failure, possibly requiring dialysis
How is GFR Measured?
Estimated GFR (eGFR)
The most common approach is the calculation of eGFR from serum creatinine. Creatinine is a waste product of muscle metabolism that is excreted exclusively by the kidneys. Rising creatinine levels in the blood indicate reduced kidney filtration capacity. The most widely used formulas are the CKD-EPI equation and the older MDRD formula.
Direct GFR Measurement
More precise measurement can be achieved through inulin clearance or iohexol clearance. These methods are more resource-intensive and are primarily used in research or complex clinical scenarios.
Causes of a Reduced GFR
A persistently reduced GFR can result from a variety of causes, including:
- Diabetes mellitus – the most common cause of chronic kidney disease
- Arterial hypertension – sustained high blood pressure damages kidney blood vessels
- Glomerulonephritis – inflammation of the glomerular filters
- Polycystic kidney disease – a hereditary condition causing cyst formation in the kidneys
- Medication side effects – e.g., from nephrotoxic substances such as certain painkillers or contrast agents
- Acute kidney injury – caused by severe infections, dehydration, or toxic exposures
Symptoms of a Reduced GFR
Impaired kidney function often produces no symptoms in the early stages. As GFR declines further, the following symptoms may appear:
- Fatigue and exhaustion
- Fluid retention (edema) in the legs or face
- Changes in urine output or color
- Foamy urine (indicating excess protein excretion)
- High blood pressure
- Nausea and loss of appetite (in advanced kidney insufficiency)
Clinical Relevance and Treatment Planning
GFR plays a critical role in medication dosing, as many drugs are excreted via the kidneys. A reduced GFR can lead to drug accumulation and increased risk of adverse effects. GFR is also a key factor in decisions regarding dialysis or kidney transplantation.
Regular GFR monitoring is particularly recommended for individuals with known risk factors such as diabetes, hypertension, or a family history of kidney disease.
References
- KDIGO 2024 CKD Guideline – Kidney Disease: Improving Global Outcomes (KDIGO). KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International, 2024.
- Levey AS, Stevens LA et al. – A New Equation to Estimate Glomerular Filtration Rate. Annals of Internal Medicine, 2009; 150(9): 604–612.
- National Kidney Foundation – K/DOQI Clinical Practice Guidelines for Chronic Kidney Disease: Evaluation, Classification, and Stratification. American Journal of Kidney Diseases, 2002; 39(2 Suppl 1): S1–S266.
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