The Glomerular Filtration Rate (GFR) is a standard measure used to assess kidney health. Kidneys filter waste products and excess fluid from the blood, and the GFR measures how efficiently they perform this duty. Regular GFR testing is important for individuals with conditions like diabetes or high blood pressure, which can damage the kidneys over time.
What GFR Represents and How It Is Calculated
The GFR represents the total volume of fluid filtered by the tiny filtering units within the kidneys, known as the glomeruli, over a specific period. It is essentially a flow rate, indicating the speed at which blood plasma is cleared of waste. Since the true GFR is difficult and impractical to measure in a clinical setting, healthcare providers rely on the estimated GFR, or eGFR.
The eGFR is calculated using a mathematical formula that incorporates a patient’s serum creatinine level, along with their age and sex. Creatinine is a natural waste product produced from the normal breakdown of muscle tissue, and healthy kidneys efficiently remove it from the bloodstream. When kidney function declines, creatinine levels in the blood rise, which the eGFR formula then translates into a filtration rate.
The result is reported in milliliters per minute per 1.73 meters squared (mL/min/1.73m\(^2\)), where the 1.73 m\(^2\) adjusts the rate to a standard body surface area. While some older formulas included a factor for race, this adjustment is increasingly being removed from clinical practice due to concerns about accuracy and health inequity. The current trend favors using race-neutral equations.
The Standard Range for Healthy Kidney Function
A GFR of 90 mL/min/1.73m\(^2\) or higher is generally considered to be in the normal range for adults. This range signifies that the kidneys are functioning optimally and efficiently removing waste products from the blood. However, a GFR between 60 and 89 may still be considered normal if there is no other evidence of kidney damage, such as protein in the urine.
The GFR naturally declines as a person ages, even without underlying kidney disease. For a young, healthy adult, the average GFR may be around 100 mL/min/1.73m\(^2\), but this rate decreases steadily over the decades. Doctors interpret the GFR result in the context of the patient’s overall health profile and age.
Interpreting GFR Levels and Kidney Disease Stages
When the GFR falls below the normal range, it indicates a reduction in the kidneys’ filtering capacity. This reduction is used to stage Chronic Kidney Disease (CKD), which is defined as kidney damage or an eGFR below 60 that persists for three months or more. The staging system uses five categories to classify the severity of the disease.
Stage 1 is defined by a GFR of 90 or greater, requiring additional signs of kidney damage, such as protein in the urine, for diagnosis. Stage 2 represents mild function loss, with a GFR between 60 and 89, also needing other evidence of damage. Stage 3 (GFR 30–59) is split into Stage 3a (45–59) and Stage 3b (30–44) to distinguish between mild-to-moderate and moderate-to-severe loss of function.
Stage 4 signifies severe loss of kidney function, with a GFR range of 15 to 29, which requires careful planning for future treatment. The final stage, Stage 5, is diagnosed when the GFR drops below 15. This indicates that the kidneys are near complete failure and suggests the need for dialysis or a kidney transplant.
Non-Disease Factors That Affect GFR Readings
An eGFR reading is an estimate, and several non-disease-related factors can influence the result, leading to temporary or misleading values. Since the calculation relies heavily on serum creatinine, anything that affects muscle mass can skew the reading. Individuals with unusually high muscle mass, such as bodybuilders, may have a higher baseline creatinine level that results in a calculated GFR that appears lower than their true kidney function.
Conversely, people with very low muscle mass, such as those who are severely malnourished, may have a lower creatinine level, leading to an artificially high eGFR. Acute conditions, particularly severe dehydration, reduce blood flow to the kidneys and can temporarily decrease the GFR. Certain medications, including nonsteroidal anti-inflammatory drugs (NSAIDs), can also interfere with the kidneys’ filtering process and affect the reading.