Glomerular Filtration Rate (GFR) is a measurement that provides a clear picture of how well your kidneys are functioning. The kidneys filter waste and excess fluid from the blood to produce urine. GFR represents the flow rate of this filtered fluid, indicating the capacity of the tiny filtering units within the kidneys, called glomeruli, to perform their job effectively. GFR is the standard used by medical professionals to determine overall kidney health and to diagnose the presence and stage of Chronic Kidney Disease (CKD).
Understanding How GFR is Measured
The Glomerular Filtration Rate is typically not measured directly in a routine clinical setting because the process is complex, time-consuming, and expensive. Instead, an Estimated Glomerular Filtration Rate (eGFR) is calculated using a simple blood test and a specific mathematical formula. This calculation uses the level of serum creatinine, a common waste product, as its primary input.
Creatinine is a breakdown product of creatine phosphate, which is naturally produced by the normal wear and tear of muscle tissue. Healthy kidneys efficiently filter creatinine from the bloodstream, so a higher level of creatinine in the blood suggests the kidneys are not filtering effectively.
The eGFR calculation adjusts for the patient’s age and sex, which correlate with the average muscle mass a person possesses. Since creatinine is derived from muscle, a young, muscular man will naturally have a higher serum creatinine level than an older woman with lower muscle mass, even if both have perfectly healthy kidneys. The eGFR formula normalizes these differences to provide a more accurate estimate of the filtration rate. The resulting eGFR is expressed in milliliters per minute per 1.73 square meters (\(\text{mL/min/1.73 m}^2\)), which standardizes the result to an average body surface area.
What is a Normal GFR for Older Adults
The GFR naturally and gradually decreases as a person ages, even in the absence of any kidney disease. This decline occurs because the number of functioning filtering units, or nephrons, gradually reduces over a lifespan. For younger, healthy adults, a GFR of 90 \(\text{mL/min/1.73 m}^2\) or higher is generally considered normal.
However, for a 75-year-old woman, the expected “normal” range is lower than this younger adult threshold. Data from large studies suggest the average eGFR for individuals aged 70 and over is approximately 75 \(\text{mL/min/1.73 m}^2\). Therefore, an eGFR above 60 \(\text{mL/min/1.73 m}^2\) is considered within the expected and acceptable range for a healthy woman in her mid-seventies.
The general rule of thumb for defining Chronic Kidney Disease is an eGFR below 60 \(\text{mL/min/1.73 m}^2\) for three months or more. When interpreting the result for a 75-year-old, it is important to understand that a slightly lower number is not necessarily a sign of disease, but rather a reflection of the normal aging process.
Interpreting Results and Stages of Kidney Function
When the eGFR falls below the expected normal range, the result is used to classify the severity of kidney function decline using the five stages of Chronic Kidney Disease (CKD). Stage 1 CKD is defined as an eGFR of 90 \(\text{mL/min/1.73 m}^2\) or higher, but with other evidence of kidney damage, such as protein in the urine. Stage 2 is characterized by a mildly reduced GFR between 60 and 89 \(\text{mL/min/1.73 m}^2\), also with signs of kidney damage.
The clinical diagnosis of kidney disease most often begins when the GFR drops to Stage 3, which represents a moderate decrease in function. Stage 3 is further divided into two sub-stages: Stage 3a is an eGFR between 45 and 59 \(\text{mL/min/1.73 m}^2\), while Stage 3b is a GFR between 30 and 44 \(\text{mL/min/1.73 m}^2\). Individuals in Stage 3 may begin to experience complications or symptoms related to impaired kidney function.
Stage 4 CKD signifies a severe loss of function, with an eGFR ranging from 15 to 29 \(\text{mL/min/1.73 m}^2\). At this point, close management by a kidney specialist is highly recommended to prepare for potential future treatment options. Stage 5 is the most severe classification, where the eGFR is below 15 \(\text{mL/min/1.73 m}^2\), indicating kidney failure. This stage often requires life-sustaining treatments like dialysis or a kidney transplant.
If a woman’s eGFR is consistently below the 60 \(\text{mL/min/1.73 m}^2\) threshold, consulting a healthcare professional is necessary. An action plan can be developed to help manage the condition and slow its progression, often involving lifestyle adjustments and medication. Regular monitoring ensures that any further decline in kidney function is detected early, allowing for timely intervention and the best possible health outcomes.