The kidneys function as a filtration system, managing waste and fluid balance. These bean-shaped organs filter toxins and excess water from the blood, producing urine and regulating substances like electrolytes and hormones that control blood pressure and red blood cell production. When the kidneys are damaged, they lose this ability, a condition known as Chronic Kidney Disease (CKD). As CKD progresses, accumulated waste and fluid can become life-threatening, requiring dialysis to take over the filtering role. The decision to begin this therapy relies on specific, measurable levels of remaining kidney function.
Quantifying Kidney Function
Kidney function is quantified using the Glomerular Filtration Rate (GFR). GFR represents the volume of blood, in milliliters, that the kidneys filter per minute (mL/min/1.73m²). This value is the most effective index for assessing the overall level of kidney function.
Since directly measuring the true GFR is time-consuming and expensive, healthcare providers rely on the estimated GFR (eGFR) for routine clinical use. The eGFR is calculated using a formula that analyzes the concentration of creatinine—a waste product of muscle metabolism—in the blood, along with the patient’s age, sex, and body size.
A GFR greater than 90 mL/min/1.73m² is considered normal function. As the kidneys sustain progressive damage, the eGFR value decreases, providing a metric of the decline in filtering capacity. Medical standards, such as those set by the Kidney Disease Outcomes Quality Initiative (KDOQI), use these eGFR values to stratify the severity of Chronic Kidney Disease.
The Five Stages of Chronic Kidney Disease
The progression of CKD is categorized into five stages defined by the eGFR measurement.
Stage 1 (eGFR 90+ mL/min/1.73m²) indicates normal function but signs of damage, such as protein in the urine. Stage 2 (eGFR 60–89) represents a mild decline in function.
Many individuals remain without noticeable symptoms during the first two stages. Stage 3 (eGFR 30–59) marks a moderate loss of function and is often subdivided into 3a (45–59) and 3b (30–44). Complications like high blood pressure and anemia may begin to appear in Stage 3.
Stage 4 (eGFR 15–29) signifies a severe loss of function. Symptoms often become more pronounced, and preparation for kidney replacement therapy is necessary. Stage 5 is diagnosed when the eGFR drops below 15 mL/min/1.73m², which is defined as End-Stage Renal Disease (ESRD) or kidney failure.
The Threshold for Starting Dialysis
Dialysis is typically considered when kidney function declines to Stage 5, corresponding to an eGFR of less than 15 mL/min/1.73m². This GFR threshold indicates that the kidneys have lost over 85% of their filtering capacity and are no longer able to adequately sustain life. However, the decision to initiate dialysis is rarely based solely on this number.
Clinical symptoms resulting from the failure to filter waste and fluid are often a more significant factor than the precise GFR value. These symptoms, known as uremia, can include intractable nausea, severe fatigue, uremic pericarditis, or altered mental status.
Dialysis becomes an immediate necessity when the kidneys cannot manage severe metabolic derangements, such as life-threatening hyperkalemia (high potassium) or fluid overload unresponsive to diuretics.
In asymptomatic patients who are carefully monitored, treatment can sometimes be safely delayed, even when the eGFR falls as low as 5 to 7 mL/min/1.73m². The timing is a shared decision between the patient and physician. The eGFR of 15 mL/min/1.73m² is better understood as the point at which planning and preparation for dialysis access must begin.
Types of Kidney Replacement Therapy
Two primary forms of kidney replacement therapy are available: hemodialysis (HD) and peritoneal dialysis (PD).
Hemodialysis cleans the blood outside the body using a specialized machine called a dialyzer, which functions as an artificial kidney. The blood is cycled through this machine, where a semipermeable membrane filters out waste products and excess fluid before returning the cleansed blood to the body.
Peritoneal dialysis uses the patient’s own body as the filter. A cleansing solution, called dialysate, is introduced into the abdominal cavity through a catheter. The peritoneum, the lining of the abdomen, acts as a natural membrane, allowing waste and fluid to diffuse into the dialysate. The spent fluid is then drained and replaced with fresh dialysate, a process often performed at home.