What is the definition of a non-functional and poor functional kidney in terms of glomerular filtration rate (GFR)?

Definitions of Non-Functional and Poor Functional Kidney

A non-functional kidney is defined as having a GFR <15 mL/min per 1.73 m² (kidney failure), while poor functional kidney refers to severely reduced GFR of 15-29 mL/min per 1.73 m² (Stage 4 CKD). 1

Non-Functional Kidney (Kidney Failure)

Kidney failure is specifically defined by either:

• GFR <15 mL/min per 1.73 m², which is accompanied in most cases by signs and symptoms of uremia, OR
• A need to start kidney replacement therapy (dialysis or transplantation) 1

Key Clinical Context:

• Approximately 98% of patients with kidney failure in the United States begin dialysis when their GFR falls below 15 mL/min per 1.73 m² 1
• This corresponds to Stage 5 CKD (G5) in the KDIGO classification system 1
• At this level, patients have lost approximately 90% or more of normal kidney function 1

Important Distinction:

• Kidney failure is not synonymous with end-stage renal disease (ESRD), which is an administrative term indicating a patient is treated with dialysis or transplantation for Medicare payment purposes 1
• The classification of ESRD does not include patients with kidney failure who are not receiving dialysis or transplantation 1

Poor Functional Kidney (Severely Reduced GFR)

Poor functional kidney is best characterized as Stage 4 CKD with GFR 15-29 mL/min per 1.73 m² 1

Clinical Significance:

• This represents severe decrease in GFR and loss of approximately 75-90% of normal kidney function 1
• At this stage, the prevalence of complications significantly increases, including hypertension, anemia, malnutrition, bone disease, neuropathy, and decreased quality of life 1
• Patients are at very high risk for progression to kidney failure and cardiovascular disease 1

Additional Context for Moderately Reduced Function

For completeness, moderately reduced kidney function (Stage 3 CKD) is defined as:

• Stage 3a (G3a): GFR 45-59 mL/min per 1.73 m²
• Stage 3b (G3b): GFR 30-44 mL/min per 1.73 m² 1

Critical Threshold:

• GFR <60 mL/min per 1.73 m² represents loss of half or more of normal adult kidney function 1
• Below this threshold, the prevalence of CKD complications increases substantially 1
• Normal GFR in young adults is approximately 120-130 mL/min per 1.73 m² 1

Common Pitfalls to Avoid

Duration Requirement:

• All CKD definitions require abnormalities to be present for ≥3 months to distinguish from acute kidney injury 1, 2, 3

Age Considerations:

• While GFR declines with age (approximately 17% of persons >60 years have eGFR <60 mL/min per 1.73 m²), decreased GFR in the elderly remains an independent predictor of adverse outcomes including death and cardiovascular disease 1
• The definition of CKD is the same regardless of age 1

Measurement Accuracy:

• GFR should be estimated using validated equations (MDRD Study equation or CKD-EPI) based on calibrated serum creatinine 1, 3
• Factors affecting eGFR accuracy include extremes of muscle mass, non-steady state conditions, and advanced age 4, 5