Definition of Renal Failure
Renal failure is defined as the inability of the kidney to secrete nitrogenous wastes and maintain fluid and electrolyte homeostasis, leading to azotemia, with acute kidney injury (AKI) defined by an increase in creatinine ≥0.3 mg/dL within 48 hours or ≥1.5 times baseline within 7 days, or urine output <0.5 mL/kg/h for 6 hours, while chronic kidney disease (CKD) is defined as abnormal renal function (GFR <60 mL/min/1.73 m² or kidney damage) present for >3 months. 1
Acute Kidney Injury (AKI) Definition
The definition of AKI uses both serum creatinine and urine output criteria, with patients meeting either criterion qualifying for the diagnosis 1, 2:
Serum Creatinine Criteria
- Increase in creatinine by ≥0.3 mg/dL within 48 hours, OR 1, 2
- Increase in serum creatinine to ≥1.5 times baseline (within prior 7 days) 1, 2
Urine Output Criteria
RIFLE Classification System
The RIFLE criteria provide a graded classification of acute renal dysfunction with three severity levels and two outcome categories 1:
Severity Stages:
- Risk: GFR decrease by 25% OR serum creatinine increase by 1.5× baseline OR urine output <0.5 mL/kg/h for 6 hours 1
- Injury: GFR decrease by 50% OR serum creatinine increase by 2× baseline OR urine output <0.5 mL/kg/h for 12 hours 1
- Failure: GFR decrease by 75% OR serum creatinine >4 mg/dL (with acute rise ≥0.5 mg/dL) OR urine output <0.3 mL/kg/h for 24 hours 1
Outcome Categories:
- Loss: Complete loss of kidney function requiring renal replacement therapy for >4 weeks 1
- End-Stage Kidney Disease: Need for dialysis for >3 months 1
Important Caveat for Baseline Creatinine
When no baseline creatinine is available, estimate a theoretical baseline assuming normal GFR of 75 mL/min/1.73 m² using the MDRD formula based on age, race, and sex 1, 2. For example, a 50-year-old Black female would have an estimated baseline creatinine of 1.0 mg/dL 1.
Chronic Kidney Disease (CKD) Definition
CKD is defined as kidney damage OR GFR <60 mL/min/1.73 m² for ≥3 months, irrespective of cause 3.
Markers of Kidney Damage
- Albuminuria: Albumin-to-creatinine ratio (ACR) ≥30 mg/g on two of three spot urine specimens 3
- Imaging findings: Reduced kidney size or cortical thinning 2
- Pathologic abnormalities on kidney biopsy 3
CKD Staging by GFR
CKD severity is classified into five stages based on GFR level 3:
- Stage 1: GFR ≥90 mL/min/1.73 m² with kidney damage 3
- Stage 2: GFR 60-89 mL/min/1.73 m² with kidney damage 3
- Stage 3a: GFR 45-59 mL/min/1.73 m² 3
- Stage 3b: GFR 30-44 mL/min/1.73 m² 3
- Stage 4: GFR 15-29 mL/min/1.73 m² 3
- Stage 5: GFR <15 mL/min/1.73 m² (kidney failure) 3
Alternative Classification System
An alternative classification uses mild, moderate, and advanced chronic renal insufficiency 4:
Critical Distinction: AKI vs CKD
Proof of chronicity requires evidence of kidney dysfunction for at least 3 months 2, 3. To differentiate:
Indicators of AKI
- Recent clinical context suggesting volume depletion (hypotension, vomiting, diarrhea) 2
- Recent nephrotoxic medication or contrast exposure 2
- No prior documentation of reduced GFR 2
Indicators of CKD
- Prior documentation of eGFR <60 mL/min/1.73 m² for >3 months 2
- Persistent albuminuria on multiple occasions 2
- Small kidneys on imaging 2
- History of diabetes, hypertension, or glomerulonephritis 2
Common Pitfalls to Avoid
- Never assume a single abnormal creatinine value represents chronic disease—repeat testing within days is mandatory to confirm the finding 2
- Urine output changes may be physiologic, making serum creatinine measurement more important than urine volume for AKI diagnosis 1
- Clinical symptoms and signs are poorly predictive of reduced renal function until GFR falls below 15 mL/min/1.73 m², as renal dysfunction remains clinically asymptomatic until late in disease 5
- Always review past measurements of GFR and creatinine to determine if findings are new or represent progression of known disease 2