Definitions of AKI, CKD, and RPRF
Acute Kidney Injury (AKI)
AKI is an abrupt decrease in kidney function occurring over 7 days or less, diagnosed when serum creatinine rises ≥0.3 mg/dL within 48 hours, OR increases to ≥1.5 times baseline within 7 days, OR urine output falls below 0.5 mL/kg/h for 6 consecutive hours. 1
Diagnostic Criteria
Creatinine criterion #1: An absolute rise of ≥0.3 mg/dL (26.5 µmol/L) occurring within any 48-hour period. 1, 2
Creatinine criterion #2: A ≥1.5-fold increase from baseline (≥50% rise) known or presumed to have occurred within the preceding 7 days. 1, 2
Urine output criterion: Urine volume <0.5 mL/kg/h sustained for ≥6 consecutive hours. 1, 2
Meeting any single criterion is sufficient for AKI diagnosis; you do not need all three. 2, 3
AKI Staging (KDIGO)
Stage 1: Creatinine 1.5–1.9 × baseline OR absolute increase ≥0.3 mg/dL, OR urine output <0.5 mL/kg/h for 6–12 hours. 1, 2, 3
Stage 2: Creatinine 2.0–2.9 × baseline, OR urine output <0.5 mL/kg/h for ≥12 hours. 1, 2, 3
Stage 3: Creatinine ≥3.0 × baseline OR absolute creatinine ≥4.0 mg/dL (with acute rise ≥0.3 mg/dL) OR initiation of renal replacement therapy, OR urine output <0.3 mL/kg/h for ≥24 hours OR anuria ≥12 hours. 1, 2, 3
Clinical Significance
Even the smallest creatinine rise (≥0.3 mg/dL) is independently associated with an approximately four-fold increase in hospital mortality, indicating that patients often die "from AKI" rather than merely "with AKI." 2, 3
Progressive advancement through KDIGO stages correlates with escalating mortality risk, with Stage 3 AKI requiring dialysis carrying the greatest risk. 2, 4
Etiologic Classification
Prerenal AKI: Volume depletion, low cardiac output, or drugs affecting glomerular hemodynamics (ACE inhibitors, ARBs, NSAIDs). 4, 5, 6
Intrinsic renal AKI: Acute tubular necrosis, glomerulonephritis, acute interstitial nephritis, atheroembolic disease. 4, 5, 6
Chronic Kidney Disease (CKD)
CKD is defined as abnormalities in kidney structure or function that persist for more than 90 days, with or without decreased eGFR. 1
Diagnostic Criteria
Structural or functional abnormalities of the kidney (proteinuria, hematuria, abnormal imaging, pathological biopsy findings) with or without decreased glomerular filtration rate (GFR), persisting for ≥3 months. 1
OR GFR <60 mL/min/1.73 m² for ≥3 months with or without other markers of kidney damage. 1
Key Features
CKD represents a gradual and progressive loss of kidney function over months to years, in contrast to the abrupt decline seen in AKI. 1
CKD in its initial stages is almost always asymptomatic and is usually detected on routine screening blood work by either an increase in serum creatinine or by the presence of protein/blood in the urine. 1
CKD is staged from 1 to 5 based on eGFR, with stage 5 representing kidney failure (with or without kidney replacement therapy). 1
Rapidly Progressive Renal Failure (RPRF) / Acute Kidney Disease (AKD)
The term "rapidly progressive renal failure" has been largely replaced by the more precise concept of Acute Kidney Disease (AKD), which describes acute or subacute kidney damage and/or loss of function occurring for a duration between 7 and 90 days after exposure to an AKI initiating event. 1
AKD Definition and Temporal Framework
AKD encompasses the 7–90 day window following an AKI initiating event, capturing the subacute recovery or progression phase. 1, 2
AKD includes AKI but also includes disorders characterized by markers of kidney damage (hematuria, pyuria, urinary tract obstruction) in which the rate of decline in GFR is not as rapid as in AKI. 1
AKD can occur with OR without preceding AKI—patients can have structural kidney damage (abnormal urinalysis, proteinuria, imaging findings) without meeting functional AKI criteria. 2
AKD without AKI is nearly 3 times more prevalent than AKI itself and carries significant long-term risk. 2
Temporal Continuum
If dysfunction persists beyond 90 days, the patient is described as CKD with a history of AKD, which confers increased risk of further CKD progression. 2
Clinical Significance of AKD
Patients with AKD without AKI have an adjusted hazard ratio of 2.26 for the composite outcome of incident CKD, kidney failure, or death. 2
Patients with AKD on CKD have markedly higher mortality (47% vs 19% for CKD alone) and an odds ratio of 16.8 for incident CKD progression. 2
The AKD framework allows clinicians to capture and manage patients in the recovery or progression phase after AKI who still have ongoing kidney dysfunction. 2
The AKI-AKD-CKD Continuum
AKI, AKD, and CKD exist on a continuum, with AKI representing the acute phase (≤7 days), AKD capturing the subacute phase (7–90 days), and CKD representing persistent dysfunction beyond 90 days. 1, 2, 7
AKI/AKD can be superimposed on pre-existing CKD (AKI/AKD on CKD), creating a particularly high-risk clinical phenotype. 1, 2
AKI is a risk factor for future loss of kidney function, cardiovascular disease, and death, even when serum creatinine appears to recover. 1, 4, 7
The term AKD was specifically created to fill the gap between AKI and CKD definitions, representing a period of heightened vulnerability following AKI. 2, 7
Common Pitfalls
Do not dismiss modest absolute creatinine rises in CKD patients as "insignificant" merely because the percentage change is small; the KDIGO absolute criterion (≥0.3 mg/dL) is designed to capture clinically relevant AKI across all baseline renal functions. 2
Relying solely on serum creatinine without considering urine output criteria may miss cases of AKI. 2, 3
Urine output criteria are unreliable in cirrhotic patients with ascites and in patients on diuretic therapy. 2, 3
Using imputed baseline creatinine (back-calculating from an eGFR of 75 mL/min/1.73 m²) may overestimate AKI incidence in populations with high CKD prevalence; known creatinine values are superior. 1, 2, 3
Failure to establish an accurate baseline creatinine can lead to misclassification of AKI. 2, 3
Not recognizing that AKI can occur in both hospital and community settings, with community-acquired AKI often going undetected. 2