Matching Acute and Chronic Kidney Disease Definitions
Chronic Kidney Disease
Chronic kidney disease is defined as kidney damage or decreased kidney function (GFR <60 mL/min per 1.73 m²) persisting for 3 or more months, commonly caused by conditions like diabetes, hypertension, and chronic decreased renal blood flow. 1
- CKD represents long-term, persisting conditions affecting kidney structure and/or function for >3 months 1
- The definition includes either markers of kidney damage (such as proteinuria with albumin-creatinine ratio >30 mg/g) or decreased GFR below 60 mL/min per 1.73 m² 1
- Common underlying causes include diabetes mellitus, hypertension, and conditions causing chronic decreased renal blood flow 1, 2
- CKD is staged from 1-5 based on GFR levels, with stage 5 representing kidney failure (GFR <15 mL/min per 1.73 m²) 1
Acute Kidney Injury
Acute kidney injury is characterized by an abrupt (occurring over days to weeks) decrease in GFR, defined by either a rise in serum creatinine ≥0.3 mg/dL within 48 hours or ≥50% increase from baseline within 7 days, and is potentially reversible. 1, 3
- AKI represents a condition of recent or sudden onset that is short-lived and reversible 1
- The functional decline occurs rapidly over intervals from 6 hours to 7 days 1
- AKI is a subset of acute kidney disease (AKD), which encompasses kidney abnormalities lasting <3 months 1, 3
- Even small increases in serum creatinine (≥0.3 mg/dL) are independently associated with approximately fourfold increase in hospital mortality 3
Prerenal Causes
Prerenal causes represent conditions affecting renal perfusion before blood reaches the kidney parenchyma, including decreased renal blood flow from volume depletion, heart failure, and hypotension. 1
- These causes involve decreased perfusion without direct parenchymal damage initially 1
- Examples include volume depletion, heart failure, and conditions causing reduced effective circulating volume 1
- Prerenal causes are potentially reversible if perfusion is restored before parenchymal injury develops 1
Intrarenal Cause
Intrarenal causes involve direct parenchymal kidney damage, with acute tubular necrosis being the most common example, affecting both kidney structure and function. 1
- Acute tubular necrosis (ATN) is the most common intrarenal cause of AKI 1
- Other intrarenal causes include glomerulonephritis and interstitial nephritis affecting kidney parenchyma directly 1
- These conditions cause both structural and functional abnormalities within the kidney tissue itself 1
Oliguric Phase
The oliguric phase during acute kidney injury is characterized by urine output <0.5 mL/kg/h for 6 hours or more, representing an abrupt decrease in GFR. 3
- Oliguria is defined as reduced urine output to less than 0.5 mL/kg/h for 6 hours or more 3
- This phase represents severe functional impairment during the acute injury period 3
- However, urine output criteria can be unreliable in certain populations, particularly patients with cirrhosis and ascites who may be oliguric despite maintaining relatively normal GFR 3
Non-Oliguric
Non-oliguric acute kidney injury occurs when kidney function declines without significant reduction in urine output, maintaining urine output ≥0.5 mL/kg/h despite rising serum creatinine. 3
- Non-oliguric AKI is diagnosed based on serum creatinine criteria alone without meeting urine output criteria 3
- This form generally carries better prognosis than oliguric AKI 3
- Relying solely on serum creatinine without considering urine output criteria may miss some cases of AKI 3
Important Clinical Considerations
- AKI can progress to acute kidney disease (AKD) if kidney abnormalities persist beyond 7 days but less than 3 months 1, 4
- If kidney dysfunction persists beyond 3 months, it transitions from AKD to CKD 1
- Ultrasound can help differentiate: normal to enlarged kidneys (>10 cm) suggest AKI, while small echogenic kidneys indicate CKD 5
- AKD occurs in approximately 25% of AKI survivors and significantly increases risk of progression to CKD 4