Causes of Acute Kidney Injury
Acute kidney injury is categorized into three main pathophysiologic mechanisms: prerenal (impaired renal perfusion), intrarenal (direct kidney parenchymal damage), and postrenal (urinary tract obstruction), with prerenal and intrarenal causes accounting for over 97% of cases. 1
Prerenal Causes (Impaired Blood Flow)
Prerenal AKI results from inadequate renal perfusion without direct kidney damage:
- Hypovolemia: Volume depletion from vomiting, diarrhea, hemorrhage, or inadequate fluid intake 1, 2
- Hypotension: Systemic arterial vasodilation, shock states, or decreased cardiac output 1
- Renal artery occlusion: Thrombosis or embolism compromising arterial blood supply 1
- Decreased effective arterial volume: Heart failure, cirrhosis with splanchnic vasodilation 3
Intrarenal (Intrinsic) Causes
Intrarenal AKI involves direct damage to kidney parenchyma and represents the most diverse category:
Acute Tubular Necrosis (Most Common)
- Ischemic injury: Prolonged prerenal states, severe hypotension, or shock 4
- Nephrotoxic injury: Direct tubular cell toxicity from filtered substances 5
Drug-Induced Kidney Injury
Drugs account for 20% of community-acquired AKI and 25% of ICU-related AKI 6. Three primary mechanisms exist:
- Direct tubular toxicity: Aminoglycosides, amphotericin B, cisplatin, contrast agents 5
- Acute interstitial nephritis: T cell-mediated immune response to antibiotics (penicillins, cephalosporins), NSAIDs, proton pump inhibitors 5
- Crystal nephropathy: Intratubular precipitation of acyclovir, methotrexate, sulfonamides causing obstruction 5
Critical drug combinations that dramatically increase AKI risk include the "triple whammy" of NSAIDs + diuretics + ACE inhibitors/ARBs 6. Each additional nephrotoxin increases AKI odds by 53%, and combining three or more nephrotoxins more than doubles AKI risk 6.
Glomerular and Vascular Diseases
- Glomerulonephritis: Immune-mediated glomerular inflammation 1
- Vasculitis: Systemic inflammatory vessel damage affecting renal vasculature 1
- Thrombotic microangiopathy: Hemolytic uremic syndrome, thrombotic thrombocytopenic purpura 7
Infection and Infiltration
- Pyelonephritis: Direct renal parenchymal infection 1
- Sepsis: Most common cause in critically ill patients, combining hemodynamic instability with inflammatory injury 8, 7
- Infiltrative diseases: Lymphoma, sarcoidosis 1
Postrenal Causes (Obstruction)
Postrenal AKI accounts for less than 3% of all AKI cases 1 but requires urgent recognition:
- Ureteral obstruction: Bilateral ureteral stones, retroperitoneal fibrosis, malignancy 1
- Bladder outlet obstruction: Benign prostatic hyperplasia, neurogenic bladder, bladder cancer 1
- Urethral obstruction: Strictures, blood clots 1
High-Risk Clinical Contexts
Hospital-Acquired AKI Risk Factors
Hospital-acquired AKI is 5-10 times more common than community-acquired AKI, affecting up to 20% of hospitalized patients and 30-60% of critically ill patients 1.
Diabetes-Related AKI
People with diabetes face elevated AKI risk from multiple mechanisms 9, 10:
- Pre-existing chronic kidney disease
- Increased susceptibility to nephrotoxic medications
- Higher infection rates
- Contrast exposure during procedures
Important caveat: Serum creatinine increases up to 30% with ACE inhibitors or ARBs in diabetic nephropathy represent hemodynamic adaptation, not true AKI, and should not prompt drug discontinuation in the absence of volume depletion 10.
Cirrhosis-Related AKI
In cirrhosis, AKI develops through 3:
- Hepatorenal syndrome (HRS-AKI): Intense renal vasoconstriction from splanchnic vasodilation and decreased effective arterial volume
- Volume depletion: From diuretics, large-volume paracentesis without albumin replacement
- Infection: Spontaneous bacterial peritonitis, other infections
- Nephrotoxic medications: NSAIDs, ACE inhibitors, ARBs
Common Pitfalls in Identifying AKI Causes
Multifactorial nature: AKI is often multifactorial rather than from a single cause 1. Systematically evaluate all three categories (prerenal, intrarenal, postrenal) rather than stopping at the first identified factor.
Drug interactions: Single nephrotoxins may be tolerated, but combinations exponentially increase risk 6. Always review the complete medication list including over-the-counter NSAIDs, which were used by 26% of CA-AKI patients versus 18% of controls 2.
Overlooking obstruction: While postrenal causes are uncommon, they are reversible with prompt intervention 1. Imaging should be obtained when clinical suspicion exists.
Confusing hemodynamic adaptation with injury: Creatinine increases with RAS blockers or SGLT2 inhibitors may represent beneficial hemodynamic effects rather than true kidney injury 10, 11.