Acute Kidney Injury

Acute kidney injury (AKI) is a sudden, often reversible drop in the kidney's ability to filter blood, clear waste, and balance fluids and electrolytes — measured over hours to days. It is one of the most common and most consequential problems in hospital medicine: up to one in five hospitalized adults and more than half of critically ill patients develop some degree of AKI, and even mild episodes raise the risk of death, prolonged admission, and future chronic kidney disease. The good news is that a large fraction of AKI is preventable or reversible if you recognize it early and reason clearly about why the kidney is failing.

The single most useful mental model in all of AKI is anatomical: is the problem before the kidney (blood not getting in), within the kidney (the filtering tissue itself is injured), or after the kidney (urine can't get out)? Get that triage right and the workup and treatment almost write themselves. This page teaches you to think like a nephrologist at the bedside.

Learning Objectives

• Define AKI using the KDIGO creatinine and urine-output criteria and stage its severity.
• Classify AKI into pre-renal, intrinsic renal, and post-renal categories with the key causes of each.
• Interpret urinary indices (fractional excretion of sodium, urine osmolality, urine microscopy) to distinguish pre-renal azotemia from acute tubular necrosis.
• Build a systematic diagnostic workup, including when to image and when to biopsy.
• Apply evidence-based management: volume optimization, removing nephrotoxins, treating the cause, and recognizing the indications for dialysis.
• Understand the historical origin of AKI as a recognized syndrome and how crush injury in wartime drove the invention of dialysis.

Quick Answer

AKI is an abrupt fall in glomerular filtration rate, defined by KDIGO as a rise in serum creatinine of at least 0.3 mg/dL within 48 hours, a 1.5-fold rise over baseline within 7 days, or urine output under 0.5 mL/kg/h for 6 hours. Causes divide into pre-renal (hypoperfusion — dehydration, hemorrhage, heart failure, sepsis), intrinsic renal (tubular, glomerular, interstitial, or vascular injury, most commonly acute tubular necrosis), and post-renal (obstruction of urine flow). Diagnosis rests on the history, volume assessment, urinalysis with microscopy, urinary indices, and renal ultrasound. Management is largely supportive: restore perfusion, stop nephrotoxins, relieve obstruction, treat the underlying cause, and correct dangerous complications such as hyperkalemia, acidosis, and fluid overload. Dialysis is reserved for refractory complications — remembered by the mnemonic AEIOU.

Where It Came From

Physicians had described sudden suppression of urine for centuries — "ischuria renalis" appears in eighteenth-century texts — but AKI as a coherent syndrome was born in the twentieth century, forged in disaster. During the German bombing of London in 1940–41, physicians Eric Bywaters and Desmond Beall at the Hammersmith Hospital noticed a pattern in people pulled from collapsed buildings. Victims trapped under rubble with crushed limbs would seem stable on rescue, then over the next day pass dark, scanty urine, swell, and die of what we now recognize as hyperkalemic cardiac arrest and uremia. Bywaters' meticulous 1941 papers established crush syndrome: prolonged muscle compression releases myoglobin, potassium, and other toxins that, on reperfusion, flood the circulation and plug and poison the renal tubules. This was the founding clinical description of what became acute tubular necrosis.

The need was now stark — young, previously healthy people were dying of a temporary, potentially survivable problem simply because their kidneys had stopped for a week or two. If you could take over the kidney's job long enough for the tubules to heal, patients would live. The answer came from the Netherlands. Willem Kolff, a Dutch physician working under Nazi occupation, built the first practical artificial kidney from cellophane sausage casing, a wooden drum, and washing-machine parts. After many failures, in 1945 he treated a 67-year-old woman in uremic coma; she woke and lived years more. Kolff smuggled his designs out after the war, seeding dialysis programs worldwide. The Korean War (1950–53) then proved dialysis could dramatically cut mortality from battlefield AKI. Later, Belding Scribner's arteriovenous shunt (1960) made repeated dialysis feasible, opening the door to chronic dialysis. Modern staging systems — RIFLE (2004), AKIN (2007), and today's KDIGO (2012) — reflect a later realization: even small, early creatinine changes matter, so we should catch AKI before it becomes crush-syndrome-severe.

Defining and Staging AKI

The old term "acute renal failure" was retired because "failure" implied the organ had already collapsed. "Injury" captures the whole spectrum, including subtle early damage. KDIGO diagnoses AKI if any of these are met:

• Serum creatinine rises by at least 0.3 mg/dL (26.5 micromol/L) within 48 hours, or
• Serum creatinine rises to at least 1.5 times baseline within the prior 7 days, or
• Urine output is less than 0.5 mL/kg/h for 6 hours.

Severity is then staged 1 to 3:

Stage	Creatinine	Urine output
1	1.5–1.9x baseline, or rise of at least 0.3 mg/dL	less than 0.5 mL/kg/h for 6–12 h
2	2.0–2.9x baseline	less than 0.5 mL/kg/h for at least 12 h
3	3.0x baseline, creatinine at least 4.0 mg/dL, or need for dialysis	less than 0.3 mL/kg/h for at least 24 h, or anuria for at least 12 h

Two cautions. First, creatinine is a lagging marker — it can take a day or two to rise after the injury, so a "normal" creatinine early does not exclude significant damage. Second, baseline creatinine may be unknown; use the lowest recent value or estimate. Urine output is often the earliest clue in the sick patient, which is why hourly output is charted in the ICU.

The Three Categories: Pre-renal, Intrinsic, Post-renal

Pre-renal AKI (roughly 40–70% of cases) is the kidney's appropriate response to poor perfusion — the tissue itself is undamaged, so it is fully reversible if perfusion is restored quickly. Causes include true volume loss (vomiting, diarrhea, hemorrhage, burns, diuretics), reduced effective circulating volume (heart failure, cirrhosis, sepsis with vasodilation), and drugs that cripple renal autoregulation. Here two drug classes deserve special respect: NSAIDs constrict the afferent (incoming) arteriole by blocking prostaglandins, and ACE inhibitors / ARBs dilate the efferent (outgoing) arteriole by blocking angiotensin II. Each alone is usually tolerated; combined with a diuretic ("triple whammy") in a dehydrated patient, they can precipitate AKI. If pre-renal hypoperfusion persists, the tubules eventually die and it converts to acute tubular necrosis.

Intrinsic renal AKI means structural damage, subdivided by the compartment injured:

• Tubular — acute tubular necrosis (ATN) is the dominant cause. Two flavors: ischemic (prolonged pre-renal states, shock, sepsis) and toxic (aminoglycosides, IV contrast, myoglobin from rhabdomyolysis, hemoglobin, chemotherapy, tumor lysis with urate crystals). This is the direct descendant of crush syndrome.
• Interstitial — acute interstitial nephritis (AIN), an allergic-type reaction most often to drugs (antibiotics like penicillins and cephalosporins, PPIs, NSAIDs), classically with rash, fever, and eosinophilia, though the full triad is uncommon.
• Glomerular — the acute glomerulonephritides (post-infectious, ANCA-associated vasculitis, anti-GBM/Goodpasture, lupus), signaled by an active urine sediment.
• Vascular — thrombotic microangiopathy (HUS/TTP), atheroembolic disease after angiography, renal artery/vein thrombosis, malignant hypertension.

Post-renal AKI (5–10%) is obstruction. Because one healthy kidney can handle the load, obstruction causing AKI must either be bilateral or affect a single functioning kidney, or be at the bladder outlet/urethra. The classic culprit is benign prostatic hyperplasia in older men (painless retention), plus bilateral stones, pelvic or retroperitoneal malignancy, and neurogenic bladder. This is the category you must never miss — a bladder scan and a catheter can reverse it in minutes.

A Worked Clinical Example

A 74-year-old man with hypertension is admitted with three days of vomiting and poor intake. He takes lisinopril, ibuprofen for knee pain, and a thiazide. Blood pressure is 96/60 lying, dropping on standing; mucous membranes are dry. Creatinine is 2.6 mg/dL (baseline 1.0), urea:creatinine ratio is high, urine output is 15 mL/h. Urinalysis is bland; urine sodium is low. This is textbook pre-renal AKI: hypovolemia plus the "triple whammy" of ACE inhibitor, NSAID, and diuretic. Management: stop all three drugs, give balanced IV crystalloid, and recheck. If creatinine falls promptly with rehydration, the diagnosis is confirmed. If it plateaus or the urine develops muddy-brown granular casts, ischemic ATN has supervened.

Diagnosis: Reading the Urine and the Vessels

Start with history and volume assessment — recent illness, drugs, contrast exposure, blood pressure, jugular venous pressure, skin turgor, and urine output trend. Then three cheap, high-yield tests:

1. Urinalysis with microscopy is the "renal biopsy of the poor man." A truly bland sediment points to pre-renal or post-renal. Muddy-brown granular casts indicate ATN. Red cell casts and dysmorphic red cells mean glomerulonephritis. White cell casts and eosinophils suggest interstitial nephritis or pyelonephritis. Heavy proteinuria points to glomerular disease.

2. Urinary indices help separate pre-renal from ATN. In pre-renal states the tubules are intact and avidly reabsorb sodium and water, so urine is concentrated and sodium-poor; in ATN the damaged tubules cannot do this.

Index	Pre-renal	ATN
Fractional excretion of sodium (FENa)	less than 1%	greater than 2%
Urine sodium	less than 20 mmol/L	greater than 40 mmol/L
Urine osmolality	greater than 500 mOsm/kg	less than 350 (isosthenuric)
Urea:creatinine ratio (serum)	high	normal
Urine sediment	bland/hyaline casts	muddy-brown granular casts

FENa is calculated as (urine sodium x plasma creatinine) / (plasma sodium x urine creatinine) x 100. A critical caveat: diuretics invalidate FENa by forcing sodium loss — use the fractional excretion of urea (FEUrea, less than 35% suggests pre-renal) instead. FENa is also low in some intrinsic causes (contrast nephropathy, early glomerulonephritis, hepatorenal syndrome), so never read it in isolation.

3. Renal ultrasound is the key structural test — it detects hydronephrosis (post-renal obstruction) and shows kidney size (small, scarred kidneys imply underlying chronic disease). Order it in nearly every AKI without an obvious pre-renal cause, and urgently if obstruction is suspected.

When to escalate: if the cause remains unclear, urine shows an active sediment, or a rapidly progressive glomerulonephritis or vasculitis is suspected, send immunologic serologies (ANA, ANCA, anti-GBM, complements) and consult nephrology for possible renal biopsy. Biopsy is the definitive test for intrinsic parenchymal disease and often changes treatment (e.g., immunosuppression for vasculitis).

Management: Support, Correct, Rescue

There is no drug that "fixes" damaged tubules — management buys time for healing while preventing death from complications.

• Optimize perfusion. For hypovolemic and most pre-renal AKI, give balanced crystalloids (Ringer's lactate/Plasma-Lyte are generally preferred over large-volume 0.9% saline, which can cause hyperchloremic acidosis). But titrate carefully: once the patient is euvolemic, more fluid only causes edema and worsens outcomes. In sepsis, restore mean arterial pressure with fluids and vasopressors (norepinephrine).
• Remove the insult. Stop nephrotoxins (NSAIDs, ACE inhibitors/ARBs, aminoglycosides), adjust all drug doses for the reduced GFR, and avoid iodinated contrast where possible.
• Relieve obstruction promptly — bladder catheter for outlet obstruction, nephrostomy or stent for upper-tract obstruction. Watch for post-obstructive diuresis afterward and replace fluids.
• Treat the specific cause — steroids and stopping the offending drug in AIN; immunosuppression in vasculitis/glomerulonephritis; aggressive fluids and urine alkalinization in rhabdomyolysis.
• Manage complications. Treat hyperkalemia (calcium gluconate to stabilize the myocardium, insulin-dextrose and beta-agonists to shift potassium intracellularly, then removal with diuretics, binders, or dialysis). Correct severe metabolic acidosis, restrict fluid and sodium in overload, and manage uremic symptoms.

Diuretics do not treat AKI — they may help manage fluid overload once you have a diagnosis, but they do not improve renal recovery or survival, and "low-dose dopamine" is now firmly discredited.

Indications for Dialysis: AEIOU

Renal replacement therapy is started for life-threatening complications refractory to medical management, remembered as AEIOU:

• A — severe metabolic Acidosis
• E — refractory Electrolyte disturbance (especially hyperkalemia)
• I — Ingestions/toxins that are dialyzable (methanol, ethylene glycol, salicylates, lithium)
• O — fluid Overload refractory to diuretics (e.g., pulmonary edema)
• U — Uremic complications (pericarditis, encephalopathy, bleeding)

There is no fixed creatinine "number" that mandates dialysis; the decision is clinical. Trials (STARRT-AKI, AKIKI) show that in the absence of these emergencies, watchful waiting is as good as early dialysis.

Real-World Applications

AKI reasoning shows up everywhere in clinical practice. On the surgical ward, a post-operative patient with falling urine output needs you to distinguish under-resuscitation (give fluid) from a blocked catheter (flush it) from developing ATN. In the emergency department, an older man with a huge palpable bladder needs a catheter, not a fluid bolus. Before ordering a contrast CT in a diabetic with reduced GFR, you weigh the (now-known-to-be-modest) risk of contrast-associated AKI and ensure hydration. In oncology, starting chemotherapy in a bulky lymphoma demands prophylaxis against tumor-lysis AKI. And everyday prescribing carries renal stakes: advising an older patient to hold their ACE inhibitor and diuretic during a bout of gastroenteritis ("sick-day rules") is a simple intervention that prevents countless admissions.

Common Mistakes

1. "The creatinine is normal, so the kidneys are fine." Creatinine lags 24–48 hours behind the injury and depends on muscle mass — a frail elderly person with a creatinine of 1.1 may already have markedly reduced GFR. Trend the value and watch urine output; don't be reassured by a single early number.

2. Giving fluids to every AKI. Fluids help hypovolemic pre-renal AKI but harm the patient who is already fluid-overloaded (decompensated heart failure, oliguric ATN). Assess volume status first; in the wrong patient, boluses cause pulmonary edema and worsen mortality.

3. Forgetting to exclude obstruction. Post-renal AKI is easy to reverse and disastrous to miss. Always consider a bladder scan and ultrasound, especially in older men, patients with pelvic cancer, or anyone with anuria (complete anuria suggests either obstruction or a catastrophic vascular event, not typical ATN).

4. (Bonus) Trusting FENa in a patient on diuretics. Diuretics artificially raise urinary sodium and FENa, mimicking ATN even when the patient is pre-renal. Use FEUrea instead.

Comparison and Connections

The most tested distinction is pre-renal azotemia versus ATN — clinically they overlap because untreated pre-renal states become ATN. The urinary indices table above is the core discriminator: intact, sodium-avid tubules (low FENa, concentrated urine, bland sediment) mean pre-renal; broken tubules (high FENa, isosthenuric urine, muddy-brown casts) mean ATN.

Also distinguish AKI from chronic kidney disease (CKD). Clues to CKD include a known long-standing elevated creatinine, small echogenic kidneys on ultrasound, anemia, and secondary hyperparathyroidism with bone changes — features that take months to develop. Remember that the two coexist: "acute-on-chronic" kidney injury is common, and CKD is a major risk factor for AKI.

Feature	Pre-renal	Intrinsic (ATN)	Post-renal
Core problem	Low perfusion	Tubular/parenchymal damage	Obstructed outflow
Reversibility	Rapid with fluids	Days to weeks	Rapid with drainage
Urine sodium	Low	High	Variable
Sediment	Bland	Granular casts	Bland or crystals/RBCs
Key test	Response to fluid	Microscopy	Ultrasound/bladder scan

Practice Questions

Recall

Q: State the three KDIGO creatinine/urine criteria for diagnosing AKI. A: A creatinine rise of at least 0.3 mg/dL within 48 hours; a rise to at least 1.5x baseline within 7 days; or urine output under 0.5 mL/kg/h for 6 hours.

Understanding

Q: Explain why FENa is low in pre-renal AKI but high in ATN. A: In pre-renal AKI the tubules are structurally intact and respond to hypoperfusion (via aldosterone and low tubular flow) by avidly reabsorbing sodium, so little sodium is excreted (FENa less than 1%). In ATN the tubular cells are damaged and cannot reabsorb sodium, so a larger fraction escapes into the urine (FENa greater than 2%).

Application

Q: A 68-year-old man presents with 18 hours of anuria and a firm, palpable suprapubic mass. Creatinine is 5.1 mg/dL. What is your immediate diagnostic and therapeutic step? A: Complete anuria with a palpable bladder points to bladder-outlet obstruction (likely BPH). Perform a bladder scan and insert a urinary catheter immediately — this is both diagnostic and therapeutic. Then arrange renal ultrasound and monitor for post-obstructive diuresis, replacing fluids as needed.

Analysis

Q: A patient on furosemide for heart failure develops AKI. Urinalysis is bland and FENa is 3%. Does the high FENa prove ATN? How would you clarify? A: No. Loop diuretics force renal sodium loss and spuriously elevate FENa, so a "high" value cannot distinguish pre-renal from ATN here. Calculate the fractional excretion of urea instead (FEUrea less than 35% supports pre-renal, since urea handling is less affected by diuretics), and interpret alongside volume status, the bland sediment, and response to careful fluid/diuretic adjustment.

FAQ

Is acute kidney injury reversible? Often, yes — especially pre-renal and post-renal causes caught early, and many cases of ATN, whose tubular cells can regenerate over 1–3 weeks. But recovery is not guaranteed; each episode raises the long-term risk of chronic kidney disease, so survivors need follow-up creatinine checks.

How is AKI different from chronic kidney disease? AKI develops over hours to days and is potentially reversible; CKD develops over months to years and is generally permanent. Small kidneys, anemia, and stable long-standing elevated creatinine point to CKD. The two frequently overlap as "acute-on-chronic."

Does contrast dye really cause kidney failure? Modern evidence suggests contrast-associated AKI is real but much less common and less severe than once feared, and much of the historical risk was attributed to the underlying illness rather than the dye. In high-risk patients (low GFR, diabetes) ensure good hydration and avoid unnecessary contrast, but do not withhold a genuinely needed scan out of fear.

Why can't you just give a diuretic to make the kidneys work again? Diuretics act on tubular cells that are already injured; they do not repair the kidney or improve survival, and forcing urine output can create a falsely reassuring picture while damage continues. They are used only to manage fluid overload, not to treat AKI itself.

What does it mean if I have muddy-brown casts in my urine? These granular casts are the microscopic signature of acute tubular necrosis — sloughed, degenerated tubular cells packed into casts. They strongly suggest structural tubular injury rather than simple dehydration.

When will I need dialysis? Only if AKI causes a complication that cannot be controlled medically — dangerous potassium levels, severe acidosis, fluid overload flooding the lungs, uremic symptoms, or certain poisonings (the AEIOU list). Many people with AKI recover without ever needing it.

Quick Revision

• AKI = abrupt fall in GFR; KDIGO: creatinine up 0.3 mg/dL in 48 h, or 1.5x in 7 days, or urine under 0.5 mL/kg/h for 6 h.
• Three buckets: pre-renal (perfusion), intrinsic (ATN, AIN, GN, vascular), post-renal (obstruction).
• ATN is the descendant of WWII crush syndrome; dialysis was invented by Kolff (1945) to bridge such patients.
• Pre-renal: FENa less than 1%, concentrated urine, bland sediment. ATN: FENa greater than 2%, isosthenuric urine, muddy-brown casts.
• Diuretics invalidate FENa — use FEUrea.
• Always exclude obstruction (bladder scan, ultrasound), especially in anuria or older men.
• Management = restore perfusion, stop nephrotoxins, relieve obstruction, treat cause, correct hyperkalemia/acidosis/overload.
• Dialysis indications: AEIOU — Acidosis, Electrolytes, Ingestions, Overload, Uremia.
• Creatinine lags; a normal early value does not exclude AKI.

Related Topics

Prerequisites

• Nephrology overview
• Renal physiology and glomerular filtration (see ../../2._Physiology/index.md)

Related Topics

• Chronic kidney disease and dialysis (nephrology branch)
• Fluid, electrolyte, and acid–base disorders — hyperkalemia and metabolic acidosis
• Shock and sepsis (see ../../24._Critical_Care_Medicine/index.md)

Next Topics

• Glomerulonephritis and the nephritic/nephrotic syndromes
• Renal replacement therapy: hemodialysis, peritoneal dialysis, and CRRT