Acute kidney injury

Background

• Majority of cases of community-acquired ARF is secondary to volume depletion although consideration of pre-renal, post renal, and renal causes are important.

AKI Staging

AKI Stage	KDIGO	UOP
1	Cr 1.5-1.9x baseline over 7d or ≥0.3 mg/dL increase over 48hrs	<0.5 mL/kg/hr for 6-12hrs
2	Cr 2.0-2.9x baseline	<0.5 mL/kg/hr for >12hrs
3	Cr ≥3.0x baseline or ≥4.0 mg/dL increase or initiation of RRT	<0.3 mL/kg/hr for >24hrs or anuria for >12hrs

Chronic Kidney Disease Stages

• Useful if patient's baseline creatinine is unknown
  • Stage 1: Kidney damage (e.g. proteinuria) and normal GFR; GFR >90
  • Stage 2: Kidney damage (e.g. proteinuria) and mild decrease in GFR; GFR 60-89
  • Stage 3: Moderate decrease in GFR; GFR >30-59
  • Stage 4: Severe decrease in GFR; GFR 15-29
  • Stage 5: Kidney failure (dialysis or kidney transplant needed); GFR <15

Risk Factors

• Radiocontrast agents
  • Especiallyif GFR <60, hypovolemic
• Atherosclerosis
• Chronic hypertension
• Chronic kidney disease
• NSAIDs
• ACEI/ARB
• Sepsis
• Hypercalcemia
• Hepatorenal syndrome

Clinical Features

• Acute renal failure itself has few symptoms until severe uremia develops:
  • Nausea/vomiting, drowsiness, fatigue, confusion, coma, pericarditis
• Patients more likely to present with symptoms related to underlying cause:
  • Prerenal
    • Thirst, orthostatic lightheadedness, decreasing urine output
  • Intrinsic
    • Flank pain, hematuria
      • Nephrolithiasis
      • Papillary necrosis
      • Crystal-induced nephropathy
    • Myalgias, seizures, recreational intoxication
      • Pigment-induced ARF (rhabdomyolysis)
    • Darkening urine and edema (esp with preceding pharyngitis or cutaneous infection)
      • Acute glomerulonephritis
    • Fever, arthralgia, rash
      • Acute interstitial nephritis
    • Cough, dyspnea, hemoptysis
      • Goodpasture syndrome, granulomatosis with polyangiitis (Wegener's)
  • Postrenal
    • Alternating oliguria and polyuria is pathognomonic of obstruction
    • Anuria

Etiologies

Prerenal

• Hypovolemia
  • GI: decreased intake, vomiting and diarrhea
  • Hemorrhage
  • Pharmacologic: diuretics
  • Third spacing
    • Pancreatitis
  • Skin losses: hyperthermia, burns
  • Miscellaneous
    • Hypoaldosteronism
    • Salt-losing nephropathy
    • Postobstructive diuresis
• Hypotension
  • Sepsis
  • Decreased cardiac output: decompensated heart failure, tamponade, massive pulmonary embolus
  • Hepatorenal Syndrome
    • Ischemia/infarction
    • Valvulopathy
  • Pharmacologic
    • Beta-blockers
    • Calcium-channel blockers
    • Antihypertensive medications
  • High output heart failure
    • Thyrotoxicosis
    • AV fistula
• Renal artery and small-vessel disease
  • Embolism: thrombotic, septic, cholesterol
  • Thrombosis: atherosclerosis, vasculitis, sickle cell disease
  • Dissection
  • Pharmacologic
    • NSAIDs
    • ACEI/ARB
      • Observed shortly after initiation of therapy
  • Microvascular thrombosis
    • Preeclampsia
    • Hemolytic Uremic Syndrome (HUS)
    • Thrombotic Thrombocytopenic Purpura (TTP)
    • Disseminated Intravascular Coagulation (DIC)
    • Vasculitis
    • Sickle Cell Disease
  • Hypercalcemia

Intrinsic

• Tubular diseases
  • Ischemic acute tubular necrosis
    • Caused by more advanced disease due to the prerenal causes
• Nephrotoxins
  • Aminoglycosides, radiocontrast, amphotericin B, heme pigments (rhabdomyolysis, hemolysis)
  • Obstruction
    • Uric acid, calcium oxalate from Ethylene Glycol Toxicity, Multiple myeloma (immunoglobin light chains), amyloidosis
    • Pharmacologic: sulfonamides, triamterene, acyclovir, indinavir
• Interstitial diseases
  • Acute interstitial nephritis: typically a drug reaction (NSAIDs, Penicillins and antibiotics, Diuretics, phenytoin)
  • Infection: bilateral pyelonephritis, Legionella, Hantavirus
  • Infiltrative disease: sarcoidosis, lymphoma
  • Autoimmune diseases: SLE
• Glomerular diseases
  • Rapidly progressive glomerulonephritis
    • Goodpasture syndrome, granulomatosis with polyangiitis (Wegener's) HSP, SLE, membranoproliferative GN
  • Postinfectious glomerulonephritis
• Small-vessel diseases
  • Microvascular thrombosis
    • Preeclampsia, HUS, DIC, TTP, vasculitis (PAN, SCD, atheroembolism)
  • Malignant hypertension
  • Scleroderma
  • Renal vein thrombosis
• Abdominal compartment syndrome
• Hepatorenal syndrome
• Cardiorenal syndrome

Postrenal

• Infants and children
  • Urethra and bladder outlet
    • Anatomic malformations
      • Urethral atresia
      • Meatal stenosis
      • Anterior and posterior urethral valves
  • Ureter
    • Anatomic malformations
      • Vesicoureteral reflux (female preponderance)
      • Ureterovesical junction obstruction
      • Ureterocele
      • Retroperitoneal tumor
• All ages
  • Various locations in GU tract
    • Trauma
    • Blood clot
  • Urethra and bladder outlet
    • Phimosis or urethral stricture (male preponderance)
    • Neurogenic bladder
      • Diabetes mellitus, spinal cord disease, multiple sclerosis, Parkinson's disease
      • Pharmacologic: anticholinergics, a-adrenergic antagonists, opioids
• Adults
  • Urethra and bladder outlet
    • BPH
    • Cancer of prostate, bladder, cervix, or colon
    • Obstructed catheters
  • Ureter
    • Ureteral calculi, uric acid crystals
    • Papillary necrosis
      • Sickle cell disease, DM, pyelonephritis
    • Tumor: Ureter, uterus, prostate, bladder, colon, rectum; retroperitoneal lymphoma
    • Retroperitoneal fibrosis: idiopathic, tuberculosis, sarcoidosis, propranolol
    • Stricture: TB, radiation, schistosomiasis, NSAIDs
    • Miscellaneous
      • Aortic aneurysm
      • Pregnant uterus
      • IBD
      • Renal or ureter trauma

Evaluation

Work-up

• Urine
• Prostate exam
• Urinalysis, urine sodium, urine creatinine, urine urea
• ECG (hyperkalemia)
• Chronic renal failure features
  • Anemia, thrombocytopenia
  • Iron studies with low Fe, low TIBC, low iron saturation, normal ferritin
  • Secondary rise in PTH, high phos, low calcium

Imaging

• CXR
  • Evidence of volume overload, pneumonia
• US: renal/bladder
  • Test of choice in setting of acute renal failure
  • Bladder size (post-void)
  • Hydronephrosis
  • IVC collapsibility (prerenal)
• CT
  • Useful to determine cause of post renal failure (identification of abdominal masses etc.)
  • Should generally not be used with IV contrast due to potential risk for CIN
  • Indicated if hydronephrosis found on ultrasound in order to define the location of obstruction

Diagnosis

Finding	Prerenal	Intrinsic	Postrenal
BUN/creatinine ratio	>20	< 12	12-20 (normal range)
FeNa^	<1% < 2% for neonates	>1% > 2.5% for neonates	>1%
Urine osm	>500	<350
Urine sodium	< 20 mEq/L	> 40 mEq/L
Specific gravity	> 1.020	< 1.020
Fractional excretion of urea	< 35%	> 50%
Microscopic analysis	Hyaline casts	Acute glomerulonephritis: RBCs, casts Acute tubular necrosis: protein, tubular epithelial cells Interstitial nephritis: eosinophils

^ (urine sodium/plasma sodium) / (urine creatinine / serum creatinine)

Management

Treat underlying cause

• Prerenal: IVF (or pRBCs if bleeding)
• Intrinsic: Depends on cause
• Obstruction:
  • Note: Postobstructive diuresis can result in significant volume loss and death
    • Typically occurs when obstruction has been prolonged / has resulted in renal failure
    • Admit patients with persistent diuresis of >250 mL/h for >2hr
  • Foley Catheter, consider Coude Catheter
  • Suprapubic (if Coude fails)

Dialysis

• Indicated for:
  • A: Acidosis (severe)
  • E: Electrolyte abnormality (e.g. uncontrolled hyperkalemia)
  • I: Ingestions (lithium, ASA, methanol, ethylene glycol, theophylline)
  • O: Overload (volume) with persistent hypoxia
  • U: Uremic pericarditis/encephalopathy/bleeding dyscrasia
  • Also:
    • Na <115 or >165 mEq/L
    • creatinine > 10
    • BUN >100
• See Dialysis catheter placement
• See Austere peritoneal dialysis

Phlebotomy to Treat Pulmonary Edema

• Possible last ditch effort to tide patient over to formal dialysis if hours away
• If traditional pulmonary edema treatments are not working for SCAPE patient, one may attempt to remove 200-300 cc of blood as a bridge to dialysis
• If Hb too low, may consider temporary venous tourniquets on each leg proximally q30 min to reduce preload volume to the heart, alternating legs

Disposition

• Admit

See Also

• Hyperkalemia
• Renal ultrasound
• Hypertensive emergency

External Links

• DDxOf: Differential Diagnosis of Acute Kidney Injury

References

📊 Fractional Excretion of Sodium (FENa) Calculator [show]

Fractional Excretion of Sodium (FENa)

FENa — Fractional Excretion of Sodium

Parameter	Value
Serum Sodium (mEq/L)
Serum Creatinine (mg/dL)
Urine Sodium (mEq/L)
Urine Creatinine (mg/dL)
FENa (%)	%

Interpretation (in setting of oliguria/AKI)
<1%	Pre-renal azotemia — Kidneys are sodium-avid (hypoperfusion, hypovolemia, heart failure, cirrhosis). Consider volume resuscitation.
>2%	Intrinsic renal disease — ATN, AIN, or glomerulonephritis. Kidneys unable to concentrate urine.
1–2%	Indeterminate — May be seen in early ATN or with mixed etiologies. Clinical correlation required.
Important Caveats
FENa is unreliable on diuretics — use FEUrea instead Low FENa (<1%) can be seen in contrast nephropathy, rhabdomyolysis, early obstruction Not validated in CKD patients

References
Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236:579-581. PMID 947239. Steiner RW. Interpreting the fractional excretion of sodium. Am J Med. 1984;77:699-702. PMID 6486145.

📊 Creatinine Clearance (Cockcroft-Gault) Calculator [show]

Creatinine Clearance (Cockcroft-Gault)

Creatinine Clearance (Cockcroft-Gault)

Parameter	Value
Age (years)
Sex	1 Male   Female
Weight (kg)
Serum Creatinine (mg/dL)
CrCl (mL/min)	mL/min

Interpretation
>90	Normal renal function.
60–89	Mildly decreased (CKD Stage 2).
30–59	Moderately decreased (CKD Stage 3). Adjust renally-dosed medications.
15–29	Severely decreased (CKD Stage 4).
<15	Kidney failure (CKD Stage 5). Consider dialysis.

References
Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron. 1976;16(1):31-41. PMID 1244564. Formula: CrCl = [(140 − age) × weight (kg)] / [72 × serum Cr (mg/dL)] × 0.85 if female. Note: Use IBW or adjusted BW in obese patients. Not validated in AKI or rapidly changing creatinine.

📊 Schwartz Equation (Pediatric GFR) Calculator [show]

Schwartz Equation (Pediatric GFR)

Schwartz Equation — Pediatric eGFR

Parameter	Value
Height (cm)
Serum Creatinine (mg/dL)
Estimated GFR	mL/min/1.73m²

References
Schwartz GJ, Muñoz A, Schneider MF, et al. New equations to estimate GFR in children with CKD. J Am Soc Nephrol. 2009;20(3):629-637. PMID 19158356. Bedside Schwartz (2009): eGFR = 0.413 × height (cm) / serum creatinine (mg/dL). Valid for ages 1-16 years.