Renal failure
Also known as: kidney failure
Renal failure is the loss of the kidneys' ability to filter waste and regulate fluid and electrolytes. It occurs as acute kidney injury, which develops rapidly and may be reversible, or chronic kidney disease, a progressive and irreversible decline.
Renal failure means the kidneys can no longer adequately filter metabolic waste, balance fluid and electrolytes, or maintain acid-base status. It presents in two forms: acute kidney injury (AKI), a rapid decline over hours to days marked by rising creatinine and often falling urine output, and chronic kidney disease (CKD), a progressive, irreversible loss of nephrons over months to years, most commonly caused by diabetes and hypertension.
AKI is classified by where the problem lies. Prerenal causes reduce blood flow to the kidney (hypovolemia, heart failure, shock) while the kidney itself is intact — classically with a BUN-to-creatinine ratio above 20:1 and concentrated urine. Intrinsic causes damage the kidney tissue itself, most often acute tubular necrosis from ischemia or nephrotoxins, with muddy brown casts in the urine. Postrenal causes obstruct urine outflow, as with stones or prostatic enlargement.
The consequences follow from lost function: fluid overload, hyperkalemia (with risk of arrhythmia), metabolic acidosis, and uremia, which can cause encephalopathy, platelet dysfunction, and pericarditis. Chronic failure adds anemia from reduced erythropoietin and renal osteodystrophy from disordered calcium and phosphate handling. Severe cases require renal replacement therapy — dialysis or transplant. Emergent indications for dialysis are captured by the AEIOU mnemonic: acidosis, electrolytes (hyperkalemia), ingestions, overload, and uremia.
Renal failure is high yield on USMLE Step 1, which tests the prerenal/intrinsic/postrenal framework, urine indices, and casts. The NCLEX approaches it through physiological adaptation, emphasizing recognition of fluid and electrolyte derangements and nursing priorities in patients with kidney failure.
Key takeaways
- Renal failure occurs as acute kidney injury (rapid, potentially reversible) or chronic kidney disease (progressive and irreversible).
- AKI is classified as prerenal (poor perfusion), intrinsic (tissue damage such as acute tubular necrosis), or postrenal (obstruction).
- A BUN:creatinine ratio above 20:1 suggests a prerenal cause, while muddy brown casts point to acute tubular necrosis.
- Key complications include hyperkalemia, fluid overload, metabolic acidosis, uremia, and, in CKD, anemia and bone disease.
- USMLE Step 1 tests the classification framework, while the NCLEX emphasizes fluid, electrolyte, and nursing management priorities.