Acute Interstitial Nephritis (AIN)

Why Stop the offending medication is correct

• The fever-rash-eosinophilia-AKI cluster with WBC casts is acute interstitial nephritis from a hypersensitivity reaction.
• Removing the trigger is the essential first step and often allows renal recovery on its own.
• Corticosteroids are added only if function fails to improve after withdrawal.

Why the others are wrong

• Start high-dose loop diuretics — Diuretics treat volume overload, not immune tubular injury, and do nothing for the mechanism; a creatinine-management trap.
• Give an IV isotonic fluid bolus — No volume depletion is described, and fluids do nothing for immune-mediated interstitial inflammation, which persists as long as the drug is continued; a reflexive-AKI trap.
• Give broad empiric antibiotics — More antibiotics could worsen the drug reaction; WBC casts here reflect inflammation, not pyelonephritis.
• Begin oral prednisone therapy — steroids are tempting because AIN is an immune hypersensitivity injury, but they are not the first step: removing the trigger often allows recovery on its own, and corticosteroids are generally reserved for kidney function that does not improve within several days of withdrawal; a steroids-first trap.

Why A 10-day course of an antistaphylococcal penicillin 2 weeks ago is correct

• Acute kidney injury with fever, rash, eosinophilia, sterile pyuria, and WHITE CELL CASTS is acute interstitial nephritis, a delayed type IV hypersensitivity reaction of the renal interstitium.
• Drugs cause more than 70 percent of cases, with beta-lactams (especially antistaphylococcal penicillins such as nafcillin and methicillin, historically the prototype), sulfonamides, NSAIDs, proton pump inhibitors, rifampin, and allopurinol the usual offenders.
• The characteristic feature is LATENCY: injury appears days to weeks after the exposure begins, typically 7-10 days for a first exposure, so a completed antibiotic course two weeks earlier fits precisely while a same-week exposure does not.
• The classic triad of fever, rash, and eosinophilia is present in fewer than a third of cases, so its absence never excludes the diagnosis.
• Stop the drug; corticosteroids are used when function does not recover.

Why the others are wrong

• A contrast-enhanced CT of the abdomen performed 3 days ago — contrast-associated acute kidney injury, which peaks at 3-5 days but produces a bland sediment with a low fractional excretion of sodium and no rash or eosinophilia.
• One dose of ibuprofen taken for a headache the previous week — NSAIDs cause interstitial nephritis, but typically after weeks to months of sustained use, and NSAID-associated disease characteristically LACKS the fever, rash, and eosinophilia seen here (trap: the right drug class in the wrong dose and duration).
• A 7-day course of intravenous gentamicin ending 3 weeks ago — aminoglycosides injure the proximal tubule and cause nonoliguric acute tubular necrosis, typically after 5 to 7 days of therapy, with muddy brown granular casts rather than white cell casts and without fever, rash, or eosinophilia.
• A coronary angiogram performed through the femoral artery 3 weeks ago — atheroembolic kidney disease, which also causes subacute kidney injury with low-grade fever and eosinophilia weeks after an arterial procedure, but its skin findings are livedo reticularis and blue toes rather than a diffuse maculopapular rash, and its sediment is usually bland without white cell casts (trap: the classic eosinophilic mimic).

Urine eosinophils are neither sensitive nor specific and have largely been abandoned.