Definition
Acute interstitial nephritis (AIN) is an immune-mediated inflammation of the renal interstitium and tubules that produces acute kidney injury. It is an important and often reversible cause of AKI, and a recurring MRCP theme because the culprit is usually a drug.
Pathophysiology
AIN is a hypersensitivity (typically T-cell mediated) reaction rather than a direct toxic effect, which is why it is idiosyncratic and not dose-related. The interstitium becomes oedematous and infiltrated with inflammatory cells, including eosinophils. Causes include:
- Drugs (the majority): antibiotics (penicillins, cephalosporins, rifampicin, ciprofloxacin), NSAIDs, proton pump inhibitors, allopurinol, diuretics
- Infections: e.g. staphylococci, streptococci, leptospirosis
- Systemic/autoimmune disease: sarcoidosis, Sjögren syndrome, SLE
Clinical presentation
- Acute kidney injury, often noticed as a rising creatinine days to weeks after starting a drug
- The classic triad of fever, rash and arthralgia occurs in only a minority — its absence does not exclude AIN
- Malaise; occasionally loin discomfort
Investigations
- Rising creatinine; peripheral eosinophilia may be seen
- Urinalysis: sterile pyuria, white cell casts, mild proteinuria, and sometimes eosinophiluria (low sensitivity and specificity)
- Renal biopsy is the gold standard — interstitial oedema with an inflammatory infiltrate rich in eosinophils
Management
- Stop the offending drug — the single most important step
- Corticosteroids are commonly given to speed recovery, especially if function fails to improve after drug withdrawal (evidence is limited)
- Supportive care and avoidance of further nephrotoxins
MRCP-specific traps
- NSAID-induced AIN frequently occurs without the fever–rash–arthralgia triad and may be accompanied by nephrotic-range proteinuria (a combined minimal-change/interstitial picture).
- AIN gives white cell casts; contrast this with the muddy-brown granular casts of acute tubular necrosis.
- Because the reaction is idiosyncratic, it can occur even at standard therapeutic doses and after prolonged, previously tolerated use (classically with PPIs).
Summary
Acute interstitial nephritis is an immune-mediated, usually drug-induced cause of AKI. Look for sterile pyuria, white cell casts and eosinophilia. Stop the culprit drug promptly; steroids are often added. Recovery is common with early recognition.