Acute Interstitial Nephritis (AIN)
• Definition: Inflammatory infiltrate in the renal interstitium, often immune-mediated
• Causes:
o Drugs (most common):
Penicillins, cephalosporins, rifampicin
NSAIDs
Proton pump inhibitors (PPIs)
Allopurinol, sulfonamides
o Infections: streptococcal, leptospirosis, TB, CMV
o Systemic diseases: SLE, Sjögren’s, sarcoidosis
• Clinical Features:
o Fever, rash, arthralgia (classic triad, uncommon)
o AKI, sterile pyuria
o Eosinophilia ± eosinophiluria
• Diagnosis:
o Urine: WBCs, WBC casts, eosinophils
o Biopsy: interstitial oedema and inflammatory infiltrate (often lymphocytes, plasma cells, eosinophils)
• Management:
o Withdraw offending agent
o Corticosteroids if no improvement
o Monitor renal function — may progress to CKD if delayed
Chronic Interstitial Nephritis
• Definition: Progressive interstitial fibrosis and tubular atrophy due to chronic injury
• Causes:
o Drugs/toxins:
Lithium (↓ concentrating ability, nephrogenic DI)
Analgesic nephropathy (NSAIDs ± paracetamol)
Heavy metals (lead, cadmium)
o Metabolic: hypercalcaemia, hyperuricaemia
o Infections: chronic pyelonephritis, TB
o Obstructive uropathy, reflux nephropathy
• Clinical Features:
o Polyuria/nocturia (due to tubular dysfunction)
o Non-nephrotic proteinuria
o Slowly progressive CKD
o Small, shrunken kidneys on imaging
• Diagnosis:
o Urinalysis: bland sediment, mild proteinuria
o Imaging: small echogenic kidneys
o Biopsy (if performed): interstitial fibrosis and tubular atrophy
• Management:
o Remove cause if possible
o Manage complications of CKD
o Avoid further nephrotoxins
Renal Papillary Necrosis
• Definition: Ischaemic necrosis of renal papillae
• Causes (mnemonic: POSTCARDS):
o Pyelonephritis
o Obstruction of urinary tract
o Sickle cell disease/trait
o Tuberculosis
o Cirrhosis
o Analgesics (especially NSAIDs)
o Renal vein thrombosis
o Diabetes mellitus
o Systemic vasculitis
• Clinical Features:
o Flank pain, haematuria, pyuria
o May cause obstruction if sloughed papillae
• Diagnosis:
o CT urography or IVU: ring sign or filling defects
o Urine: necrotic tissue or clumps of cells
• Management:
o Treat underlying cause
o Supportive care; may require stenting if obstructive
Renal Involvement in Sarcoidosis
• Pathophysiology:
o Non-caseating granulomas infiltrate interstitium
o Associated with hypercalcaemia (↑ 1α-hydroxylase activity in macrophages → ↑ vitamin D)
• Renal Manifestations:
o Hypercalcaemic nephropathy → nephrocalcinosis
o Granulomatous interstitial nephritis
o Nephrogenic diabetes insipidus
o Rarely glomerular disease (e.g. FSGS)
• Diagnosis:
o Elevated serum calcium, low PTH
o Imaging: nephrocalcinosis
o Renal biopsy: non-caseating granulomas
• Management:
o Corticosteroids first-line
o Treat hypercalcaemia (hydration, avoid vitamin D supplements)
