Interstitial Nephritis

Interstitial nephritis is inflammation of the kidney tubules and the supporting tissue surrounding them (the interstitium) — the tiny filters and structural framework that allow the kidneys to function. This inflammation impairs the kidneys' ability to filter waste and regulate fluid and electrolytes. Acute interstitial nephritis (AIN) develops over days to weeks, while chronic interstitial nephritis (CIN) progresses slowly over months to years. Drug-induced AIN is the most common form and accounts for a significant proportion of unexplained acute kidney injury.

Types

Acute interstitial nephritis (AIN): Develops over days to weeks, most commonly as an immune reaction to a medication. Kidney function can decline rapidly but is often reversible if the cause is identified and removed promptly — the sooner, the better.

Chronic interstitial nephritis (CIN): Develops gradually, often without obvious symptoms until kidney function is significantly reduced. Because CIN progresses silently, it is often only detected through blood or urine tests. It is more likely to cause lasting kidney damage and chronic kidney disease than AIN.

Causes

Medications — the most common cause. Responsible for around 70–75% of AIN cases. This is not a complete list, but common culprits include:

• NSAIDs (ibuprofen, naproxen, diclofenac)
• Antibiotics — particularly penicillins, cephalosporins, sulphonamides, fluoroquinolones, and rifampicin
• Proton pump inhibitors (PPIs) — omeprazole, lansoprazole, and related drugs; an increasingly recognised cause, even after months or years of use
• Diuretics — furosemide and thiazides
• Allopurinol
• Immune checkpoint inhibitors — used in cancer treatment

Infections — bacterial (including Legionella, Streptococcus, and E. coli), viral (including EBV, CMV, and hantavirus), and certain parasitic infections can trigger AIN directly.

Autoimmune and systemic conditions — systemic lupus erythematosus (SLE), Sjögren's syndrome, sarcoidosis, IgG4-related disease, and tubulointerstitial nephritis with uveitis (TINU syndrome).

Other causes — heavy metal exposure (lead, cadmium), chronic analgesic overuse, herbal remedies containing aristolochic acid, prolonged urinary tract obstruction, and other less common exposures.

Symptoms

AIN often has no specific symptoms. Many people feel generally unwell or have only vague symptoms, which is why blood and urine tests are crucial for diagnosis. When symptoms do occur, they may include:

• Decreased urine output or changes in urine appearance, such as darker or foamy urine
• Swelling in the legs or ankles (fluid retention)
• Fatigue and general malaise
• Nausea and loss of appetite
• Flank or back pain (less common)
• Fever, rash, and joint pain — a classic triad in drug-induced AIN, but present in only a minority of cases and cannot reliably confirm or exclude the diagnosis

CIN progresses silently in most cases. By the time symptoms appear, significant kidney damage and chronic kidney disease may already be present.

Diagnosis

Interstitial nephritis is often suspected when kidney function declines unexpectedly, particularly after starting a new medication.

• Blood tests — elevated creatinine and urea indicate reduced kidney function. Eosinophilia (raised eosinophil count) is sometimes present in drug-induced AIN but is neither sensitive nor specific; its absence does not rule out the diagnosis
• Urinalysis — may show white blood cells in urine without bacteria (sterile pyuria), red blood cells, white blood cell casts, and low-grade proteinuria; eosinophils in urine (eosinophiluria) can suggest drug-induced AIN but has limited diagnostic accuracy
• Urine protein/creatinine ratio — tubular proteinuria is common; nephrotic-range proteinuria (heavy protein loss) may occur specifically with NSAID-associated AIN
• Imaging — kidney ultrasound may show enlarged kidneys in acute cases, or scarring and shrinkage in chronic disease
• Kidney biopsy — the definitive diagnostic test, showing inflammatory cell infiltration of the interstitium and tubules. The decision to biopsy depends on the severity, tempo, and clarity of the clinical picture, and is typically undertaken when the diagnosis is uncertain or the cause is unclear

Treatment

Remove the causative agent In drug-induced AIN, stopping the offending medication is the first and most important step. The sooner the medication is stopped, the better the chance of recovery. Kidney function often begins to improve within days to weeks, though full recovery is not always achieved, particularly if the diagnosis is delayed.

Corticosteroids Prednisolone or methylprednisolone is commonly used in moderate to severe AIN to reduce inflammation and speed recovery. The evidence supporting this is observational rather than from randomised trials, and practice varies between centres. The decision is typically guided by the severity of kidney function decline and biopsy findings.

Treat the underlying condition In infection-related AIN, treating the infection is the primary intervention. In autoimmune-related AIN, immunosuppressive therapy targeting the underlying condition may be required.

Supportive care Managing fluid balance, electrolyte abnormalities (particularly high potassium and acidosis), and blood pressure. Temporary dialysis may be needed in severe acute cases.

Monitoring Regular follow-up of kidney function (creatinine, eGFR) is essential to assess recovery and detect progression to chronic kidney disease. Long-term avoidance of the causative agent — and often the entire drug class — is recommended. This should be clearly documented in the medical record and communicated to all treating clinicians.

FAQ

Is interstitial nephritis reversible? In acute drug-induced cases, kidney function often recovers substantially once the causative medication is stopped, particularly when identified early. Recovery is less complete when diagnosis is delayed or when chronic changes are already present. CIN is less likely to fully reverse and may progress to chronic kidney disease.

Which drugs most commonly cause interstitial nephritis? NSAIDs, antibiotics (particularly penicillins and cephalosporins), and proton pump inhibitors (PPIs) are the most frequently implicated. Many other medications have been reported as causes. If you develop unexplained kidney function decline after starting a new medication, discuss it with your doctor promptly.

Related biomarkers

Key lab markers for diagnosing and monitoring interstitial nephritis include serum creatinine, eGFR, and urine protein/creatinine ratio for kidney function, alongside urinalysis findings such as white blood cells, casts, and eosinophils. In autoimmune cases, ANA, anti-dsDNA, and complement levels may also be relevant. Tracking these markers in HealthMatters can highlight early changes in kidney function, show how quickly you are recovering after treatment, and help you and your clinician decide when further investigation is needed.