Medicine – Acute Interstitial Nephritis

Acute interstitial nephritis (AIN) is an inflammatory disorder affecting primarily the renal interstitium and tubules. It is an important and potentially reversible cause of acute kidney injury (AKI).

The most common cause is a drug-induced immune or hypersensitivity reaction, although infections, autoimmune disorders, and other systemic diseases can also produce tubulointerstitial inflammation.

1. Site of Renal Injury

AIN primarily affects the:

Renal interstitium

and

Renal tubules.

This distinguishes it from glomerulonephritis, in which the principal inflammatory injury occurs within the:

Glomeruli.

2. Pathophysiology

In many cases, particularly drug-induced AIN, the offending agent triggers an:

Immune-mediated inflammatory response.

Inflammatory cells infiltrate the renal interstitium and produce:

Interstitial oedema.

Tubular inflammation.

Tubular dysfunction.

This reduces renal function and may cause AKI.

3. Drug-Induced Acute Interstitial Nephritis

Drugs are among the most important causes of AIN.

The reaction is generally:

Idiosyncratic and immune mediated

rather than simply the result of dose-dependent drug toxicity.

Therefore, AIN can occur even at ordinary therapeutic doses.

4. Clinical Presentation

The original notes list:

Mild renal impairment

and

Hypertension.

However, the presentation is considerably more variable.

The most important renal manifestation is:

Acute kidney injury.

The rise in serum creatinine may be mild or severe.

5. Acute Kidney Injury

Patients typically develop:

Increasing serum creatinine

with a reduction in:

GFR.

The onset may occur days to weeks after exposure to the causative drug, although the timing varies considerably between medications and previous exposures.

6. Urinary Features

Urinalysis may demonstrate:

Sterile pyuria.

Mild-to-moderate proteinuria.

Microscopic haematuria.

White blood cell casts.

These findings reflect inflammation of the tubulointerstitial compartment.

7. Sterile Pyuria

A particularly useful clue is:

Sterile pyuria.

This means:

White blood cells in the urine despite a negative routine bacterial urine culture.

AIN is therefore an important cause of sterile pyuria.

8. White Cell Casts

Inflammatory white cells may enter the renal tubules and form:

White blood cell casts.

These support an inflammatory process within the kidney rather than simple contamination of the urine.

However, WBC casts are not specific to AIN and can also occur in disorders such as:

Pyelonephritis.

9. Proteinuria

Proteinuria is usually:

Mild or moderate.

Heavy nephrotic-range proteinuria is less typical.

An important exception is:

NSAID-associated AIN,

which can occasionally be associated with substantial or nephrotic-range proteinuria, sometimes alongside minimal-change disease.

10. Haematuria

Patients may develop:

Microscopic haematuria.

However, prominent glomerular haematuria with numerous dysmorphic red cells and red-cell casts should raise suspicion for:

Glomerulonephritis

rather than uncomplicated AIN.

11. Hypertension

Hypertension can occur, particularly when there is:

Sodium and water retention

from impaired renal function.

However, hypertension is not the defining presentation.

The more important pattern is:

Recent drug exposure + unexplained AKI + inflammatory urinary findings.

12. Hypersensitivity Features

Drug-induced AIN may produce systemic manifestations of hypersensitivity, including:

Fever.

Skin rash.

Peripheral eosinophilia.

The traditional teaching describes a triad of:

Fever + rash + eosinophilia.

13. The Classic Triad Is Often Absent

An important clinical point is that the complete hypersensitivity triad occurs in only a minority of patients.

Therefore:

No fever, rash, or eosinophilia does NOT exclude AIN.

This is particularly important because many patients present only with:

Unexplained AKI.

14. Eosinophilia

Peripheral blood:

Eosinophilia

may support drug-induced AIN, particularly when accompanied by rash or fever.

However, it is neither sufficiently sensitive nor specific to establish the diagnosis.

15. Urinary Eosinophils

Older teaching often emphasised:

Eosinophils in the urine.

However, eosinophiluria has poor diagnostic accuracy.

Therefore:

Urinary eosinophils should not be relied upon to diagnose or exclude AIN.

16. Drug Causes

A very wide range of drugs can cause AIN.

Important groups include:

Antibiotics.

NSAIDs.

Proton-pump inhibitors.

Certain diuretics.

Some anticonvulsants and other medications.

A careful medication history is essential.

17. Penicillins

Penicillins are classical causes of drug-induced AIN.

Examples include:

Methicillin historically.

Amoxicillin.

Other beta-lactam antibiotics.

The mechanism is usually immune mediated.

18. Sulfonamides

Sulfonamide-containing drugs are another traditional association.

They can produce:

Hypersensitivity reactions

including tubulointerstitial inflammation.

19. NSAIDs

Non-steroidal anti-inflammatory drugs (NSAIDs) are important causes of AIN.

NSAID-associated disease may differ from the classic hypersensitivity pattern because:

Fever, rash, and eosinophilia may be less prominent.

Proteinuria may be more substantial.

20. Proton-Pump Inhibitors

An important modern addition is:

Proton-pump inhibitors (PPIs).

Examples include:

Omeprazole.

Lansoprazole.

Pantoprazole.

PPI-associated AIN may develop relatively insidiously and without obvious hypersensitivity symptoms.

21. Other Antibiotics

AIN may occur with several antimicrobial drugs, including:

Cephalosporins.

Rifampicin.

Fluoroquinolones.

and others.

Therefore, the entire medication history should be reviewed rather than looking only for penicillin exposure.

22. Infections

AIN can also occur in association with infection.

The inflammatory process may result from:

Direct infection

or

An immune response associated with infection.

23. CMV

Cytomegalovirus (CMV) can be associated with tubulointerstitial nephritis, particularly in:

Immunocompromised patients.

The clinical context is therefore important.

24. Leptospirosis

Leptospirosis can cause significant renal involvement, including:

Tubulointerstitial nephritis

and

Acute kidney injury.

Patients may also have:

Fever.

Myalgia.

Jaundice in severe disease.

25. Mycobacterial Infection

Mycobacterial disease, particularly:

Tuberculosis,

can involve the kidney and cause chronic granulomatous tubulointerstitial inflammation.

A classic urinary clue is:

Persistent sterile pyuria.

26. Autoimmune Causes

Several systemic immune-mediated disorders can cause interstitial nephritis.

Important examples include:

Sjögren syndrome.

Sarcoidosis.

Systemic lupus erythematosus.

IgG4-related disease.

27. Sjögren Syndrome

Sjögren syndrome is an important cause of chronic or acute tubulointerstitial nephritis.

Tubular dysfunction may produce:

Distal renal tubular acidosis.

Therefore:

Sjögren + metabolic acidosis + alkaline urine/hypokalaemia → consider distal RTA from tubulointerstitial disease.

28. Sarcoidosis

Sarcoidosis can produce:

Granulomatous interstitial nephritis.

It may also cause renal problems through:

Hypercalcaemia and hypercalciuria

due to increased vitamin D activation by granulomas.

29. SLE

Systemic lupus erythematosus (SLE) can involve the renal interstitium.

However, the classic and much more important renal manifestation of SLE is:

Immune-complex glomerulonephritis – lupus nephritis.

Therefore, SLE should not be thought of primarily as a typical cause of isolated AIN.

30. Goodpasture Syndrome – Important Correction

The original notes list:

Goodpasture’s syndrome

as a cause of AIN.

This is not the typical renal lesion.

Goodpasture disease, or anti-GBM disease, primarily causes:

Rapidly progressive glomerulonephritis

due to antibodies directed against the glomerular basement membrane.

It may occur with:

Pulmonary haemorrhage.

Therefore:

GOODPASTURE / ANTI-GBM DISEASE → GLOMERULONEPHRITIS, NOT CLASSIC AIN.

31. TINU Syndrome

Another important cause is:

Tubulointerstitial nephritis and uveitis syndrome (TINU).

This combines:

Acute interstitial nephritis

with

Uveitis.

It is particularly important in younger patients.

32. Kidney Biopsy

AIN can often be suspected clinically, particularly when AKI follows exposure to a known causative drug.

However, when the diagnosis is uncertain or kidney dysfunction is severe or persistent:

Renal biopsy

may be considered.

33. Histology

The characteristic biopsy findings include:

Interstitial inflammatory cell infiltration.

Interstitial oedema.

Tubulitis.

The glomeruli are often relatively preserved.

34. Tubulitis

Tubulitis means inflammatory cells infiltrating the tubular epithelium.

This is an important histological feature of:

Tubulointerstitial inflammation.

35. Eosinophils on Biopsy

In drug-induced disease, the interstitial infiltrate may contain:

Eosinophils.

However, eosinophils are not always present and are not unique to AIN.

The infiltrate may also contain:

Lymphocytes.

Monocytes.

Plasma cells.

36. Granulomatous Interstitial Nephritis

Granulomas may occur in selected causes of interstitial nephritis.

Important associations include:

Sarcoidosis.

Tuberculosis.

Certain drugs.

Therefore, granulomatous inflammation on biopsy changes the differential diagnosis.

37. Treatment – Remove the Cause

The most important treatment of drug-induced AIN is:

Immediately stop the suspected offending drug whenever possible.

Early withdrawal reduces continued inflammatory injury and improves the likelihood of renal recovery.

38. Supportive Treatment

Patients should also receive appropriate management of AKI.

This may include:

Monitoring renal function.

Monitoring potassium and other electrolytes.

Maintaining appropriate fluid balance.

Avoiding additional nephrotoxic drugs.

Adjusting drug doses for renal function.

39. Corticosteroids

The original note states:

“Some require a small dose of steroids.”

This needs qualification.

Corticosteroids may be considered in selected patients with:

Drug-induced AIN and significant or persistent renal dysfunction, particularly when renal function fails to improve promptly after withdrawal of the causative drug.

40. Steroid Evidence

Steroids are not automatically required in every case.

Their use depends on factors such as:

Severity of AKI.

Biopsy findings.

Likelihood of drug-induced disease.

Response after drug withdrawal.

Risk of steroid treatment.

If steroids are appropriate, early treatment may be more useful than waiting until extensive fibrosis develops.

41. Treat Infection Appropriately

If AIN is caused by an infection, treatment should target:

The underlying infection.

Immunosuppression should not simply be given without considering whether active infection is present.

42. Dialysis

Severe AIN can occasionally produce AKI requiring:

Temporary dialysis.

Indications are the same as for other forms of severe AKI, such as refractory:

Hyperkalaemia.

Metabolic acidosis.

Fluid overload.

Uraemic complications.

The need for dialysis does not necessarily mean that renal failure will be permanent.

43. Prognosis

The original statement that:

Most patients recover completely

is broadly reassuring but somewhat optimistic.

Many patients experience substantial renal recovery, particularly when the offending drug is identified and stopped early.

However, recovery is not always complete.

44. Incomplete Recovery

Some patients are left with:

Reduced GFR

or

Chronic kidney disease.

The risk of incomplete recovery increases with:

Delayed diagnosis.

Prolonged exposure to the offending drug.

Severe initial kidney injury.

Older age.

Interstitial fibrosis on biopsy.

45. Why Early Diagnosis Matters

Persistent inflammation can eventually lead to:

Tubular atrophy

and

Interstitial fibrosis.

Once significant fibrosis has developed, renal damage becomes less reversible.

Therefore:

Early recognition + removal of cause → greater chance of renal recovery.

46. Acute Interstitial Nephritis – Note Form

Definition:

Inflammatory disorder of the renal interstitium and tubules.

Major renal presentation:

Acute kidney injury.

Urinalysis:

Sterile pyuria.

Mild-to-moderate proteinuria.

Microscopic haematuria.

White blood cell casts.

Classic hypersensitivity features:

Fever.

Rash.

Eosinophilia.

However, the complete triad is often absent.

Most important cause:

Drug-induced immune reaction.

Important drugs:

NSAIDs.

Penicillins and other antibiotics.

Sulfonamides.

Proton-pump inhibitors.

Other medications.

Infectious causes:

CMV.

Leptospirosis.

Mycobacterial infection.

Immune/systemic causes:

Sjögren syndrome.

Sarcoidosis.

SLE can cause tubulointerstitial disease, although lupus nephritis is primarily glomerular.

TINU syndrome.

Biopsy:

Interstitial oedema.

Inflammatory infiltrate.

Tubulitis.

± eosinophils.

Treatment:

Stop the offending drug.

Treat the underlying infection or systemic cause.

Supportive AKI management.

Consider corticosteroids in selected persistent or severe drug-induced cases.

47. Important Corrections to the Original Notes

The original presentation:

“Mild renal impairment”

is too restrictive.

AIN can cause:

MILD TO SEVERE ACUTE KIDNEY INJURY.

The classic triad:

FEVER + RASH + EOSINOPHILIA

is useful for exams but occurs in only a minority of patients.

Its absence does not exclude AIN.

The original list includes:

Goodpasture syndrome.

This should be corrected:

ANTI-GBM / GOODPASTURE DISEASE → RAPIDLY PROGRESSIVE GLOMERULONEPHRITIS, NOT CLASSIC ACUTE INTERSTITIAL NEPHRITIS.

Important modern drug causes missing from the original list include:

PROTON-PUMP INHIBITORS.

The statement:

“Complete recovery in most people”

should be qualified.

Many recover substantially, but some are left with:

Persistent renal impairment or CKD.

Key Clinical Pattern

The classic pattern is:

NEW DRUG → AKI + STERILE PYURIA ± WBC CASTS ± FEVER/RASH/EOSINOPHILIA → THINK ACUTE INTERSTITIAL NEPHRITIS.

Remember:

AIN = INTERSTITIUM + TUBULES.

GLOMERULONEPHRITIS = GLOMERULI.

For treatment:

STOP THE OFFENDING DRUG FIRST.

And for prognosis:

EARLY DRUG WITHDRAWAL → BETTER RENAL RECOVERY.

PROLONGED INFLAMMATION → INTERSTITIAL FIBROSIS + TUBULAR ATROPHY → POSSIBLE CKD.