Acute interstitial nephritis

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

AIN is often missed because its presentation overlaps with sepsis-associated tubular injury, urinary infection and glomerular disease. A patient may have only fatigue, nausea and a creatinine rise. The highest-yield diagnostic act is a meticulous exposure timeline including prescriptions started weeks earlier, intermittent antibiotics, over-the-counter NSAIDs, acid-suppression therapy and oncology treatment. The temporal association supports causality but does not prove it, especially in a patient with several concurrent kidney insults.

The urine pattern reflects interstitial inflammation rather than a single pathognomonic marker. Pyuria without a positive culture, mild protein and blood are compatible; nephrotic-range protein or dysmorphic red cells should broaden the differential. Eosinophilia may strengthen a hypersensitivity story, yet normal eosinophils and absence of rash do not exclude AIN. Ultrasound mainly excludes obstruction and may be normal.

Early culprit withdrawal offers the clearest modifiable benefit. The place, timing and regimen of corticosteroids are not settled by a bedside pattern alone and depend on diagnostic confidence, biopsy, degree and duration of injury, infection risk and comorbidity. Immunosuppression requires nephrology and relevant-specialty ownership under the current biopsy and glucocorticoid protocols.

Key points

Acute interstitial nephritis is an inflammatory tubulointerstitial cause of AKI, most often triggered by medicines but also associated with infection, immune disease and less common infiltrative disorders.
Common medication groups include beta-lactam and other antibiotics, proton-pump inhibitors, NSAIDs and allopurinol; immune-checkpoint inhibitors create a specialist oncology–renal presentation.
The classic triad of fever, rash and eosinophilia is uncommon. Its absence should not reassure when creatinine rises after a plausible exposure and urine findings suggest tubulointerstitial inflammation.
Latency varies by agent and previous sensitisation: antibiotics may cause a relatively abrupt syndrome, whereas proton-pump inhibitor-associated disease can be subtle and delayed.
Urinalysis may show sterile pyuria, microscopic haematuria and modest proteinuria; white-cell casts can support the diagnosis but are neither sensitive nor specific, and urine eosinophils are unreliable.
NSAID-associated AIN may differ by having less systemic hypersensitivity and more marked proteinuria, occasionally with a minimal-change-type nephrotic presentation.
Stop the likely culprit promptly, document the suspected reaction and avoid casual rechallenge. Also stop unnecessary co-nephrotoxins and adjust medicines that accumulate in AKI.
Kidney biopsy is the diagnostic reference when uncertainty matters, recovery is absent or immunosuppression is contemplated; corticosteroids are a nephrology-led risk–benefit decision, not an automatic response to pyuria.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Medicine-associated hypersensitivity

Antibiotics, proton-pump inhibitors, NSAIDs, allopurinol and immune-checkpoint inhibitors can provoke a delayed interstitial immune reaction; latency varies with the agent, previous exposure and host susceptibility.

Infection-associated inflammation

Bacterial, viral or other infections may inflame the renal interstitium directly or through immune responses, sometimes producing fever and pyuria that resemble urinary sepsis.

Systemic immune or infiltrative disease

Autoimmune and less common infiltrative disorders can involve the tubulointerstitium, particularly when kidney injury accompanies inflammatory, glandular, pulmonary or multisystem features without a convincing medicine trigger.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Interstitial immune recruitment
A medicine-derived antigen, infection or systemic immune process activates inflammatory cells that accumulate around renal tubules and within the interstitium.
2
Tubulitis
Inflammatory infiltrates cross tubular basement membranes, producing tubulitis; biopsy may confirm this pattern when diagnostic uncertainty affects treatment.
3
Tubular dysfunction and AKI
Interstitial inflammation and tubulitis impair kidney function, so creatinine may continue rising briefly after the suspected exposure is stopped.
4
Fibrotic repair
Persistent or severe inflammation can progress to tubular atrophy and interstitial fibrosis, explaining incomplete recovery and later chronic kidney impairment despite removal of the trigger.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Medication-linked syndrome

Creatinine rises after a new antibiotic, proton-pump inhibitor, NSAID, allopurinol or immune therapy, particularly when no sustained shock or obstruction explains the trajectory. Previous exposure may shorten the apparent latency.

Hypersensitivity featuresRed flag

Fever, a new maculopapular eruption, eosinophilia, arthralgia or liver-test disturbance can support a systemic drug reaction. Assess mucosa, skin pain, blistering and organ involvement because severe cutaneous reactions require emergency specialist care.

Sterile inflammatory urine

Leukocytes or white-cell casts with negative culture, microscopic blood and usually non-nephrotic protein fit AIN. Recent antibiotics can sterilise a genuine urinary infection, so culture interpretation must include exposure and symptoms.

Non-recovering suspected AINRed flag

Creatinine continues to rise or fails to improve after trigger withdrawal, particularly with an uncertain multi-causal illness. This should prompt early renal discussion about biopsy, prognostic fibrosis and whether specialist treatment is justified.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Dated prescription, dispensing and administration historyFirst step
Why
Identify a plausible culprit and estimate latency, dose exposure, dechallenge and competing nephrotoxins.
Interpretation and limitations
Include medicines started in the preceding weeks or months, recent short antibiotic courses, non-prescription NSAIDs and immune therapy. More than one candidate is common; rank causality explicitly rather than deleting the entire drug list.
02
Urinalysis, urine albumin-to-creatinine ratio and microscopy
Why
Characterise interstitial inflammation and screen for glomerular or pigment alternatives.
Interpretation and limitations
Sterile pyuria and white-cell casts may support AIN, but lack adequate sensitivity. High albuminuria or red-cell casts changes the pathway; haem positivity with few erythrocytes suggests pigment rather than interstitial bleeding.
03
Urine culture and infection assessment
Why
Exclude bacterial urinary inflammation before attributing leukocytes to drug hypersensitivity or starting immunosuppression.
Interpretation and limitations
A positive culture with compatible symptoms supports infection; a negative result after antibiotics is less decisive. Fever may be drug related or infectious, so integrate cultures, imaging and systemic observations.
04
Full blood count, eosinophils, CRP and liver tests
Why
Find supportive hypersensitivity or systemic drug-reaction features and alternative inflammatory illness.
Interpretation and limitations
Peripheral eosinophilia strengthens but does not establish AIN, and normal eosinophils do not exclude it. Cytopenias, marked hepatic injury or high inflammatory markers require broader drug-reaction, infection or immune assessment.
05
Renal immunology screen selected by phenotype
Why
Investigate vasculitis, anti-GBM disease, lupus or complement-mediated disease when urine and systemic findings suggest a glomerular process.
Interpretation and limitations
ANCA, anti-GBM antibody, ANA, complement and paraprotein tests should be targeted; negative serology cannot exclude every glomerulonephritis, and positive results need clinical and often biopsy correlation.
06
Kidney biopsy
Why
Confirm interstitial inflammation, assess fibrosis and exclude competing intrinsic disease when the result will alter treatment.
Interpretation and limitations
Interstitial inflammatory infiltrates and tubulitis support AIN, while fibrosis predicts less reversible disease. Biopsy timing and bleeding risk require nephrology assessment; empirical steroids can reduce later diagnostic yield.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Acute tubular injury

Shock, sepsis, surgery or dose-related toxins favour tubular injury; renal epithelial cells and coarse granular casts support it, whereas a compatible exposure with sterile pyuria favours interstitial nephritis.

Pyelonephritis

Urinary symptoms, flank tenderness, bacteriuria, positive culture or focal imaging support infection, although recent antibiotics may sterilise cultures and require interpretation alongside the clinical course.

Glomerulonephritis

Marked albuminuria, dysmorphic erythrocytes, red-cell casts, low complement, severe hypertension or pulmonary haemorrhage suggest glomerular inflammation rather than isolated interstitial disease.

Urinary obstruction

Retention symptoms, a distended bladder or collecting-system dilatation support post-renal injury; ultrasound may be normal early, so persistent clinical suspicion still requires review.

Additional chapter-specific clues

Glomerular mimicRed flag

Substantial albuminuria, dysmorphic haematuria, red-cell casts, severe hypertension, low complement or pulmonary haemorrhage point away from isolated interstitial nephritis and demand an urgent glomerulonephritis work-up.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01SuspectBuild a defensible AIN diagnosisFirst stepUnexplained AKI follows a plausible medicine or systemic inflammatory exposure.

11. Reconstruct every medicine and illness exposure with start, stop and administration dates, including over-the-counter analgesia and recent courses completed before admission.
22. Examine for rash, fever, arthralgia, lymphadenopathy, mucosal or hepatic involvement and measure urine findings, culture, albuminuria and renal trajectory.
33. Exclude haemodynamic injury and obstruction, and seek glomerular, infectious or systemic clues that would make AIN an incomplete explanation.
44. Discuss early with nephrology when AKI is severe, worsening, atypical or likely to need biopsy or immunosuppression.

02WithdrawRemove the likely trigger safelyA medicine is reasonably suspected of causing interstitial nephritis.

1Alternative1. Stop the most likely non-essential culprit promptly and ask the relevant specialty for an alternative when infection, reflux disease, gout or cancer treatment still needs therapy.
22. Review additional nephrotoxins and renally cleared drugs, manage volume and electrolyte complications, and avoid further exposure while causality is assessed.
33. Record the agent, latency, renal phenotype and degree of certainty in the notes and discharge record; distinguish suspected AIN from immediate IgE allergy.
44. Trend creatinine and urine output after withdrawal, recognising that improvement may take days and incomplete recovery is possible.

03EscalateBiopsy and specialist treatment decisionEscalationRenal function fails to improve, diagnosis remains uncertain or steroid benefit is being considered.

11. Reassess for persistent infection, obstruction, tubular injury and glomerular disease, and complete targeted serology before assuming refractory AIN.
22. Nephrology weighs biopsy benefit against bleeding risk and considers how delay, fibrosis and diagnostic uncertainty affect the value of corticosteroid treatment.
33. If immunosuppression is chosen, use a specialist regimen with infection screening, glucose, blood-pressure, bone and gastrointestinal risk mitigation appropriate to duration.
44. Set objective response and stopping criteria; lack of renal improvement should trigger diagnostic review rather than indefinite steroid exposure.

Key medicines and prescribing safety1 treatment · regimens, roles and cautions

May reduce ongoing interstitial inflammation in carefully selected patients, particularly when renal recovery has not followed prompt culprit withdrawal.

Systemic corticosteroid for selected biopsy-supported or highly probable AIN

The agent, route, weight-based or fixed regimen and taper are prescribed by nephrology under the current local protocol after infection and diagnostic uncertainty have been addressed.

Evidence is not equivalent to a universal indication. Steroids can worsen infection, hyperglycaemia, delirium, gastrointestinal and bone risk, and may obscure biopsy findings; do not start from pyuria or eosinophilia alone.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Progressive acute kidney injury

Ongoing inflammation can rapidly reduce filtration, producing oliguria and a rising creatinine even after the suspected medicine has been withdrawn.

Electrolyte and volume emergencies

Severe filtration and tubular failure may cause hyperkalaemia, metabolic acidosis or fluid overload, each requiring immediate treatment independent of the diagnostic uncertainty.

Incomplete renal recovery

Delayed recognition or established fibrosis can leave residual chronic kidney disease, persistent proteinuria or a greater susceptibility to future acute kidney injury.

Systemic drug reaction

When interstitial nephritis forms part of a broader hypersensitivity syndrome, hepatic injury, mucosal disease or severe cutaneous involvement can create additional organ-threatening illness.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Plot creatinine and urine output from the date of culprit withdrawal, looking for plateau or recovery while remembering that biochemical improvement may not be immediate.
Repeat potassium, bicarbonate and fluid-status assessment because the urgent risks of AIN are the same AKI complications seen with other mechanisms.
Follow rash, temperature, eosinophil count and liver tests when a systemic drug reaction is possible, escalating rapidly if mucosal or multi-organ disease appears.
If corticosteroids are used, monitor infection, capillary glucose, blood pressure, mental state and treatment-specific prophylaxis through the responsible specialist team.
At discharge, reconcile the suspected medicine reaction in the GP record and provide a renal-function follow-up date, recovery status and explicit avoidance or specialist-rechallenge advice.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

The famous triad is insensitive

Fever, rash and eosinophilia together make a memorable examination pattern but occur in only a minority. Depending on the triad creates late diagnoses, especially with proton-pump inhibitor-associated AIN.

Urine eosinophils disappoint

Eosinophiluria is neither sufficiently sensitive nor specific to confirm AIN and can occur in other urinary inflammation. The overall exposure, phenotype and sometimes biopsy carry more weight.

NSAID disease may look different

NSAID-related interstitial nephritis can have little fever or rash and disproportionately high proteinuria, sometimes alongside minimal-change pathology, so a bland systemic examination does not exclude it.

Dechallenge is not instant proof

Creatinine may continue rising briefly after a culprit is stopped because inflammation and marker kinetics persist. Conversely, improvement after withdrawal may also reflect treatment of sepsis or hypovolaemia.

Documentation determines future safety

Recording only ‘allergy’ loses the renal phenotype and certainty. A specific note—suspected delayed interstitial nephritis with timing and biopsy status—supports safer future antimicrobial and prescribing decisions.

11Common pitfallsFrequent interpretation and management errors.

01
Waiting for fever, rash and eosinophilia before considering AIN misses the common incomplete presentation and prolongs exposure to the culprit.
02
Treating sterile pyuria as diagnostic ignores partially treated urinary infection, stones, instrumentation and other inflammatory causes.
03
Starting corticosteroids before infection and glomerular mimics are assessed can worsen sepsis and make later tissue diagnosis harder.
04
Stopping an essential antimicrobial without arranging an effective alternative can trade renal risk for uncontrolled life-threatening infection.
05
Reintroducing the suspected agent casually to test causality can provoke recurrent, faster and more severe renal injury.