# 💧 Acute Interstitial Nephritis (AIN)

## 🔍 How to Read This Page — Evidence Provenance

AIN has **no randomized controlled trials** guiding treatment, with one exception noted below. Rather than blur that, every substantive claim on this page is tagged:

EVIDENCE-BASED

Anchored to a named, citable study with a PMID. The number came from the paper.

PRACTICE-BASED

Standard nephrology teaching, pharmacovigilance reporting, and clinical experience. Real and useful — but not trial-anchored. Treat as a differential-generating checklist.

#### ⚠️ Why this matters here more than usual

The entire second-line drug literature in AIN rests on case series in the **single digits**. When you see PRACTICE-BASED on a treatment recommendation, that is not a hedge — it is the actual state of the field.

## 🔍 Definition & Recognition

**Acute interstitial nephritis** = immune-mediated inflammation of the renal **interstitium and tubules**, with the glomeruli spared. It accounts for **5–15% of AKI** in hospitalized patients.

The mechanism is **T-cell mediated delayed-type hypersensitivity** — not dose-dependent, and it recurs rapidly on re-exposure. Remember this one fact; it drives the entire treatment section, including why B-cell drugs like rituximab do not belong here.

#### ⚠️ The latency trap

Do **not** anchor on “2 to 8 weeks.” Latency is agent-specific: **antibiotics 1–3 weeks**, **PPIs 3–6 months**, **checkpoint inhibitors 3–12 months**, **mesalamine months to years**. The most commonly missed AIN is a slow creatinine drift on a chronic PPI, written off as CKD progression.

## 💊 Causes 1: Drugs (70–75% of all AIN)

EVIDENCE-BASED — class split from pooled biopsy series **49%**

**Antibiotics**

1–3 week latency

**14%**

**PPIs**

3–6 month latency

**11%**

**NSAIDs**

Steroid-unresponsive

Omeprazole is the single most implicated agent (approximately 12% of drug-induced AIN); amoxicillin second (approximately 8%).

PRACTICE-BASED — agent lists below

### 💉 Antibiotics

- **Beta-lactams** — penicillins, all cephalosporin generations, carbapenems. Cross-reactive: avoid both classes after either
- **Sulfonamides** — TMP-SMX, sulfadiazine. Remember this when picking PJP prophylaxis
- **Fluoroquinolones** — ciprofloxacin most reported; can follow a single dose
- **Vancomycin** — overlapping AIN *and* ATN on biopsy
- **Rifampin** — distinct mechanism (anti-rifampin antibodies); flu-like illness, hemolysis, often dialysis-requiring
- Macrolides, tetracyclines, linezolid, daptomycin

### 🪨 Gastrointestinal

- **All PPIs** — omeprazole, esomeprazole, pantoprazole, lansoprazole, rabeprazole, dexlansoprazole. **Cross-reactivity is near-universal — never switch to another PPI.** Use famotidine
- **5-ASA agents** — mesalamine, sulfasalazine, balsalazide. Insidious over months to years and **frequently irreversible**. Needs baseline and periodic creatinine
- H2 blockers — cimetidine, ranitidine (uncommon)

### 💦 Analgesics

- All non-selective NSAIDs and COX-2 selective agents
- **Topical NSAIDs are systemically absorbed** — patients and clinicians both discount these
- May carry nephrotic-range proteinuria from minimal-change overlap

### 🏹 Immuno-Oncology

- **PD-1** — pembrolizumab, nivolumab, cemiplimab
- **PD-L1** — atezolizumab, durvalumab, avelumab
- **CTLA-4** — ipilimumab. **Combination therapy carries the highest risk**
- Frequently no rash and no eosinophilia

### ⚖️ Other Agents

- **Allopurinol** — among the most severe. Higher risk with CKD, concurrent thiazides, and **HLA-B\*58:01** carriers
- **Thiazides and loop diuretics** (sulfonamide-based)
- **Aromatic anticonvulsants** — phenytoin, carbamazepine, phenobarbital; cross-reactive, switch to a non-aromatic agent
- Hydralazine (also drug-induced lupus/ANCA), statins, ACEi/ARB (rare)

### 🔥 DRESS Triggers

Renal involvement in DRESS *is* AIN. The high-risk agents cluster tightly:

**Allopurinol · aromatic anticonvulsants · sulfonamides · vancomycin · minocycline · dapsone · abacavir · nevirapine**

Latency 2–8 weeks — longer than ordinary drug hypersensitivity. Needs longer steroid courses and relapses on fast tapers.

#### 🎯 The checkpoint-inhibitor attribution trap

Most patients on a checkpoint inhibitor are **also on a PPI**. Before you attribute AIN to a cancer drug the patient needs — and stop it — account for the PPI they do not need. Stopping the PPI is free. Stopping immunotherapy is not.

## 🦠 Causes 2: Infection, Autoimmune, and the Rest

PRACTICE-BASED

### 🦠 Infection (5–10%)

**Bacterial:** Streptococcus, Staphylococcus, Legionella, Brucella, Salmonella, Mycoplasma, Leptospira, TB, syphilis, Q fever

**Viral:** EBV, CMV, HIV, hantavirus, BK polyomavirus, adenovirus, parvovirus B19, hepatitis B/C, SARS-CoV-2

**Fungal/parasitic:** Histoplasma, Coccidioides, Leishmania, Toxoplasma

### 🧿 Systemic / Autoimmune

- **Sarcoidosis** — granulomas, hypercalcemia
- **Sjögren** — distal RTA, hypokalemia
- **SLE** — can be isolated interstitial disease
- **IgG4-related disease** — storiform fibrosis, mass lesions
- **TINU** — bilateral anterior uveitis, young women
- **Anti-TBM** and **ABBA** disease
- **IBD** — as an extraintestinal manifestation

### 🧬 Malignancy & Genetic

- **Lymphoma/leukemia** — direct interstitial infiltration; monoclonality distinguishes it
- **Myeloma/light chain** — check free light chains
- **Karyomegalic interstitial nephritis** — *FAN1* mutations, enlarged tubular nuclei
- **ADTKD** — *UMOD*, *MUC1*, *REN*, *HNF1B*

#### ⚠️ The IBD fork — get this right

A Crohn or UC patient on mesalamine with interstitial nephritis has two possible causes pointing in **opposite therapeutic directions**: mesalamine toxicity, where you stop the drug — or an extraintestinal manifestation of active IBD, where you intensify treatment. Disease activity, drug timing, and biopsy separate them. Guessing wrong makes the patient worse either way.

## 🔬 Diagnosis

### The classic triad appears in under 10% of cases

Fever, maculopapular rash, peripheral eosinophilia. **Absent in over two-thirds of PPI-AIN.** Its absence does not exclude AIN — and waiting for it is how the diagnosis gets missed.

#### ✅ What actually helps — the urine sediment

- **Sterile pyuria + WBC casts** — the most useful bedside finding
- **Sub-nephrotic proteinuria**, usually under 1 g/day. Nephrotic range suggests NSAID-associated minimal change
- Cast logic: **RBC casts = GN · WBC casts = AIN · muddy brown casts = ATN**

#### 🚫 Urine eosinophils — a debunked test. Do not order.

EVIDENCE-BASED — PMID 24052222

Muriithi et al. (*CJASN* 2013) tested it properly — 566 patients with both a urine eosinophil test and a native kidney biopsy within a week of each other, 91 with AIN. At a 1% Hansel-stain cutoff:

**30.8%**

Sensitivity

**68.2%**

Specificity

**15.6%**

PPV

**0.97**

LR+

**1.01**

LR−

**The likelihood ratios are the whole argument.** Both sit at 1.0, so the result moves post-test probability essentially nowhere in either direction. A test that cannot revise your estimate is not a diagnostic test. It fails in *both* directions — a negative does not rule out, a positive does not rule in.

Why older sources disagree: the four largest earlier series reported sensitivity 40–91% and specificity 52–95% — but none used kidney biopsy as the gold standard.

#### 🔭 Urinary CXCL9 — the emerging replacement

EVIDENCE-BASED — PMID 37395276

Tubular epithelium secretes CXCL9 in response to IFN-gamma from infiltrating T cells — so it tracks the actual lesion. Stable at room temperature, unlike beta-2 microglobulin. Identified and validated by Moledina et al. in *J Clin Invest*. Availability outside research and reference labs remains limited.

#### 🧪 What to demand from the biopsy report

Three items drive management more than the creatinine does:

- **Degree of interstitial fibrosis and tubular atrophy (IFTA)**
- **Presence or absence of an active inflammatory infiltrate**
- **Granulomas** — raises sarcoid, TB, fungal, PPI, fluoroquinolones

Light microscopy: dense infiltrate, tubulitis, interstitial edema, **normal glomeruli and vessels**. IF usually negative — except linear TBM staining in anti-TBM disease and IgG4-rich plasma cells in IgG4 disease.

## 💊 Treatment — Sorted by Etiology

### Step 1 — Universal, every case

- **Withdraw the offending agent.** Sequentially if AKI is mild, **simultaneously if severe**
- **Avoid the whole class and cross-reactive classes**
- **Hunt the OTC exposures** — PPIs and NSAIDs go unreported because patients do not consider them medications
- Correct the tubular consequences; remove other nephrotoxins

**Do not delay withdrawal awaiting biopsy.** Withdrawal is reversible. The fibrosis from a two-week delay is not.

| Etiology | First line | Then | Do NOT |
|----|----|----|----|
| **Drug-induced (general)** | Withdraw; prednisone 1 mg/kg/day if severe or not recovering | Extend steroids to 8–12 weeks if active infiltrate → then MMF 1–2 g/day | Escalate when IFTA is heavy and infiltrate absent |
| **NSAID-induced** | Withdrawal + supportive only | — | **Give steroids — this subtype does not respond** |
| **PPI-induced** | Withdraw; famotidine if acid suppression needed | Steroids if not recovering → MMF | **Switch to another PPI** |
| **Mesalamine / 5-ASA** | Withdraw permanently; steroids if inflammation active | — | Rechallenge; confuse with IBD-associated nephritis |
| **Allopurinol / DRESS** | Withdraw; prednisone 1 mg/kg/day, **longer course** with slow taper | MMF or cyclosporine if steroid-dependent | Rechallenge; use a 2-week taper (DRESS relapses) |
| **Checkpoint inhibitor** | Hold ICI; prednisone 0.5–1 mg/kg/day | MMF → infliximab or cyclophosphamide (case-level) | Stop the ICI before excluding a concurrent PPI |
| **Infection-associated** | **Treat the organism** | Supportive | **Give steroids — generally contraindicated** |
| **BK nephropathy (allograft)** | **Reduce immunosuppression** | IVIG, leflunomide, cidofovir (weak evidence) | Add steroids for “rejection” without excluding BK |
| **Sarcoidosis** | Prednisone 0.5–1 mg/kg/day, slow taper over 6–12 months | Methotrexate / azathioprine / MMF → infliximab | Reach for rituximab; forget the hypercalcemia |
| **Sjögren** | Prednisone | MMF or azathioprine; HCQ adjunct | **Neglect the distal RTA** — bicarbonate and K⁺ repletion are treatment |
| **IgG4-related disease** | Prednisone — but **most flare on taper** | **Rituximab or inebilizumab**; obexelimab | Expect steroids alone to hold remission; delay (fibrotic disease will not respond) |
| **TINU** | Systemic steroids **+ topical steroids for the uveitis** | MMF or azathioprine for relapsing uveitis | Manage without ophthalmology — the courses run independently |
| **Anti-TBM / ABBA** | Prednisone | **Rituximab** — mechanistically rational here (antibody-mediated) | — |
| **Lymphoma / myeloma** | **Treat the malignancy** | — | Immunosuppress for “AIN” |
| **Karyomegalic / genetic** | Supportive; CKD management | Transplant | **Immunosuppress — no role** |

PRACTICE-BASED — except the IgG4 row, which has randomized data

## ⛔ The Non-Responder at 3–4 Weeks

If the patient has not recovered 3–4 weeks after withdrawal plus steroids, work it in this order:

1.  **Question the diagnosis.** Consider alternative AKI etiologies, especially if no biopsy was done. Biopsy now if feasible.
2.  **If biopsy-confirmed, read the chronicity.** This is the decision point.
3.  **If there IS an active infiltrate** — extend steroids to a total of **8–12 weeks**. Do *not* add a second agent yet.

#### 🛑 The most commonly missed decision in AIN

**Severe chronic changes (heavy IFTA) with no acute inflammatory infiltrate: the patient will not improve. Taper and stop the glucocorticoids.** Adding immunosuppression here purchases infection risk, not GFR.

## 💉 Second-Line Immunosuppression

#### Mycophenolate mofetil — the best-supported option

EVIDENCE-BASED — PMID 17699278

- **Dose: 1 g/day divided, titrate to 2 g/day if tolerated**
- Preddie et al. (*CJASN* 2006) — **n=8**, treated 13–34 months. These patients had **responded to steroids but could not tolerate withdrawal** — steroid-*dependent*, not steroid-*resistant*. All discontinued glucocorticoids
- **Critical limitation: only two of the eight had drug-induced AIN.** Applying this to classic drug AIN is a genuine extrapolation

#### ❌ Rituximab — where it belongs and where it does not

**Not for drug-induced AIN.** Drug-induced AIN is T-cell mediated, so B-cell depletion targets the wrong effector. Major reference sources do not list it for this indication.

**Rational only when the driver is antibody- or B-cell-mediated:** IgG4-related disease (best evidence), anti-TBM disease, ABBA disease, and at case-report level Sjögren-associated TIN.

**Not standard for sarcoid** — that pathway runs methotrexate, azathioprine, or MMF, then anti-TNF.

## ⚠️ The PJP Prophylaxis Trap

Prophylaxis applies at prednisone 20 mg/day or more for four or more weeks, especially alongside MMF. TMP-SMX is the default agent — and here it carries two specific problems:

**1. Sulfonamides are a classic AIN trigger**

If sulfa is anywhere on the suspect list, do not use it.

**2. Trimethoprim inhibits tubular creatinine secretion**

It raises creatinine 0.1–0.3 mg/dL with no change in true GFR — contaminating the exact measurement you are using to judge treatment response.

**✅ Atovaquone 1500 mg daily avoids both.** Dapsone requires G6PD testing first — and dapsone is itself a DRESS trigger.

## 🎯 Key Takeaways

#### 🔍 Recognition

- Classic triad in under 10% — do not wait for it
- Latency is agent-specific, not a fixed window
- Sterile pyuria + WBC casts is the useful finding
- Hunt the OTC PPI and NSAID

#### ⚡ Practice-Changing

- **Do NOT order urine eosinophils** — LR 0.97/1.01
- NSAID-AIN does not respond to steroids
- Never switch to a different PPI
- Rituximab has no role in ordinary drug AIN

#### 📋 Management

- Withdrawal is the therapy — do not await biopsy
- Extend steroids to 8–12 weeks before adding a drug
- **Heavy IFTA + no infiltrate = stop, do not escalate**
- Atovaquone, not Bactrim, for PJP prophylaxis

## 📈 Prognosis

**50–75%**

Complete recovery

**20–30%**

Partial recovery

**5–10%**

Progress to ESKD

#### Predictors of poor recovery

- **Kidney failure lasting more than three weeks** before treatment
- **NSAID** as the causative agent
- **Interstitial granulomas, fibrosis, and tubular atrophy** on biopsy
- Mesalamine as the agent — frequently irreversible

**The one thing to take away:** time-to-withdrawal is the dominant modifiable variable. Steroids may accelerate recovery when started early; nothing rescues a kidney that has already fibrosed. When deciding whether to escalate immunosuppression, the biopsy chronicity — not the creatinine — casts the deciding vote.

## 📚 Sources

1.  **Muriithi AK, Nasr SH, Leung N.** Utility of urine eosinophils in the diagnosis of acute interstitial nephritis. *Clin J Am Soc Nephrol.* 2013;8(11):1857-1862. [PMID: 24052222](https://pubmed.ncbi.nlm.nih.gov/24052222/). *— n=566 with paired biopsy. Sensitivity 30.8%, specificity 68.2%, PPV 15.6%, LR+ 0.97, LR− 1.01. Verified full-text.*
2.  **González E, Gutiérrez E, Galeano C, et al.** Early steroid treatment improves the recovery of renal function in patients with drug-induced acute interstitial nephritis. *Kidney Int.* 2008;73(8):940-946. [PMID: 18185501](https://pubmed.ncbi.nlm.nih.gov/18185501/). *— Retrospective, only 9 untreated controls; OR 6.6 (95% CI 1.3–33.6). Hypothesis-generating, not a treatment effect estimate.*
3.  **Muriithi AK, Leung N, Valeri AM, et al.** Biopsy-proven acute interstitial nephritis, 1993-2011: a case series. *Am J Kidney Dis.* 2014;64(4):558-566. [PMID: 24927897](https://pubmed.ncbi.nlm.nih.gov/24927897/). *— n=95; found NO difference in recovery with prednisone at 6 months. The counterweight to reference 2.*
4.  **Preddie DC, Markowitz GS, Radhakrishnan J, et al.** Mycophenolate mofetil for the treatment of interstitial nephritis. *Clin J Am Soc Nephrol.* 2006;1(4):718-722. [PMID: 17699278](https://pubmed.ncbi.nlm.nih.gov/17699278/). *— n=8, steroid-dependent; only 2 of 8 drug-induced.*
5.  **Fernández-Juárez G, Pérez JV, Caravaca-Fontán F, et al.** Duration of treatment with corticosteroids and recovery of kidney function in acute interstitial nephritis. *Clin J Am Soc Nephrol.* 2018;13(12):1851-1858. [PMID: 30397027](https://pubmed.ncbi.nlm.nih.gov/30397027/).
6.  **Moledina DG, Obeid W, Smith RN, et al.** Identification and validation of urinary CXCL9 as a biomarker for diagnosis of acute interstitial nephritis. *J Clin Invest.* 2023. [PMID: 37395276](https://pubmed.ncbi.nlm.nih.gov/37395276/).
7.  **Della-Torre E, et al.** Obexelimab for the treatment of IgG4-related disease. *N Engl J Med.* 2026. [PMID: 42233621](https://pubmed.ncbi.nlm.nih.gov/42233621/). *— n=194; 52-week flare 26.8% vs 54.6% placebo (ARR 27.8%, NNT 4). Endpoint is disease flare, an intermediate outcome.*
8.  **Carruthers MN, Topazian MD, Khosroshahi A, et al.** Rituximab for IgG4-related disease: a prospective, open-label trial. *Ann Rheum Dis.* 2015;74(6):1171-1177. [PMID: 25667206](https://pubmed.ncbi.nlm.nih.gov/25667206/). *— Open-label, single-arm, n=30. No control group.*
9.  **Sahutoglu T, et al.** Update on acute tubulointerstitial nephritis: clinical features, immunologic insights, and diagnostic and treatment approaches. *Kidney Int Rep.* 2025. [PMID: 40630288](https://pubmed.ncbi.nlm.nih.gov/40630288/).
10. **Sadarangani S, et al.** Diagnosing acute tubulointerstitial nephritis: novel biomarkers address an important clinical challenge. *Curr Opin Nephrol Hypertens.* 2026. [PMID: 41460039](https://pubmed.ncbi.nlm.nih.gov/41460039/).

📚 For Educational Purposes Only

© 2026 Andrew Bland MD - All Rights Reserved

### Also on this topic

The same subject at other levels of depth.

- **Interactive case:** [Acute Interstitial Nephritis](https://urinenephrology.org/2025_UDPA_Lectures_Live/cases/case3_enhanced.html)
- **Student handout:** [Acute Interstitial Nephritis: Diagnosis and Management](https://urinenephrology.org/student-resources/aki/ain-student-handout.html)


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