# Acute Interstitial Nephritis (AIN): Student Handout > **Updated March 2026 to reflect KDIGO 2026 draft guideline** ## Learning Objectives By the end of this handout, you should be able to: - Recognize AIN as a major cause of AKI (accounting for 15-27% of cases) - Identify drug and non-drug causes of AIN - Understand the pathophysiology of drug-induced immune-mediated AIN - Recognize diagnostic limitations and pitfalls - Interpret urinalysis and novel biomarkers - Distinguish AIN from acute tubular necrosis - Recognize TINU syndrome (tubulointerstitial nephritis + uveitis) - Implement appropriate diagnostic and therapeutic approaches ------------------------------------------------------------------------ ## Definition and Epidemiology **Acute Interstitial Nephritis:** Immune-mediated inflammatory response affecting the kidney tubulointerstitium (interstitial space and tubules), sparing glomeruli (in primary AIN). **Incidence:** - Accounts for **15-27% of kidney biopsies** performed for unexplained AKI - Represents **1-3% of all AKI** in general population - **70-75% are drug-induced** (medication-related) **Mortality & Morbidity:** - In-hospital mortality: ~20% of severe cases requiring dialysis - Many patients progress to chronic kidney disease if untreated - Early recognition and intervention critical for outcomes ------------------------------------------------------------------------ ## Pathophysiology: The Immune Mechanism ### The Pathogenic Process **Step 1: Initial Injury** - Medication acts as hapten (chemical conjugates with protein) - Creates neo-antigen complex recognized as “foreign” **Step 2: Sensitization** - Antigen-presenting cells process the hapten-protein complex - Helper T cells become activated - T cell–mediated hypersensitivity reaction develops **Step 3: Inflammatory Infiltrate** - Lymphocytes (predominantly T cells) infiltrate interstitium - Eosinophils may be present (but inconsistently) - Inflammatory cytokines cause cellular injury **Step 4: Tubulointerstitial Damage** - Tubule epithelial cells injured first - Interstitial edema and inflammation develop - Progressive fibrosis if untreated **Key Feature:** Primary damage is **immune-mediated**, not dose-dependent. This explains why AIN can occur at therapeutic doses and after brief exposure. ------------------------------------------------------------------------ ## Causes: Drug and Non-Drug Etiologies ### Drug-Induced AIN (Most Common Cause) **Most Common Drug Causes (In Descending Frequency):** | Drug Class | Examples | Frequency | |----|----|----| | Antibiotics | Amoxicillin, ciprofloxacin, TMP-SMX, cephalosporins | 49% of drug-induced AIN | | Proton pump inhibitors | Omeprazole, pantoprazole, lansoprazole | 14% of drug-induced AIN | | NSAIDs | Ibuprofen, naproxen, indomethacin | 11% of drug-induced AIN | | Diuretics | Thiazides, furosemide | ~5% | | Anticonvulsants | Phenytoin, carbamazepine, allopurinol | Rare | | Biologics | TNF-alpha inhibitors, monoclonal antibodies | Emerging cause | **Timing Patterns:** - **Antibiotics:** Early onset (typically first 1-2 weeks) - **NSAIDs:** Intermediate timing (weeks 2-4) - **PPIs:** Delayed onset (weeks 3-8 or even months of exposure) ------------------------------------------------------------------------ ## Clinical Presentation: The Classic Triad Is Uncommon ### Classic Triad (Absent in ~2/3 of Cases) 1. **Fever** (~20-30% of patients) 2. **Rash** (~25% of patients) 3. **Eosinophilia** (~20% of patients) **Critical Point:** The **absence** of the classic triad does NOT exclude AIN. Many patients have AIN without any of these findings. ### More Common Presentation **Most Typical Scenario:** - **Nonoliguric AKI** (preserved or only mildly reduced urine output despite rising creatinine) - **AKI developing days to weeks after new medication initiation** - **Bland or nonspecific symptoms** (fatigue, anorexia, nausea) - **Physical exam unremarkable** (no fever, no rash visible) **Clinical Pearl:** The key clue is **nonoliguric AKI temporally related to new medication**—this should raise high suspicion for AIN. ------------------------------------------------------------------------ ## Diagnostic Approach: Urinalysis Findings ### Urinalysis Pattern in AIN | Finding | Typical Pattern | Significance | |----|----|----| | Proteinuria | Mild (usually \<1-2 g/day) | Heavy proteinuria suggests glomerular disease | | Hematuria | Mild to moderate | RBCs and WBCs present | | Leukocyte esterase | 2+ (positive) | Indicates WBCs in urine | | WBC count | Elevated (\>5/hpf, often 13-138/µL) | Strong finding in AIN | | WBC casts | May be present | Support diagnosis but not required | | RBC casts | Absent (or rare) | Suggests GN instead; if present, consider dual pathology | ### Urine Microscopy Interpretation **Characteristic Pattern:** - Numerous WBCs without bacteria (sterile pyuria) - Occasional RBCs - White cell casts when present - Mild proteinuria - **ABSENCE of RBC casts** (helps differentiate from glomerulonephritis) **Important Distinction:** - **RBC casts = GN/RPGN** (glomerular disease) - **WBC casts = AIN** (interstitial disease) - **Muddy brown casts = ATN** (tubular disease) ### Tests That DON’T Help (Important Limitations) **Urine Eosinophils — do not order.** `[EVIDENCE-BASED — PMID 24052222]` Muriithi et al. (*CJASN* 2013) tested this properly: 566 patients who had 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: | Measure | Value | What it means | |-------------|-----------|--------------------------------------------------| | Sensitivity | **30.8%** | Misses roughly 70% of biopsy-proven AIN | | Specificity | **68.2%** | Nearly a third of non-AIN patients test positive | | PPV | **15.6%** | Most positive results are wrong | | NPV | 83.7% | Only moderately reassuring | | **LR+** | **0.97** | **Changes nothing** | | **LR−** | **1.01** | **Changes nothing** | - **The likelihood ratios are the whole argument.** Both sit at 1.0. A test that cannot move post-test probability in either direction is not a diagnostic test — it is a coin flip you paid for. - **Critical teaching point:** the test fails in BOTH directions. A negative does not rule out AIN, and a positive does not rule it in. Older teaching emphasized only the rule-out failure; that framing understates the problem. - Raising the cutoff to 5% drops sensitivity to 19.8% while specificity rises to 91.2% — still not clinically useful. - **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.** Those numbers are an artifact of unvalidated reference standards. **Peripheral Eosinophilia:** - Occurs in only minority of drug-induced AIN cases - Absence doesn’t exclude diagnosis - No diagnostic value as screening test ------------------------------------------------------------------------ ## Novel Diagnostic Biomarkers ### Beta-2 Microglobulin (B2M) **What It Is:** Low-molecular-weight protein freely filtered and reabsorbed by proximal tubule. Elevation indicates tubular dysfunction. **Why It’s Mentioned:** Historically discussed as AIN marker, but limited utility for general AIN diagnosis. **Current Role:** - **Moderate utility for TINU syndrome** (tubulointerstitial nephritis + uveitis) - Limited for differentiation of AIN from ATN (both cause elevated B2M) - Pre-analytical instability (degrades at pH \<6.0) limits reliability **Bottom Line:** B2M is outdated for general AIN diagnosis; newer biomarkers superior. ### CXCL9: The Emerging Gold Standard **What It Is:** C-X-C motif chemokine ligand 9—a T cell–recruiting chemokine directly involved in immune response. **Why It’s Revolutionary:** - **Directly mechanistically linked** to T cell–mediated inflammation (the actual AIN pathophysiology) - Not elevated in ATN (unlike B2M, NGAL) - Excellent stability (remains stable \>24 hours at room temperature) - Fewer pre-analytical issues **Diagnostic Performance:** \| Metric \| Value \| \|——–\|——-\| \| AUC for AIN vs ATN \| 0.84-0.95 (excellent) \| \| Sensitivity \| 85-92% \| \| Specificity \| 80-92% \| \| Outperforms all traditional biomarkers \| YES \| **Cutoff Interpretation:** - High CXCL9 supports AIN diagnosis - Can distinguish AIN from other AKI causes - Increasingly available in specialized centers **Clinical Pearl:** CXCL9 is the future of non-invasive AIN diagnosis; traditional markers (urinary eosinophils) are outdated. ### Urinary Inflammatory Cytokines (KDIGO 2026 draft, PP 2.5.3) **Emerging role:** Urinary inflammatory cytokines (including but not limited to CXCL9) may be more useful than traditional markers for **distinguishing AIN from other causes of AKI**. The KDIGO 2026 draft guideline highlights this as a promising diagnostic approach, though availability remains limited outside research settings. ### Drug Classification Framework and AIN Detection **KDIGO 2026 draft context:** The guideline introduces a classification framework for drug effects on the kidney based on functional and damage biomarkers: - **Function(+)/Damage(+):** Drug causes both GFR decline and tubular damage (e.g., AIN with concurrent ATN features) - **Function(-)/Damage(+):** Drug causes tubular damage without immediate GFR change (subclinical AIN) AIN-causing drugs fall into these damage-positive categories. **Clinical implication:** Biomarkers of tubular damage (NGAL, KIM-1, \[TIMP-2\]x\[IGFBP-7\]) may help identify drug-induced AIN earlier than creatinine alone, particularly in the Function(-)/Damage(+) window before GFR decline is apparent. ------------------------------------------------------------------------ ## Kidney Biopsy: Definitive Diagnosis **Indication:** When clinical diagnosis uncertain and diagnosis would change management. **Histologic Findings:** - Dense lymphoplasmacytic infiltration of interstitium - Sparing of glomeruli (primary AIN) - Eosinophils may or may not be present - Tubular epithelial cell injury - Possible interstitial edema **Timing:** Biopsy within 2-3 weeks of symptom onset optimal for diagnosis and prognosis assessment. **Not Always Necessary:** In obvious cases (classic timeline + medication history + urinalysis pattern + clinical context), biopsy may be deferred if patient improving with drug discontinuation. ------------------------------------------------------------------------ ## TINU Syndrome: A Special Entity ### Definition and Epidemiology **TINU = Tubulointerstitial Nephritis + Uveitis Syndrome** **Key Statistics:** - Represents ~1-2% of uveitis cases in specialized centers - **NOT just a pediatric disease:** Affects patients aged 9-76 years - Female predominance (3:1 ratio) - Often misdiagnosed as isolated uveitis or drug-induced AIN alone **Important Note:** Historical literature emphasized adolescent presentation; modern recognition shows substantial adult involvement (30-40% of cases are adults). ### Clinical Presentation **Dual System Involvement:** 1. **Renal:** Acute interstitial nephritis (elevated creatinine, pyuria, RBC+WBC casts possible) 2. **Ocular:** Bilateral anterior uveitis (eye pain, photophobia, visual symptoms) 3. **Systemic:** Often nonspecific (fever, malaise, arthralgia) **Timeline:** - Simultaneous presentation common - Ocular findings may precede renal manifestations (diagnosed as isolated uveitis first) - Renal manifestations may occur months after ocular symptoms ### Diagnostic Criteria **Definite TINU:** 1. Kidney biopsy confirming AIN 2. Bilateral anterior uveitis on ophthalmology exam 3. No other systemic disease explaining both **Probable TINU:** 1. Clinical AIN (elevated Cr + compatible urinalysis; biopsy not required if clinical picture clear) 2. Bilateral anterior uveitis confirmed 3. No alternative diagnosis ### Role of B2M in TINU **This is where B2M has established utility:** In TINU syndrome specifically, urinary B2M has superior sensitivity/specificity compared to general AIN. **Performance in TINU:** - Sensitivity: 88-100% - Specificity: 70-80% - Elevated B2M + bilateral uveitis = high probability TINU **Practical Application:** - Screen for bilateral anterior uveitis in any patient with AKI (ophthalmology exam) - If uveitis present + elevated B2M, TINU diagnosis highly likely - Refer to rheumatology/immunology for systemic workup ------------------------------------------------------------------------ ## Management Principles ### Immediate Steps **1. Discontinue Offending Medication** - **Most critical intervention** - Discontinue as soon as AIN suspected - Many cases improve significantly with drug withdrawal alone **2. Supportive Care** - Fluid and electrolyte management - Renal function monitoring - RRT if necessary (same indications as ATN) ### Corticosteroid Therapy **Evidence & Recommendation:** Steroid evidence in drug-induced AIN is mainly observational. Earlier treatment may help selected patients, but no elapsed-day or single-creatinine rule mandates treatment. After prompt culprit withdrawal, use recovery trajectory, diagnostic confidence, biopsy activity/chronicity, infection, and treatment risks to decide with nephrology. Dose, duration, and taper are individualized; a common regimen is not a universally validated protocol. [Treatment-duration cohort](https://pubmed.ncbi.nlm.nih.gov/30397027/). **Outcome interpretation:** Recovery ranges from complete to partial or absent and depends on cause, baseline function, scarring, and timing. Do not present one cohort’s percentages as an individual prognosis or as proof of a causal steroid effect. **Factors Associated with Better Outcomes:** - Earlier steroid initiation (within 1-2 weeks) - Shorter duration of drug exposure before diagnosis - Lower peak creatinine at presentation **Clinical Pearl:** Prompt withdrawal and timely diagnostic reassessment matter. Observational associations with earlier steroids support avoiding unnecessary delay in selected active AIN, while substantial chronic scarring or infection can change the balance against immunosuppression. ### When to Escalate to Immunosuppression Some cases with severe AIN or inadequate response to steroids may require: - Mycophenolate mofetil (MMF) - Tacrolimus - Other immunosuppressive agents - Typically reserved for severe/refractory cases ------------------------------------------------------------------------ ## Differential Diagnosis: AIN vs. ATN vs. RPGN | Feature | AIN | ATN | RPGN | |----|----|----|----| | **Mechanism** | Immune-mediated inflammation | Tubular necrosis | Glomerular crescents | | **Urine output** | Nonoliguric (preserved) | Oliguric or nonoliguric | Often oliguric | | **Proteinuria** | Mild (\<1-2 g/day) | Mild | Heavy (often \>3 g/day) | | **RBC casts** | Uncommon; reported in biopsy-proven AIN ([case report](https://pubmed.ncbi.nlm.nih.gov/22607916/)) | Unusual; consider coexisting glomerular disease | Strong clue when present; absence does not exclude GN | | **WBC casts** | May be present | Absent | May be present | | **Muddy brown casts** | Absent | PRESENT | Absent | | **WBC in urine** | Elevated (13-138/µL) | Low | Low | | **Timeline** | Days-weeks post-medication | Hours-days post-injury | Days-weeks | | **Classic triad** | Rare | N/A | Hemoptysis possible | ------------------------------------------------------------------------ ## Practice Questions **Question 1:** A 54-year-old on omeprazole (3 months) for GERD develops AKI (Cr 1.1 to 2.8 in 1 week). Urinalysis: WBC 45/hpf, RBCs 8/hpf, protein 1+, no casts. Most likely diagnosis? A) Acute tubular necrosis B) Acute interstitial nephritis C) Rapidly progressive glomerulonephritis D) Prerenal AKI **Answer:** B) Acute interstitial nephritis. The timeline (3-month PPI use with recent AKI), nonoliguric pattern (Cr rise despite presumably preserved urine output), elevated WBCs in urine without casts, and mild proteinuria are classic for drug-induced AIN. PPIs are 2nd-most common drug cause of AIN. Absence of RBC casts does not exclude glomerulonephritis, and absence of muddy-brown casts does not exclude tubular injury. Integrate the creatinine trajectory, protein quantity, systemic findings, and targeted tests; discuss biopsy when uncertainty changes treatment. ------------------------------------------------------------------------ **Question 2:** A 32-year-old woman presents with red eyes (bilateral anterior uveitis, confirmed by ophthalmology) and AKI (Cr 2.1). Urinalysis: WBCs, mild proteinuria, no RBC casts. Urine B2M elevated. Most likely diagnosis? A) Goodpasture syndrome B) Systemic lupus erythematosus C) TINU syndrome D) Medication-induced AIN only **Answer:** C) TINU syndrome. The combination of bilateral anterior uveitis (ocular finding) + AIN (renal finding) + elevated B2M = TINU syndrome diagnosis. This is the specific scenario where B2M has excellent diagnostic utility. Anti-GBM disease may cause pulmonary hemorrhage and glomerular sediment, but neither hemoptysis nor RBC casts is obligatory. SLE possible but bilateral uveitis less typical. ------------------------------------------------------------------------ **Question 3:** A 68-year-old on amoxicillin (day 7) develops AKI. Biopsy confirms AIN. Current Cr is 3.2 (baseline 1.0). Which statement represents best management? A) Discontinue amoxicillin and observe; most cases self-limited B) Discontinue or replace amoxicillin and obtain nephrology assessment for individualized steroid treatment C) Discontinue amoxicillin + start dialysis; steroids contraindicated in infections D) Continue amoxicillin (needed for infection); add corticosteroids for kidney protection **Answer:** B) Discontinue or replace amoxicillin and obtain nephrology assessment for individualized steroid treatment. Preserve effective infection therapy if antibiotics remain needed. Biopsy-confirmed AIN with marked dysfunction can support steroid consideration, but day 7 or creatinine 3.2 alone does not mandate it. Review biopsy activity/chronicity, recovery after withdrawal, and infection/treatment harms. Dialysis is reserved for clinical complications, not for the AIN label alone. ------------------------------------------------------------------------ ## Key Takeaways 1. **AIN accounts for 15-27% of biopsy-proven AKI** — must maintain high suspicion 2. **Classic triad absent in 2/3 of cases** — don’t rely on fever/rash/eosinophilia 3. **Nonoliguric AKI + new medication = suspect AIN** until proven otherwise 4. **Urine eosinophils unreliable** (outdated test; don’t order) 5. **CXCL9 emerging biomarker** for non-invasive AIN diagnosis 6. **Discontinue offending agent immediately** — most critical intervention 7. **Earlier steroids may help selected drug-induced AIN** in observational studies; individualize rather than applying an automatic time cutoff 8. **TINU is not just pediatric** — affects adults; requires ophthalmology screening ------------------------------------------------------------------------ ## See Also ### Related Student Handouts - AKI Workup and Diagnostic Approach - Drug-Induced AKI - AKI Biomarkers and Early Detection - Magnesium Disorders ### Clinical Content (01-Clinical-Medicine/Nephrology) - AKI Hub - Full Clinical Reference - Essential Renal Laboratory Tests ### Butler-COM Resources - Butler COM - Nephrology Deep Dive ------------------------------------------------------------------------ ## Related Resources - AKI Workup and Diagnostic Framework - Detailed B2M and TINU Discussion - Drug-Induced AKI Overview - PPI-Associated Kidney Disease ------------------------------------------------------------------------ **Document Type:** Student Handout **Level:** PA/Medical Student **Created:** 2026-02-12 **Updated:** 2026-03-29 (KDIGO 2026 draft AKI/AKD guideline) **Source:** AIN comprehensive literature, TINU syndrome guidelines, contemporary biomarker evidence, KDIGO 2026 draft AKI/AKD guideline ### Also on this topic The same subject at other levels of depth. - **Lecture:** [\💧 Acute Interstitial Nephritis (AIN)](https://urinenephrology.org/2025_UDPA_Lectures_Live/aki/interstitial-nephritis.html) - **Interactive case:** [Acute Interstitial Nephritis](https://urinenephrology.org/2025_UDPA_Lectures_Live/cases/case3_enhanced.html) --- [Website version](https://urinenephrology.org/student-resources/aki/ain-student-handout.html) · Markdown synchronized October 3, 2026.