Nephritis / Glomerulonephritis / Interstitial Nephritis — Clinical Documentation Guide (2026)

🔍 Definition

Nephritis is a broad term encompassing inflammation of the kidney, primarily classified into two major anatomical patterns: glomerulonephritis (GN), affecting the glomeruli (the filtering units), and interstitial nephritis (tubulointerstitial nephritis), affecting the tubules and surrounding interstitium. These processes share the common pathophysiologic endpoint of impaired renal filtration but differ markedly in etiology, clinical presentation, and ICD-10-CM coding pathways.

Glomerulonephritis results from immune-mediated injury to the glomerular capillary tuft, manifesting as one of several histopathologic patterns (membranous, mesangial proliferative, endocapillary proliferative, mesangiocapillary, dense deposit disease, crescentic, etc.). Clinical syndromes include acute nephritic syndrome, rapidly progressive GN (RPGN), recurrent/persistent hematuria, chronic nephritic syndrome, and nephrotic syndrome — each corresponding to distinct ICD-10-CM subcategories (N00–N07) with morphology-specific fourth-character digits.

Interstitial nephritis involves inflammation of the tubulo-interstitial compartment, often triggered by drugs (NSAIDs, antibiotics, proton pump inhibitors), infections, obstructive uropathy, or metabolic disorders. Acute interstitial nephritis (AIN) may present as AKI; chronic interstitial nephritis (CIN) progresses to CKD. ICD-10-CM codes N10–N16 capture the spectrum from acute pyelonephritis to drug-induced tubular damage.

Accurate documentation requires the clinician to specify: (1) the clinical syndrome (nephritic vs. nephrotic, acute vs. chronic, recurrent vs. rapidly progressive); (2) the morphologic pattern on biopsy when available; (3) the underlying etiology (diabetic, lupus, hypertensive, drug-induced, hereditary); and (4) the presence and stage of CKD or AKI. Each of these dimensions drives ICD-10-CM code selection, HCC capture, and MS-DRG assignment.

🗂️ Alternative Terminology

🩺 Signs & Symptoms

Clinical presentations vary by syndrome and must be explicitly documented to support specific coding:

• Nephritic syndrome features (N00, N01, N03, N05): Hematuria (gross or microscopic with RBC casts), oliguria, hypertension, edema (periorbital, dependent), azotemia, proteinuria (sub-nephrotic range <3.5 g/day). In RPGN (N01), renal function declines by ≥50% over weeks.
• Nephrotic syndrome features (N04): Heavy proteinuria ≥3.5 g/day, hypoalbuminemia (<3.5 g/dL), edema (anasarca, ascites, pleural effusion), hyperlipidemia, lipiduria (oval fat bodies, fatty casts). Thrombophilia risk (renal vein thrombosis).
• Recurrent/persistent hematuria (N02): Episodic gross hematuria (often with upper respiratory infections in IgA nephropathy), persistent microscopic hematuria, flank discomfort, mild proteinuria.
• Interstitial nephritis (N10–N12): AIN classic triad of fever, rash, eosinophilia (now recognized in <10% of cases); AKI with sterile pyuria, eosinophiluria, subnephrotic proteinuria, tubular dysfunction (Fanconi syndrome). Chronic interstitial nephritis presents with slowly progressive CKD, hyperkalemia, non-anion gap metabolic acidosis, polyuria.
• Pyelonephritis (N10): Flank pain, costovertebral angle tenderness, fever, chills, dysuria, frequency, bacteriuria/pyuria on urinalysis.
• Drug-induced nephropathy (N14.x): Sudden rise in serum creatinine following introduction of offending drug, non-oliguric AKI, tubular casts, hyperkalemia.

🧭 Differential Diagnosis

📋 Clinical Indicators for Coders/CDI

🦴 Anatomy & Pathophysiology

The kidney contains approximately 1 million nephrons per organ, each comprising a glomerulus and a tubule. The glomerulus — a specialized capillary tuft enclosed in Bowman’s capsule — filters approximately 180 L of plasma per day, driven by hydrostatic pressure. The filtration barrier consists of three layers: fenestrated glomerular endothelium, the glomerular basement membrane (GBM), and podocytes with their slit diaphragms. Disruption to any of these layers by immune deposits, antibodies, or cytokines initiates the glomerulonephritis cascade.

Glomerulonephritis Pathophysiology: In immune-complex GN (IgA nephropathy, lupus nephritis, post-infectious GN), immune complexes deposit in the mesangium, subendothelial, or subepithelial spaces, activating complement (C3a, C5a), recruiting neutrophils and monocytes, and triggering mesangial/endothelial cell proliferation. In anti-GBM disease, linear IgG antibodies against the NC1 domain of collagen IV directly attack the GBM. In ANCA-associated GN, activated neutrophils degranulate within glomerular capillaries, producing necrotizing lesions and crescent formation (parietal epithelial cell proliferation into Bowman’s space — the hallmark of RPGN).

Tubulo-Interstitial Pathophysiology: Drug-induced AIN is a type IV hypersensitivity reaction in which a hapten-drug complex bound to tubular antigens recruits T-lymphocytes, causing tubular epithelial injury and interstitial edema. Ischemic or nephrotoxic AKI disrupts proximal tubular epithelium, causing ATN (N17.0) with tubular cell sloughing and cast obstruction. Chronic interstitial nephritis leads to tubular atrophy, interstitial fibrosis, and ultimately nephron loss with progressive CKD. In obstructive nephropathy (N13.x), increased intratubular pressure causes tubular dilation, collecting system distension, and — if sustained — irreversible medullary and cortical damage.

Etiology/Manifestation Principle: ICD-10-CM recognizes that many nephritides are manifestations of systemic disease. Under the FY2026 ICD-10-CM Official Guidelines, certain conditions require the underlying disease to be sequenced first (e.g., code M32.14 before N08 for lupus nephritis; code E11.21/E11.22 as the combination code for diabetic nephropathy, not N08).

💊 Medication Impact / Treatment

Pharmacotherapy for nephritis conditions has direct coding implications — both for HCC validation through treatment patterns and for adverse effect/underdosing codes when drug-induced nephropathy is present.

Immunosuppressive and Biologic Therapies

• Corticosteroids (prednisone, methylprednisolone): First-line for MCD, FSGS, acute interstitial nephritis, lupus nephritis flare. Long-term use requires documentation of complications (osteoporosis, diabetes, adrenal insufficiency).
• Mycophenolate mofetil (MMF): Induction and maintenance for lupus nephritis (Class III–V); used in FSGS and MCD steroid-dependent cases.
• Cyclophosphamide: Induction therapy for ANCA vasculitis GN, severe lupus nephritis; high-dose IV or oral protocols.
• Rituximab (J9312): Anti-CD20 biologic for ANCA-GN, membranous nephropathy (PLA2R+), refractory lupus nephritis.
• Belimumab (J0490): FDA-approved 2020 specifically for active lupus nephritis; IV formulation; HCPCS J0490.
• Anifrolumab (J0491): Type I interferon receptor antagonist; approved for moderate-to-severe SLE including lupus nephritis manifestations.
• Tacrolimus / Cyclosporine: Calcineurin inhibitors used in steroid-resistant FSGS and membranous nephropathy.
• Voclosporin (Lupkynis): Calcineurin inhibitor FDA-approved 2021 specifically for active lupus nephritis in combination with MMF.

RAAS Blockade and Nephroprotection

• ACE inhibitors / ARBs: Mandatory in proteinuric CKD (diabetic and non-diabetic nephropathy) to reduce intraglomerular pressure and proteinuria. Underdosing or discontinuation despite proteinuria should be queried.
• SGLT2 inhibitors (dapagliflozin, empagliflozin): FDA-approved 2020–2021 for CKD with or without T2DM; reduce progression in IgA nephropathy (dapagliflozin 2023 approval) and FSGS (investigational).
• Sparsentan: Dual endothelin/angiotensin receptor antagonist; FDA-approved 2023 for IgA nephropathy.
• Iptacopan: Factor B inhibitor; FDA-approved 2024 for paroxysmal nocturnal hemoglobinuria nephropathy and C3 glomerulopathy.

Supportive Agents

• Furosemide / torsemide: Management of nephrotic edema and volume overload in advanced CKD.
• ESAs (epoetin alfa Q4081, darbepoetin J0882): Anemia of ESRD/CKD; covered under ESRD dialysis bundle or separately billed per HCPCS codes.
• Phosphate binders, vitamin D analogs, calcimimetics: Mineral bone disorder management in advanced CKD/ESRD.

Preview ends here. The full guide continues with FY2026 ICD-10-CM code sets, CPT surgical coding, MS-DRG mapping, reimbursement guidance, CDI query templates, and an audit checklist — all available to CCO Members.

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📘 ICD-10-CM Guidelines (FY2026)

The following guidelines govern code selection for nephritis conditions under FY2026 ICD-10-CM Official Guidelines for Coding and Reporting and the FY2026 ICD-10-CM Tabular List (NCHS/CDC):

1. Etiology/Manifestation Coding — Critical Sequencing Rules

• Diabetic nephropathy (Section I.C.4.a): Use combination codes E11.21 (T2DM with diabetic nephropathy) or E11.22 (T2DM with diabetic CKD). The combination code subsumes the manifestation — do NOT add N08 when using E11.21. When CKD stage is documented, use E11.22 as the diabetes code (captures nephropathy + CKD link) and add N18.x for the CKD stage. Never use E11.21 with E11.22 simultaneously — they are mutually exclusive.
• Lupus nephritis: Code M32.14 (glomerular disease in SLE) as the principal/first-listed code per the “code first” instruction at N08. M32.14 is the most specific code and does not require a separate N08 in most scenarios. Add N18.x if CKD stage is documented.
• Hypertensive CKD (Section I.C.9.a): ICD-10-CM presumes a causal relationship between hypertension and CKD — no physician statement of causality is required. Use I12.x combination codes (I12.9 HTN with CKD stage 1–4 or unspecified; I12.0 HTN with CKD stage 5 or ESRD). Add N18.x for CKD stage. If heart failure is also present, use I13.x codes (hypertensive heart and CKD combination codes).
• HIV nephropathy: Code B20 (HIV disease) first, then N08 as manifestation per the “code first” instruction.
• Amyloid nephropathy: Code the amyloidosis type (E85.x) first, then N08.

2. Morphology-Specific Fourth Digits (N00–N07)

The categories N00–N07 each use a common set of fourth-digit subcategories representing the histopathologic pattern documented on renal biopsy or clinically established:

• .0 — Minor glomerular abnormality (thin basement membrane, minimal change)
• .1 — Focal and segmental glomerular lesions (FSGS, focal sclerosis, hyalinosis)
• .2 — Diffuse membranous glomerulonephritis
• .3 — Diffuse mesangial proliferative GN (IgA nephropathy maps here)
• .4 — Diffuse endocapillary proliferative GN (post-infectious GN)
• .5 — Diffuse mesangiocapillary GN (MPGN type I)
• .6 — Dense deposit disease (MPGN type II / C3 glomerulopathy)
• .7 — Diffuse crescentic GN (RPGN — hallmark of N01.7)
• .8 — Other specified morphologic changes (fibrillary GN, immunotactoid GN)
• .9 — Unspecified morphologic lesion (use when biopsy not done or pattern not specified)

Documentation tip: When pathology reports specify a pattern (e.g., “IgA nephropathy with mesangial proliferative changes”), the coder should select N02.3 (recurrent/persistent hematuria with diffuse mesangial proliferative GN) rather than N02.9. This requires clear physician documentation linking clinical syndrome to morphologic pattern.

3. AKI vs. CKD vs. AKI on CKD

Per FY2026 guidelines Section I.C.14:

• Assign N17.x for AKI and N18.x for CKD when both are documented as concurrent; they are not mutually exclusive.
• AKI (N17.x) is coded when the physician documents “acute kidney injury,” “acute renal failure,” or “acute kidney failure” — not simply elevated creatinine.
• “Acute on chronic kidney disease” or “AKI superimposed on CKD”: assign both N17.x and N18.x with appropriate stage.
• AKI alone (N17.x) does not generate HCC value; AKI paired with CKD stages 3–5 allows both codes and the N18.x drives HCC capture.

4. Sequencing Principal Diagnosis — Inpatient

• For admitted nephritis workup (biopsy, initiation of immunosuppression): the nephritic/nephrotic syndrome category (N00–N07) is typically the principal diagnosis; the underlying etiology coded additionally if separately identified.
• When admission is for a complication of CKD (volume overload, hyperkalemia, dialysis access), sequence the complication as principal with N18.x as secondary.
• For AKI requiring hospitalization: N17.x may be principal if it is the reason for admission; add causative codes (sepsis, ischemia, drug) and N18.x if CKD co-exists.

🔢 ICD-10-CM Code Set (FY2026)

N00–N07: Glomerular Diseases — Nephritic/Nephrotic Syndromes

N10–N16: Renal Tubulo-Interstitial Diseases

Etiology/Manifestation Combination Codes — Critical Sequencing

Related Codes — AKI, CKD, and Special Status

🔎 Indexing

Key Alphabetic Index entries in the FY2026 ICD-10-CM Index to Diseases and Injuries:

• Nephritis → N05.9 (default lead term); subterms: acute → N00.9; chronic → N03.9; rapidly progressive → N01.9; nephrotic → N04.9
• Glomerulonephritis → see also Nephritis; acute → N00.9; chronic → N03.9; IgA → see Nephropathy, IgA
• Nephropathy — lead term: IgA → N02.9; diabetic (see also E10–E13 with .21 or .22); hypertensive → see Hypertension, kidney; analgesic → N14.0; drug-induced → N14.1; hereditary → N07.9; Alport → Q87.81-related
• Nephrotic syndrome → N04.9; with specified morphology see N04.0–N04.8
• Pyelonephritis → acute → N10; chronic → N11.9; nonobstructive reflux-assoc → N11.0; obstructive → N11.1
• Nephropathy, toxic → N14.4; heavy-metal → N14.3; analgesic → N14.0
• Interstitial nephritis → acute → N10; chronic → N11.9; drug-induced → N14.1
• Failure, kidney → acute → N17.9; chronic → N18.9; stage-specific → N18.1–N18.6; ESRD → N18.6
• Balkan nephropathy → N15.0
• Syndrome, nephrotic → N04.9; see also N04.0–N04.8 by morphology
• Hematuria, recurrent and persistent → N02.9; with specified morphology → N02.0–N02.8

🏥 CPT (2026)

🧾 HCPCS (2026)

📚 AHA Coding Clinic (Recent Guidance)

The following summarizes relevant AHA Coding Clinic guidance applicable to nephritis coding. Coders should verify access to current editions for full guidance text.

💰 HCC / Risk Adjustment (v28)

Under CMS-HCC Model v28 (effective 2024–2026), nephritis and renal disease codes map to the following hierarchical condition categories:

✍️ CDI Query Templates

All queries below follow ACDIS/AHIMA CDI query guidelines: non-leading, offer multiple clinically plausible options including “other” and “clinically undetermined,” formatted as multiple choice or open-ended as appropriate.

🧑‍⚕️ Treatments (Clinical)

Glomerulonephritis — Disease-Specific Management

• IgA Nephropathy (N02.3): RAAS blockade (ACE inhibitor/ARB) for proteinuria ≥0.5 g/day; optimized BP control; fish oil (adjunct); corticosteroids for high-risk patients; targeted-release budesonide (Nefecon/Tarpeyo — FDA-approved 2021 for IgA nephropathy); dapagliflozin; sparsentan (FDA 2023). Tonsillectomy in select cases (Japanese guidelines).
• Minimal Change Disease (N04.0): High-dose prednisone (first-line); relapsing course managed with mycophenolate, calcineurin inhibitors, or rituximab for steroid-dependent/resistant cases; typically excellent prognosis with steroid response.
• FSGS (N04.1): Prednisone for primary FSGS; consider underlying cause (secondary FSGS from obesity, reflux, sickle cell, medications); cyclosporine/tacrolimus for steroid-resistant; rituximab; plasma exchange for FSGS recurrence post-transplant.
• Membranous Nephropathy (N04.2): PLA2R antibody-guided management; spontaneous remission ~30%; rituximab (anti-PLA2R based); KDIGO 2021 guidelines recommend rituximab or cyclophosphamide/calcineurin inhibitor-based protocols; RAAS blockade mandatory.
• RPGN/ANCA Vasculitis (N01.7): Urgent immunosuppression with high-dose methylprednisolone IV + cyclophosphamide or rituximab (induction); plasma exchange for severe disease; maintenance azathioprine or rituximab; close renal function monitoring.
• Lupus Nephritis (M32.14): Class III/IV: high-dose steroids + MMF or cyclophosphamide induction; belimumab add-on; voclosporin + MMF (FDA 2021); anifrolumab (FDA 2021 for systemic SLE); maintenance MMF or azathioprine; hydroxychloroquine backbone for all SLE.

Tubulointerstitial Nephritis Management

• Acute Pyelonephritis (N10): IV antibiotics (fluoroquinolone or beta-lactam based on culture/sensitivity); oral step-down; 7–14 days total course; drainage if obstructed (N13.x); blood and urine cultures guide therapy.
• Drug-Induced AIN (N14.1): Immediate discontinuation of offending agent; corticosteroid pulse (0.5–1 mg/kg prednisone) if no improvement within 5–7 days or severe AKI; recovery of kidney function often within weeks if agent removed promptly.
• Analgesic Nephropathy (N14.0): Discontinue all analgesics; treat CKD if progression; avoid rechallenging with NSAIDs; consider renal papillary necrosis imaging (contrast CT).

CKD Progression Management in Nephritis Patients

• RAAS blockade (ACE inhibitor/ARB) — mandatory for proteinuria ≥300 mg/day
• SGLT2 inhibitors (dapagliflozin, empagliflozin) — FDA-approved CKD indication regardless of diabetes status (2020–2021)
• BP target <130/80 mmHg; dietary protein restriction 0.8 g/kg/day in CKD 3–5
• Mineral-bone disorder management: phosphate binders, vitamin D analogs, cinacalcet (CKD 5/ESRD)
• Anemia management: ESAs (Q4081, J0882) + iron replacement (IV iron for dialysis patients)
• Renal replacement therapy planning: AV fistula creation (CPT 36821) at CKD 4; peritoneal dialysis catheter (CPT 49421); transplant referral at CKD 4–5

🎓 Patient Education / Summary

The following is a plain-language summary suitable for patient education materials, CDI team briefings, or provider communication aids:

What is Nephritis? Nephritis means inflammation (“itis”) of the kidneys (“nephros”). There are two main types: glomerulonephritis (GN) affects the tiny filtering units (glomeruli) that clean the blood, and interstitial nephritis affects the supporting tissue around the kidney’s tubules. Both can be caused by the immune system attacking the kidneys, infections, certain medications, or other diseases like diabetes or lupus.

Why Does Accurate Documentation Matter?

• For patients: Accurate diagnosis drives correct treatment selection. Knowing whether the nephritis is “nephrotic” (with heavy protein loss) vs. “nephritic” (with blood cells and high blood pressure) helps doctors choose the right medications and predict outcomes.
• For coders and CDI: ICD-10-CM has very specific codes requiring the clinical syndrome (acute, chronic, rapidly progressive), the microscopic pattern seen on biopsy (membranous, mesangial, crescentic, etc.), and the underlying cause (diabetes, lupus, drugs). Missing any of these components leads to codes that underrepresent disease severity, miss HCC capture, or result in audit findings.
• For quality and reimbursement: CKD stage documentation at every encounter is essential for Medicare Advantage risk adjustment. CKD stages 3–5 and ESRD generate significant RAF values (HCC 326, 327, 328) while stages 1–2 carry no risk-adjustment value. Under-coding CKD stage is one of the most common and impactful documentation gaps in nephrology practices and hospitalist medicine.

Key Takeaways for Clinical Documentation:

1. Always document the clinical syndrome (nephritic vs. nephrotic; acute vs. chronic; RPGN).
2. Include the morphologic diagnosis from the renal biopsy report (e.g., “diffuse membranous GN,” “mesangial proliferative GN with IgA deposits”).
3. Specify the underlying etiology and link it explicitly: “nephrotic syndrome DUE TO diabetic nephropathy in T2DM.”
4. Document CKD stage at every encounter using GFR-based criteria — not just “CKD.”
5. When both AKI and CKD are present, document “AKI on CKD stage X” — both codes can and should be captured.
6. For drug-induced nephropathy, name the offending drug and document causality explicitly.
7. For ESRD patients on dialysis, document both “ESRD” and “dialysis-dependent” to capture the highest HCC tier (HCC 328).

For additional coding guidance, see the FY2026 ICD-10-CM Official Guidelines (CMS.gov), FY2026 ICD-10-CM Tabular List (CDC/NCHS), AHA Coding Clinic, and the CMS HCC Model v28 documentation.

About this Guide

This Clinical Documentation Guide is published by CCO Academy and is intended for credentialed coding, CDI, and clinical documentation professionals. Content is updated for FY2026 ICD-10-CM (effective October 1, 2025). All code assignments should be verified against the official ICD-10-CM Tabular List, AHA Coding Clinic, and applicable payer-specific policies. This guide does not constitute legal, medical, or compliance advice.

Last reviewed: April 2026 · Next scheduled review: October 2026 (FY2027 update)