Crystal's Coder Hub🧬 ICD-10-CM N18.31 — Chronic Kidney Disease, Stage 3a
Billable Code Confirmed
ICD-10-CM N18.31 is a valid, billable 6-character ICD-10-CM diagnosis code for FY2026. Characters 1-3 (N18) define the CKD category; character 4 (.3) identifies stage 3 (moderate); character 5 (1) specifies substage 3a, distinguishing this code from N18.30 (stage 3, unspecified) and N18.32 (stage 3b). All six characters are required for submission; no additional characters are available or needed.
Non-Billable Parent Codes — Never Submit These
- ❌
N18— 3-character header — no stage, substage, or type specificity present- ❌
N18.3— 5-character header — stage 3 identified but substage (a, b, or unspecified) not definedAlways submit N18.31 (all 6 characters) when chronic kidney disease is documented as stage 3a, or when the GFR is confirmed in the 45-59 range and the provider specifies or confirms stage 3a.
Clinical Context: N18.31 Is the Most Specific Available Code When GFR Is 45-59
ICD-10-CM N18.31 captures CKD stage 3a — defined by an eGFR of 45-59 mL/min/1.73 m² persisting ≥3 months. It is a more specific code than N18.30 (unspecified substage) and reflects mild-to-moderate kidney impairment with a lower short-term progression risk compared to stage 3b. If GFR is available in the medical record and falls in the 45-59 range, N18.31 should be used over N18.30 — but only when the provider has documented or confirmed the stage. Never independently assign a substage based solely on lab values without provider documentation or a completed CDI query response.
Code Classification
ICD-10-CM Diagnosis Code — wRVU, assistant-at-surgery payable status, and global period fields are not applicable to ICD-10-CM diagnosis codes. For associated procedures performed in the inpatient or outpatient setting, refer to the CPT Procedural Crosswalk and ICD-10-PCS Crosswalk sections below.
🔍 Code Description
[ICD-10-CM] N18.31 classifies chronic kidney disease, stage 3a — a mild-to-moderate reduction in kidney function defined by an estimated glomerular filtration rate (eGFR) of 45-59 mL/min/1.73 m² that has persisted for three months or more, as confirmed by repeat testing to exclude acute kidney injury.1,2
At stage 3a, kidney function is reduced to approximately 45-59% of normal, and patients may be largely asymptomatic or present with mild fatigue, early anemia, or mildly elevated blood pressure. The underlying etiologies most commonly include diabetic nephropathy, hypertensive nephrosclerosis, and glomerulonephritis, and ICD-10-CM tabular instructions require these to be coded first when they are the documented cause.1,3 Stage 3a carries an elevated lifetime risk for cardiovascular events, heart failure, and progression to later CKD stages, underscoring the importance of annual documentation and comorbidity capture even in the absence of HCC payment weight under CMS-HCC v28.2,3
🌳 Code Tree / Hierarchy
N18 Chronic kidney disease (CKD) ❌ Non-billable
│
├── N18.1 Chronic kidney disease, stage 1 ✅ Billable
├── N18.2 Chronic kidney disease, stage 2 (mild) ✅ Billable
│
├── N18.3 Chronic kidney disease, stage 3 (moderate) ❌ Non-billable
│ │
│ ├── N18.30 Chronic kidney disease, stage 3 unspecified ✅ Billable
│ ├── N18.31 Chronic kidney disease, stage 3a ◀ THIS CODE ✅ Billable
│ └── N18.32 Chronic kidney disease, stage 3b ✅ Billable
│
├── N18.4 Chronic kidney disease, stage 4 (severe) ✅ Billable
├── N18.5 Chronic kidney disease, stage 5 ✅ Billable
├── N18.6 End stage renal disease ✅ Billable
└── N18.9 Chronic kidney disease, unspecified ✅ Billable
N18.31 Over N18.30 — Always Pursue the Most Specific Code
When the GFR is documented or available in the record and falls in the 45-59 range, N18.31 is the correct code — do not default to N18.30 (unspecified) unless the provider has not specified or confirmed the substage. A brief CDI query or review of the lab flowsheet may be all that is needed to justify the more precise code. Neither N18.31 nor N18.30 carries HCC payment weight under CMS-HCC v28, but documentation specificity strengthens the record and prepares for future model updates.
✅ Includes
The following clinical terms and scenarios map to N18.31 when documented:
- Chronic kidney disease, stage 3a — eGFR 45-59 mL/min/1.73 m², confirmed by provider documentation or CDI query response
- Mild-to-moderate CKD, substage 3a, due to diabetic nephropathy (code E11.22 first)
- Mild-to-moderate CKD, substage 3a, due to hypertensive nephrosclerosis (code I12.- or I13.- first)
- CKD stage 3a with albuminuria categories A1, A2, or A3 — albuminuria degree does not change the stage code but may be documented as a comorbidity modifier
❌ Excludes
Excludes 1 — Cannot Be Coded Simultaneously with N18.31
| Code | Description | Note |
|---|---|---|
| N18.9 | Chronic kidney disease, unspecified | Once a stage is documented, N18.9 cannot also be reported — the staged code entirely replaces the unspecified code |
Excludes 1 Violation Risk
Excludes 2 — May Be Coded in Addition if Separately Present
| Code | Description | Note |
|---|---|---|
| No formal Excludes 2 at N18.31 — see “Code First” and “Use Additional Code” instructions |
"Code First" and "Use Additional Code" Instructions
ICD-10-CM tabular instructions at the N18 category level apply fully to N18.31:1
- Code first any associated diabetic CKD: E08.22, E09.22, E10.22, E11.22, E13.22
- Code first any associated hypertensive CKD: I12.9 (hypertensive CKD) or I13.- (hypertensive heart and CKD)
- Use additional code to identify associated cachexia: E88.A
- Use additional code to identify kidney transplant status if applicable: Z94.0
📋 Clinical Overview
CKD Stage 3 Substage Comparison
The table below clarifies when N18.31 is the correct choice versus its sibling codes within the stage 3 subcategory.
| Feature | N18.30 — Stage 3 Unspecified | N18.31 — Stage 3a | N18.32 — Stage 3b |
|---|---|---|---|
| eGFR Range | 30-59 (substage not documented) | 45-59 mL/min/1.73 m² | 30-44 mL/min/1.73 m² |
| Kidney Function Level | Moderate, NOS | Mild-to-moderate reduction | Moderate-to-severe reduction |
| Symptom Burden | Variable/NOS | Often mild or asymptomatic | More overt fatigue, anemia, BP elevation |
| Cardiovascular Risk | Elevated | Elevated | Higher than 3a |
| Short-Term Progression Risk | Unknown/unspecified | Lower within stage 3 | Higher within stage 3 |
| CMS-HCC v28 Mapping | ❌ No HCC | ❌ No HCC | ❌ No HCC |
| Best Used When | Substage not clarified by provider | GFR 45-59 confirmed or provider documents 3a | GFR 30-44 confirmed or provider documents 3b |
CDI Query Trigger — Use Lab Data to Prompt Substage Specificity
If the GFR is documented in the record in the 45-59 range but the provider has only written “CKD stage 3,” initiate a CDI query asking the provider to confirm whether the stage is 3a (eGFR 45-59) or 3b (eGFR 30-44). Use the actual GFR value in the query to make it easy for the provider to confirm. Once confirmed, N18.31 replaces N18.30.
Manifestations & Symptom Burden
Common manifestations associated with CKD stage 3a that should be separately coded when documented:
- Anemia of chronic kidney disease: Decreased erythropoietin synthesis; code with D63.1 — Anemia in chronic kidney disease
- Secondary hyperparathyroidism: Phosphate retention and reduced 1,25(OH)₂ vitamin D production; code with E21.1 when documented
- Metabolic acidosis: Impaired renal acid excretion, typically mild at stage 3a; code with E87.2 when documented
- Hypertension: Nearly universal comorbidity in CKD 3a; when documented as causal, sequence I12.- or I13.- before N18.31
- Proteinuria/albuminuria: Code with R80.- codes (e.g., R80.1 microalbuminuria, R80.2 macroalbuminuria) when separately documented
Coding Manifestations
Always code documented manifestations to fully capture patient complexity. Key manifestation codes to consider alongside N18.31:
- D63.1 — Anemia in chronic kidney disease
- E87.2 — Acidosis
- E21.1 — Secondary hyperparathyroidism, not elsewhere classified
- R80.1 or R80.2 — Albuminuria, when documented
- Z94.0 — Kidney transplant status, if applicable
💰 HCC Risk Adjustment (CMS-HCC v28)
| Field | Detail |
|---|---|
| CMS-HCC Model Version | v28 (Fully operative as of January 1, 2026) |
| HCC Assignment | ❌ Not HCC-Mapped (PY2026) |
| HCC Category | N/A — CKD stages 1-3 carry no payment HCC under v28 |
| Prior v24 Mapping | Previously mapped to HCC 138 (CKD, Stage 4) under v24 — no longer applicable |
| RAF Coefficient | N/A — No current RAF contribution under v28 |
N18.31 does not map to a payment HCC under CMS-HCC v28 and therefore contributes no RAF coefficient to the patient’s risk score.4
Watch for 2027 Model Update — CKD 3a Is on CMS's Radar
The CMS 2027 Advance Notice (February 2026) proposes a 50.4% coefficient reduction for CKD Stage 3a — which implies CMS is considering assigning a small coefficient to this substage in PY2027. This is not confirmed, but coders and CDI specialists should monitor the final CMS 2027 Announcement. More urgently: if the patient’s GFR trends into the 15-29 range, upgrade to N18.4, which maps to HCC 330 under v28 and carries meaningful RAF weight (~0.289 community, non-dual, aged). Never miss a stage 4 upgrade.4,5
🏥 MS-DRG Assignment
MDC 11 — Diseases and Disorders of the Kidney & Urinary Tract
| DRG | Title | Est. Relative Weight* |
|---|---|---|
| DRG 682 | Renal Failure with MCC | ~2.68 |
| DRG 683 | Renal Failure with CC | ~1.48 |
| DRG 684 | Renal Failure without CC/MCC | ~0.97 |
Approximate. Verify against IPPS FY2026 Final Rule Table 5.
Sequencing and DRG Impact
When N18.31 is the principal diagnosis, the case groups to the DRG 682/683/684 triplet under MDC 11 — identical to N18.30 and N18.32. N18.31 itself carries no CC or MCC designation as a secondary diagnosis and will not independently raise the DRG tier of a different principal. To achieve DRG 682 (with MCC) or 683 (with CC), qualifying secondary diagnoses must be present and documented. Document all associated comorbidities — anemia of CKD (D63.1), metabolic acidosis (E87.2), hyperkalemia (E87.5), or fluid overload — as these may carry CC or MCC status and change the DRG assignment.
🔗 Related ICD-10-CM Codes
CKD Stage Progression Variants
| Code | Description |
|---|---|
| N18.1 | Chronic kidney disease, stage 1 |
| N18.2 | Chronic kidney disease, stage 2 (mild) |
| N18.30 | Chronic kidney disease, stage 3 unspecified |
| N18.31 | Chronic kidney disease, stage 3a ← This Code |
| N18.32 | Chronic kidney disease, stage 3b |
| N18.4 | Chronic kidney disease, stage 4 (severe) — HCC 330 under v28 |
| N18.5 | Chronic kidney disease, stage 5 — HCC 329 under v28 |
| N18.6 | End stage renal disease — HCC 329 under v28 |
| N18.9 | Chronic kidney disease, unspecified |
CKD With Underlying Etiology — Code First Codes
| Code | Description |
|---|---|
| I12.9 | Hypertensive chronic kidney disease with stage 1-4 CKD, or unspecified CKD |
| I13.10 | Hypertensive heart and chronic kidney disease without heart failure, with stage 1-4 or unspecified CKD |
| E11.22 | Type 2 diabetes mellitus with diabetic chronic kidney disease |
| E10.22 | Type 1 diabetes mellitus with diabetic chronic kidney disease |
🛠️ Commonly Associated CPT Codes (Nephrology / Inpatient)
Outpatient and Inpatient Setting Context
CKD stage 3a is most frequently managed in the nephrology and primary care outpatient setting with periodic labs and medication adjustment. Inpatient encounters arise when a complication — acute decompensation, volume overload, or electrolyte emergency — precipitates admission. The CPT codes below reflect services commonly billed in association with this diagnosis across both settings.
| CPT Code | Description | Profee Coding Notes |
|---|---|---|
| 99213 | Office/outpatient E/M, established patient, moderate complexity | Most common outpatient E/M level for CKD 3a monitoring; document MDM elements or total time |
| 99214 | Office/outpatient E/M, established patient, moderate-high complexity | Appropriate when labs show progression concerns or medication changes require complex decision-making |
| 50200 | Renal biopsy, percutaneous, needle | Percutaneous approach to confirm CKD etiology; requires Modifier -26 for interpretation-only profee billing |
| 90935 | Hemodialysis procedure, single evaluation by physician | If CKD 3a decompensates acutely and HD is initiated; per-session professional fee billing |
| 90945 | Dialysis procedure (non-ESRD), single evaluation | For acute non-ESRD dialysis if electrolyte or fluid emergency warrants intervention |
| 36800 | Insertion of cannula for hemodialysis | Vascular access planning in the setting of anticipated disease progression |
NCCI Bundling Considerations
- E/M codes (99213, 99214) billed on the same date as a procedure (e.g., renal biopsy 50200) require Modifier [[-25]] on the E/M to confirm a separately identifiable evaluation and management service supported by the documentation.
- Renal biopsy (50200) and ultrasound imaging guidance (76942) — imaging guidance is typically separately reportable, but verify payer-specific bundling policies before billing both.
🔬 ICD-10-PCS Crosswalk (Inpatient Procedures)
When N18.31 is an inpatient diagnosis, these PCS codes are relevant for associated inpatient procedures.
| PCS Section | Body System | Root Operation | Clinical Application |
|---|---|---|---|
| 5 (Extracorporeal or Systemic Assistance and Performance) | A (Physiological Systems) | 1 (Performance) | Hemodialysis: 5A1D00Z (single) or 5A1D60Z (multiple) — for inpatient HD initiated due to acute-on-chronic decompensation of CKD 3a |
| 0 (Medical and Surgical) | T (Urinary System) | B (Excision) | Percutaneous renal biopsy: 0TB03ZX — diagnostic to confirm CKD etiology (e.g., IgA nephropathy, focal segmental glomerulosclerosis) |
| 0 (Medical and Surgical) | T (Urinary System) | Y (Transplantation) | Kidney transplant, allogeneic, open approach: 0TY00Z0 (right) / 0TY10Z0 (left) — for patients progressing toward ESRD with transplant planning initiated at stage 3a |
💊 Coding Scenarios and Examples
Scenario 1 — Outpatient/Profee: Nephrology Follow-Up, CKD 3a With Diabetic Etiology
Clinical Vignette: A 61-year-old female with a 14-year history of type 2 diabetes and hypertension presents for her quarterly nephrology visit. Her most recent eGFR is 52 mL/min/1.73 m², unchanged from the prior visit. Microalbuminuria is documented at 80 mg/g creatinine. The nephrologist documents “CKD stage 3a due to diabetic nephropathy, stable, continue ACE inhibitor and dietary sodium restriction.” No acute findings are noted.
CPT (Profee):
- 99213 — Office/outpatient E/M, established patient (Document MDM: moderate complexity — chronic condition with stable status, medication review, lab interpretation)
ICD-10-CM Diagnoses:
- E11.22 — Type 2 diabetes mellitus with diabetic chronic kidney disease (Code first — diabetic etiology documented; ICD-10-CM tabular “Code First” instruction applies)
- N18.31 — Chronic kidney disease, stage 3a (Sequenced after E11.22 per tabular instruction)
- R80.1 — Microalbuminuria (Documented separately — separately reportable)
- I10 — Essential (primary) hypertension (Documented comorbidity; note: if hypertensive CKD relationship is documented as causal, replace I10 with I12.9 or I13.- + N18.31)
Scenario 2 — Inpatient: CKD 3a With Fluid Overload and Hyponatremia
Clinical Vignette: A 74-year-old male with known CKD stage 3a (eGFR 48) and heart failure with reduced ejection fraction is admitted for acute volume overload. Lab values on admission show serum sodium of 129 mEq/L. The attending documents “acute decompensated heart failure, CKD stage 3a contributing to diuretic resistance.” Inpatient diuresis is initiated.
Principal Diagnosis:
- I50.31 — Acute on chronic diastolic (congestive) heart failure (Reason for admission — principal diagnosis)
Secondary Diagnoses:
- N18.31 — Chronic kidney disease, stage 3a (Documented contributing factor — sequenced secondarily; does not carry CC/MCC status alone)
- E87.1 — Hypo-osmolality and hyponatremia (Documented lab value — may function as CC depending on grouper)
- I12.9 — Hypertensive chronic kidney disease with stage 1-4 CKD (If hypertension also documented as causal for CKD — code I12.9 and retain N18.31 per “use additional code” instruction at I12)
MS-DRG Assignment: Principal heart failure diagnosis routes to MDC 05 (Circulatory System); DRG tier determined by whether secondary diagnoses qualify as MCC or CC. N18.31 alone does not elevate the tier.
Scenario 3 — CDI Query: Upgrading from N18.30 to N18.31
Clinical Vignette: Inpatient discharge documentation reads: “Chronic kidney disease, stage 3, moderate, secondary to hypertensive nephrosclerosis.” Lab flowsheet from the admission shows eGFR values of 54, 56, and 51 on consecutive days. No substage is specified in any note. The coder has assigned N18.30 but suspects the eGFR range supports stage 3a.
Action / Outcome: The documented eGFR values of 51-56 mL/min/1.73 m² fall squarely in the stage 3a range (45-59). The coder cannot independently upgrade to N18.31 — a completed CDI query is required. Query recommended: “The patient’s eGFR values during this admission ranged from 51 to 56 mL/min/1.73 m², which corresponds to CKD stage 3a (eGFR 45-59). Can you confirm whether the patient’s CKD is stage 3a or further specify the substage to support accurate ICD-10-CM code assignment?”
Query Response: Provider addends: “CKD stage 3a confirmed based on eGFR 51-56.”
Corrected ICD-10-CM Coding:
- I12.9 — Hypertensive chronic kidney disease with stage 1-4 CKD (Code first — hypertensive CKD documented)
- N18.31 — Chronic kidney disease, stage 3a (Upgraded from N18.30 following CDI clarification)
⚠️ Coding Pitfalls and Tips
| Pitfall or Tip | |
|---|---|
| ❌ | Using N18.30 when GFR supports 3a. When eGFR is documented or available in the 45-59 range, N18.30 is less specific than N18.31. Always pursue a CDI query or verify provider documentation before defaulting to the unspecified substage code. |
| ❌ | Assigning N18.31 without provider documentation. Even when lab data clearly supports stage 3a, coders cannot independently assign a substage — the provider must document or confirm it through a CDI query response. |
| ❌ | Coding N18.9 alongside N18.31. N18.9 is an Excludes 1 condition when a stage is documented. Do not report both codes; N18.31 replaces N18.9 entirely. |
| ❌ | Forgetting the “Code First” rule. When CKD is attributed to diabetes (E11.22, etc.) or hypertension (I12.-, I13.-), the combination or etiology code must be sequenced before N18.31. Coding N18.31 as principal in those cases is a sequencing error. |
| ✅ | Code all documented manifestations. Anemia of CKD (D63.1), secondary hyperparathyroidism (E21.1), metabolic acidosis (E87.2), and albuminuria (R80.1/R80.2) are separately reportable when documented — each adds clinical complexity and may carry CC status. |
| ✅ | Watch GFR trends at every encounter. If eGFR is falling toward or into the 30-44 range, flag for a CDI query to confirm progression to stage 3b (N18.32). If eGFR drops below 30, query for stage 4 (N18.4) — which maps to HCC 330 under CMS-HCC v28 and significantly impacts the RAF score. |
| ✅ | Monitor the 2027 CMS Advance Notice. The proposed 50.4% coefficient adjustment for CKD stage 3a in the 2027 model suggests CMS may assign a small payment HCC to N18.31 in future years. Annual capture and substage documentation now positions the record for that potential change. |
📚 Sources
- CMS/NCHS. ICD-10-CM Official Guidelines for Coding and Reporting, FY2026. Section I.C.14.a — Chronic kidney disease (CKD). cms.gov
- National Kidney Foundation. Stage 3a Chronic Kidney Disease (CKD). kidney.org/kidney-topics/stage-3a-chronic-kidney-disease-ckd. Reviewed May 2026.
- BCBS Illinois. Chronic Kidney Disease Coding Reference Guide. bcbsil.com — CKD stage eGFR classification table.
- HCC Buddy. Chronic Kidney Disease HCC Coding Guide — CMS-HCC V28 ICD-10 Mapping (Updated April 25, 2026). hccbuddy.com/conditions/chronic-kidney-disease
- RAAPID / Clayton W. CMS-HCC Model V28: Full List of Chronic Conditions & Key Updates. raapidinc.com/blogs/cms-hcc-model-v28 (Updated March 2026). (Source for 2027 Advance Notice CKD 3a coefficient reduction proposal of −50.4%.)
- CMS. IPPS Final Rule FY2026 — MS-DRG Definitions Manual v43.1. MDC 11 logic tables; DRG 682/683/684 Renal Failure triplet.
- AMA. CPT Professional Edition 2026. Surgery / Urinary System and Evaluation & Management sections.