N28.81 - Hypertrophy of Kidney

Short Description

N28.81: Use for hypertrophy (enlargement) of the kidney - most commonly seen as compensatory hypertrophy when one kidney enlarges to compensate for reduced function or absence of the contralateral kidney (e.g., solitary kidney, congenital absence, post-nephrectomy, or contralateral kidney dysfunction). This represents adaptive renal enlargement, not pathological mass or tumor.

Full Description & Clinical Context

N28.81 - Hypertrophy of kidney describes abnormal enlargement of the kidney, most commonly representing compensatory hypertrophy where a functioning kidney increases in size to maintain adequate renal function when the opposite kidney is absent, dysfunctional, or hypoplastic.

Key Clinical Features:

Compensatory hypertrophy - most common etiology (single functioning kidney)
Adaptive response - kidney enlarges to increase nephron workload capacity
Benign condition - not a tumor or malignancy
Usually asymptomatic - often incidental finding on imaging
Functional significance - enlarged kidney maintains GFR despite loss of contralateral function

Common causes of compensatory hypertrophy:

Unilateral nephrectomy - surgical removal of opposite kidney (cancer, donation, trauma)
Congenital solitary kidney - born with one kidney
Congenital renal hypoplasia - opposite kidney underdeveloped/small
Congenital renal agenesis - opposite kidney absent
Severe contralateral kidney disease - opposite kidney non-functional (chronic disease, obstruction)
Post-traumatic loss - opposite kidney injured/removed

Pathophysiology:

Loss of one kidney → remaining kidney compensates
Increased nephron size (hypertrophy) rather than number (hyperplasia)
Single kidney typically increases 40-50% in size
Increased glomerular filtration per nephron
Usually maintains 70-80% of normal bilateral kidney function
Adaptation occurs over weeks to months

Clinical significance:

Generally well-tolerated with single hypertrophied kidney
Important to protect remaining kidney function
Monitor for proteinuria, hypertension (can develop with single kidney)
Avoid nephrotoxic agents
Counsel on hydration, BP control, lifestyle modifications

Code Details

Code set: ICD-10-CM
Full code: N28.81
Title: Hypertrophy of kidney
Code type: Billable/specific diagnosis code
Clinical category: Other disorders of kidney and ureter, not elsewhere classified
Parent code: N28.8 (Other specified disorders of kidney and ureter)
Grandparent code: N28 (Other disorders of kidney and ureter, not elsewhere classified)

Coding notes from N28.8 (parent):

Excludes 1:
- Hydroureter → N13.4
- Ureteric stricture with hydronephrosis → N13.1
- Ureteric stricture without hydronephrosis → N13.5

Important note on laterality:

N28.81 does NOT specify laterality (right vs left)
Code describes hypertrophy of “kidney” (singular or in general)
No separate codes for right vs left kidney hypertrophy
Documentation should specify which kidney is hypertrophied, but code remains N28.81

Related code note:

R93.422 (Abnormal radiologic findings on diagnostic imaging of left kidney) has Excludes2 for hypertrophy of kidney (N28.81)
If imaging shows kidney enlargement, code the hypertrophy (N28.81), not just abnormal imaging

Code	Description	Key Difference
N28.81	Hypertrophy of kidney	Enlarged kidney (compensatory) ← YOU ARE HERE
N27.0	Small kidney, unilateral	Shrunken/small kidney (one side)
N27.1	Small kidney, bilateral	Shrunken/small kidneys (both)
N27.9	Small kidney, unspecified	Small kidney, side not specified
Q60.0	Renal agenesis, unilateral	Congenital absence of kidney
Q60.3	Renal hypoplasia, unilateral	Congenital underdeveloped kidney
Z90.5	Acquired absence of kidney	History of nephrectomy (status code)
N28.1	Cyst of kidney, acquired	Acquired kidney cyst
Q61.x	Cystic kidney disease	Congenital cystic disease

Clinical distinction:

N28.81 = Enlarged kidney (typically compensatory)
N27.x = Small/shrunken kidney
These can coexist (one kidney small, other hypertrophied)

Complete N28 Family - Other Kidney/Ureter Disorders

Code	Description	Key Feature
N28.0	Ischemia and infarction of kidney	Blood flow compromise
N28.1	Cyst of kidney, acquired	Acquired kidney cyst
N28.8-	Other specified disorders	Parent category
N28.81	Hypertrophy of kidney	Kidney enlargement ← YOU ARE HERE
N28.82	Megaloureter	Dilated ureter
N28.83	Nephroptosis	Dropped/mobile kidney
N28.84	Pyelitis	Renal pelvis inflammation
N28.85	Pyeloureteritis	Renal pelvis + ureter inflammation
N28.86	Ureteritis	Ureter inflammation
N28.89	Other specified disorders	Other
N28.9	Disorder of kidney and ureter, unspecified	Unspecified

When to Use N28.81

Use N28.81 ONLY when:

Kidney hypertrophy documented:
- “Hypertrophy of kidney” OR
- “Enlarged kidney” OR
- “Compensatory hypertrophy of kidney” OR
- Imaging reports kidney larger than normal
Benign enlargement:
- Not a tumor or mass (would use neoplasm codes)
- Not hydronephrosis (fluid-filled, would be N13.x)
- Not polycystic kidney disease (would be Q61.x)
Appropriate clinical context:
- Solitary kidney (congenital or acquired)
- Contralateral kidney absent, small, or dysfunctional
- Compensatory response documented
Not excluded conditions:
- Not hydroureter (N13.4)
- Not ureteric stricture with/without hydronephrosis (N13.1, N13.5)

Typical scenarios for N28.81:

“Status post left nephrectomy 5 years ago; imaging shows compensatory hypertrophy of right kidney”
“Congenital solitary left kidney with compensatory hypertrophy”
“CT abdomen: Right kidney measures 14 cm (enlarged); left kidney congenitally absent; diagnosis: Compensatory hypertrophy of right kidney”
“Born with unilateral renal agenesis; hypertrophy of remaining kidney”

When NOT to Use N28.81

Do NOT use N28.81 when:

Scenario	Use Instead	Why
Kidney tumor/mass	C64.x (malignant) or D30.0x (benign)	Neoplasm, not hypertrophy
Hydronephrosis	N13.x	Fluid distention, not true hypertrophy
Polycystic kidney disease	Q61.x	Cystic disease, not compensatory hypertrophy
Normal-sized kidney	No code needed	No pathology
Status post nephrectomy (alone)	Z90.5	Status code, not hypertrophy unless documented
Renal cell carcinoma causing enlargement	C64.x	Malignancy primary
Acute kidney injury with swelling	N17.x	Acute injury, not chronic hypertrophy

Critical distinction:

Hypertrophy = adaptive, benign enlargement (N28.81)
Mass/tumor = neoplastic growth (C64.x or D30.x)
Hydronephrosis = fluid-filled dilation (N13.x)

Documentation Requirements for N28.81

MINIMUM documentation needed to assign N28.81:

✅ MUST include:

“Hypertrophy of kidney” OR “enlarged kidney” documented:
- Provider diagnosis or imaging report
- Kidney size documented as enlarged
Clinical context supporting hypertrophy:
- Solitary kidney (congenital or acquired)
- Contralateral kidney absent, small, or dysfunctional
- Compensatory mechanism documented or implied
Not a tumor/mass:
- Benign enlargement, not neoplastic
- May need to explicitly state “no mass” if imaging done

❌ CANNOT use if:

Only “abnormal imaging” without hypertrophy specified
Kidney tumor/mass present (code the neoplasm)
Hydronephrosis causing enlargement (code hydronephrosis)
Normal kidney size

✅ SHOULD document (best practice):

Which kidney is hypertrophied (right vs left)
Kidney dimensions/size on imaging
Reason for hypertrophy (compensatory to absent/dysfunctional contralateral kidney)
Status of contralateral kidney (absent, small, dysfunctional)
Renal function (GFR, creatinine) - shows functional compensation
Proteinuria status (monitor for hyperfiltration injury)
Blood pressure (single kidney at risk for HTN)
Underlying cause (nephrectomy, congenital agenesis, trauma, etc.)

Clinical Evaluation

Typical workup when N28.81 suspected or diagnosed:

History:

Prior nephrectomy (cancer, donation, trauma)?
Known congenital kidney abnormality?
History of kidney disease, obstruction, infection?
Family history of kidney anomalies?
Symptoms (usually none, but assess flank pain, hematuria)?

Physical examination:

Blood pressure (hypertension screening)
Abdominal exam (palpable kidney if very enlarged)
Volume status assessment

Diagnostic studies:

Imaging (confirms hypertrophy):

Renal ultrasound:
- Measures kidney length (normal adult: 9-13 cm)
- Enlarged if >13-14 cm
- Assesses echogenicity, masses, hydronephrosis
- Evaluates contralateral kidney (if present)
CT abdomen/pelvis with or without contrast:
- Precise kidney measurements
- Rules out mass, hydronephrosis, obstruction
- Assesses vascular anatomy
- Identifies cause (absent/small contralateral kidney)
MRI abdomen:
- Alternative to CT (no radiation, good soft tissue detail)
- Useful if contrast-enhanced CT contraindicated

Laboratory (assesses renal function):

BMP/CMP:
- Creatinine (renal function)
- eGFR (should be maintained with compensatory hypertrophy)
- Electrolytes
Urinalysis:
- Proteinuria (monitor for hyperfiltration injury)
- Hematuria
24-hour urine collection:
- Creatinine clearance
- Protein quantification (if proteinuria present)

Follow-up monitoring (for single kidney):

Annual or biannual renal function checks
BP monitoring (risk of hypertension)
Urinalysis for proteinuria
Imaging if symptoms develop

Management & Monitoring

Management of compensatory kidney hypertrophy (N28.81):

Generally no specific treatment needed - focus on kidney protection:

1. Blood pressure control:

Target <130/80 mmHg (or per guidelines)
ACE inhibitors or ARBs if HTN or proteinuria present
Single kidney at increased risk for hypertension

2. Proteinuria management:

If proteinuria develops → ACE-I/ARB therapy
Monitor for hyperfiltration injury (single kidney working harder)
Target urine protein <500 mg/day

3. Avoid nephrotoxins:

NSAIDs (minimize use)
Aminoglycosides
Contrast agents (pre-hydrate, use lowest dose)
Nephrotoxic drugs when possible

4. Lifestyle modifications:

Adequate hydration
Moderate protein intake (not excessive high-protein diet)
Avoid contact sports if solitary kidney (protect from trauma)
Weight management, healthy diet

5. Infection prevention:

Prompt treatment of UTIs
Good hygiene practices

6. Regular monitoring:

Annual renal function (Cr, eGFR)
Annual urinalysis (proteinuria, hematuria)
BP checks
Imaging if symptoms or function decline

7. Patient education:

Inform about solitary kidney status
Medical alert bracelet recommended
Inform all healthcare providers
Avoid nephrotoxic medications
Hydrate before contrast studies

Prognosis:

Most patients with single hypertrophied kidney do well
Life expectancy generally normal with proper care
Maintain 70-80% of normal bilateral kidney function
Long-term risk of proteinuria, hypertension, CKD (20-30% over decades)
Regular monitoring essential

Special Considerations

Compensatory hypertrophy by cause:

Post-nephrectomy (kidney cancer, donation):

Code N28.81 (hypertrophy) + Z90.5 (acquired absence of kidney)
Also code reason for nephrectomy if relevant (e.g., C64.x history if cancer)

Congenital solitary kidney:

Code N28.81 (hypertrophy) + Q60.0 (renal agenesis, unilateral)
Higher risk of hypertension, proteinuria long-term

Contralateral renal hypoplasia:

Code N28.81 (hypertrophy of normal kidney)
Code Q60.3 (renal hypoplasia) for small kidney
Example: “Hypertrophy of left kidney due to congenital hypoplasia of right kidney”

Kidney donors:

Pre-donation: two normal kidneys
Post-donation: remaining kidney hypertrophies
Code Z90.5 (acquired absence of kidney) primary
Add N28.81 if hypertrophy documented

HCC Information

N28.81 does NOT map to a CMS-HCC - kidney hypertrophy is not a risk-adjusted condition
No direct HCC weight or RAF score impact
Used for accurate diagnosis documentation and monitoring

Note: If kidney hypertrophy is associated with chronic kidney disease (CKD), the CKD code (N18.x) may have HCC implications, but N28.81 itself does not.

RVU / wRVU Information

ICD-10-CM codes (including N28.81) do NOT carry RVUs or wRVUs
RVUs apply to CPT/HCPCS procedure codes only
N28.81 supports medical necessity for:
- Renal imaging (ultrasound, CT, MRI)
- Renal function testing
- Follow-up monitoring
- Nephrology consultations

Common CPT Procedure Pairings with N28.81

E/M Services:

99202-99205 - Office visit, new patient
99211-99215 - Office visit, established patient
99242-99245 - Office consultation
99291-99292 - Critical care (if acute complication)

Imaging (most common):

76700 - Ultrasound, abdominal, real time with image documentation; complete
76705 - Ultrasound, abdominal, real time with image documentation; limited
76770 - Ultrasound, retroperitoneal (includes renal), real time with image documentation; complete
76775 - Ultrasound, retroperitoneal, real time with image documentation; limited
74150 - CT abdomen without contrast
74160 - CT abdomen with contrast
74170 - CT abdomen without contrast, followed by contrast
74176 - CT abdomen and pelvis without contrast
74177 - CT abdomen and pelvis with contrast
74178 - CT abdomen and pelvis without contrast, followed by contrast
74181 - MRI abdomen without contrast
74182 - MRI abdomen with contrast
74183 - MRI abdomen without contrast, followed by contrast

Nuclear Medicine:

78700 - Kidney imaging morphology
78707 - Kidney imaging morphology with vascular flow
78708 - Kidney function study

Laboratory:

80047 - Basic metabolic panel (BMP)
80053 - Comprehensive metabolic panel (CMP)
82565 - Creatinine, blood
82570 - Creatinine clearance
84155 - Protein, total, serum
84156 - Protein, total, urine

Common Associated ICD-10-CM Codes

Often coded WITH N28.81:

Underlying causes/associated conditions:

Z90.5 - Acquired absence of kidney (post-nephrectomy)[web:280]
Q60.0 - Renal agenesis, unilateral (congenital absence)
Q60.3 - Renal hypoplasia, unilateral (congenital small kidney)[web:280]
N27.0 - Small kidney, unilateral (opposite kidney)[web:280]
Z94.0 - Kidney transplant status (if transplant recipient)

Complications/monitoring:

I10 - Essential hypertension (common with single kidney)
N18.x - Chronic kidney disease (if develops)
R80.x - Proteinuria (monitor for hyperfiltration)
R31.x - Hematuria (if present)

History codes:

Z85.528 - Personal history of malignant neoplasm of kidney
Z52.4 - Kidney donor (if living donor)

Reason for nephrectomy (if applicable):

C64.x - Malignant neoplasm of kidney (if cancer was reason)
S37.0xx - Injury of kidney (if trauma)
N20.0 - Calculus of kidney (if stone disease)

Clinical Examples: When to Use N28.81

✅ Example 1 - Post-Nephrectomy Compensatory Hypertrophy

SCENARIO:
62-year-old male, 5 years post left nephrectomy for renal cell carcinoma.
Annual surveillance imaging.

Imaging:
- CT abdomen: Right kidney measures 14 cm (enlarged for adult)
- No masses or hydronephrosis
- Left nephrectomy bed noted, no recurrence

Lab:
- Creatinine 1.1 mg/dL (normal)
- eGFR 68 mL/min/1.73m² (adequate for single kidney)
- Urinalysis: Trace protein, no hematuria

Assessment:
- Compensatory hypertrophy of right kidney
- History of left renal cell carcinoma, status post nephrectomy
- Stable renal function

Plan:
- Continue annual surveillance
- Monitor BP and renal function

CODES:
- N28.81 (Hypertrophy of kidney) - PRIMARY
- Z90.5 (Acquired absence of kidney)
- Z85.528 (Personal history of kidney cancer)
- Monitor with Z08 (encounter for follow-up after cancer treatment)

RATIONALE:
├─ Right kidney enlarged on imaging (compensatory)
├─ Left kidney surgically removed
├─ N28.81 appropriate for documented hypertrophy
├─ Z90.5 documents acquired absence
└─ Z85.528 captures cancer history

✅ Example 2 - Congenital Solitary Kidney with Hypertrophy

SCENARIO:
28-year-old female with known congenital absence of right kidney.
Routine nephrology follow-up.

History:
- Born with unilateral renal agenesis (right)
- Diagnosed in childhood
- No history of UTIs or kidney problems

Exam:
- BP 128/82 (borderline elevated)
- Otherwise normal

Imaging (renal ultrasound):
- Left kidney measures 13.5 cm (enlarged)
- Normal echotexture, no masses
- No hydronephrosis
- Right renal fossa empty (agenesis)

Lab:
- Creatinine 0.9 mg/dL
- eGFR 85 mL/min/1.73m²
- Urinalysis: Normal, no proteinuria

Assessment:
- Congenital solitary left kidney with compensatory hypertrophy
- Borderline hypertension
- Stable renal function

Plan:
- Lifestyle modifications for BP
- Monitor BP and renal function annually
- Repeat urinalysis in 6 months (screen for proteinuria)
- Counsel on kidney protection

CODES:
- N28.81 (Hypertrophy of kidney)
- Q60.0 (Renal agenesis, unilateral)
- I10 (Hypertension) if BP criteria met

RATIONALE:
├─ Left kidney enlarged (compensatory)
├─ Congenital absence of right kidney
├─ N28.81 documents hypertrophy
├─ Q60.0 documents congenital cause
└─ Important to monitor for complications

✅ Example 3 - Living Kidney Donor Follow-Up

SCENARIO:
45-year-old male, 2 years post living kidney donation.
Donor follow-up clinic.

History:
- Donated left kidney to sibling 2 years ago
- Uncomplicated donation and recovery

Imaging (renal ultrasound):
- Right kidney measures 13.2 cm (enlarged from baseline 11 cm)
- Normal echotexture, no masses or hydronephrosis
- Left nephrectomy site unremarkable

Lab:
- Creatinine 1.2 mg/dL (stable)
- eGFR 65 mL/min/1.73m²
- Urinalysis: Negative

Assessment:
- Compensatory hypertrophy of right kidney post left nephrectomy
- History of living kidney donation
- Excellent renal function for single kidney

Plan:
- Continue annual monitoring
- BP management
- Kidney protection counseling

CODES:
- Z90.5 (Acquired absence of kidney) - PRIMARY
- N28.81 (Hypertrophy of kidney) - if documented
- Z52.4 (Kidney donor)

RATIONALE:
├─ Kidney removed for donation
├─ Right kidney compensating well
├─ Z90.5 primary status code
├─ N28.81 if hypertrophy explicitly documented
└─ Z52.4 documents donor status

❌ Example 4 - WRONG: Renal Cell Carcinoma, Not Hypertrophy

SCENARIO:
Patient with 8 cm renal mass on CT.

Imaging:
- CT: 8 cm heterogeneous mass in left kidney
- Kidney appears enlarged due to mass effect
- Biopsy: Renal cell carcinoma

WRONG CODE: N28.81 (hypertrophy)
CORRECT CODE: C64.2 (Malignant neoplasm of left kidney)

WHY:
├─ Enlargement due to TUMOR, not hypertrophy
├─ N28.81 is for benign compensatory enlargement
├─ Neoplasm takes precedence
└─ Code the malignancy, not "hypertrophy"

KEY POINT: Tumor ≠ hypertrophy!

❌ Example 5 - WRONG: Hydronephrosis, Not Hypertrophy

SCENARIO:
Patient with kidney obstruction.

Imaging:
- Ultrasound: Right kidney measures 15 cm
- Severe hydronephrosis (fluid-filled collecting system)
- Dilated renal pelvis and calyces
- Ureteral obstruction from stone

WRONG CODE: N28.81 (hypertrophy)
CORRECT CODE: N13.0 or N13.2 (Hydronephrosis with obstruction)

WHY:
├─ Kidney enlarged due to FLUID (hydronephrosis)
├─ NOT true tissue hypertrophy
├─ N13.x codes for hydronephrosis
├─ Obstruction is the problem, not compensatory growth
└─ Different pathophysiology entirely

KEY POINT: Fluid dilation ≠ hypertrophy!

❌ Example 6 - WRONG: Just Status Post Nephrectomy

SCENARIO:
Patient 1 year post nephrectomy for cancer.
No recent imaging documented.

Documentation:
"Status post left nephrectomy for RCC"
No mention of remaining kidney size or hypertrophy.

WRONG: Automatically coding N28.81
CORRECT: Z90.5 (Acquired absence of kidney)
         Z85.528 (History of kidney cancer)

WHY:
├─ No documentation of hypertrophy
├─ Cannot assume hypertrophy without documentation
├─ Status code (Z90.5) appropriate for post-nephrectomy
├─ N28.81 requires explicit documentation of enlargement
└─ Don't code what's not documented!

BEST PRACTICE: Query provider or wait for imaging that documents size

Common Documentation Errors to AVOID

❌ Error 1: Coding Without Documented Hypertrophy

WRONG: Coding N28.81 for solitary kidney without documented enlargement
├─ Solitary kidney doesn't automatically = hypertrophy
├─ Need imaging or documentation of enlarged kidney
└─ CORRECT: Need "hypertrophy" or enlarged size documented

✅ CORRECT: Only code N28.81 when hypertrophy documented

❌ Error 2: Confusing Hypertrophy with Tumor

WRONG: Using N28.81 for kidney mass/tumor
├─ Tumor = neoplasm (C64.x or D30.x)
├─ Hypertrophy = adaptive benign enlargement
└─ CORRECT: Code neoplasm, not hypertrophy

✅ CORRECT: Hypertrophy = no mass; Tumor = code as neoplasm

❌ Error 3: Confusing Hypertrophy with Hydronephrosis

WRONG: Using N28.81 for fluid-distended kidney
├─ Hydronephrosis = fluid dilation (N13.x)
├─ Hypertrophy = tissue enlargement
└─ CORRECT: Hydronephrosis codes for fluid obstruction

✅ CORRECT: Check if enlargement is tissue or fluid

❌ Error 4: Not Documenting Which Kidney

WRONG: Documentation only says "kidney hypertrophy"
├─ Should specify right vs left kidney
├─ Helps clarify which kidney is absent/dysfunctional
└─ CORRECT: Document "right kidney hypertrophy" or "left kidney hypertrophy"

✅ CORRECT: Always specify laterality in documentation
(even though code doesn't differentiate)

❌ Error 5: Not Coding Underlying Cause

WRONG: Coding only N28.81 without Z90.5, Q60.0, etc.
├─ Should code why kidney is hypertrophied
├─ Tells complete clinical story
└─ CORRECT: N28.81 + underlying cause code

✅ CORRECT: Code both hypertrophy and underlying reason

Compliance Checklist

Before coding N28.81, verify:

Quick Reference Card

ICD-10-CM N28.81 - Hypertrophy of Kidney
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
USE WHEN:
• Kidney hypertrophy/enlargement documented
• Imaging shows enlarged kidney (>13-14 cm in adult)
• Compensatory hypertrophy (single functioning kidney)
• Benign adaptive enlargement (NOT tumor)
• Common causes: post-nephrectomy, congenital solitary kidney, 
  contralateral renal hypoplasia/agenesis
 
DON'T USE WHEN:
• Kidney tumor/mass → code neoplasm (C64.x, D30.x)
• Hydronephrosis (fluid-filled) → N13.x
• Polycystic kidney disease → Q61.x
• Normal kidney size (no hypertrophy documented)
 
CRITICAL DISTINCTIONS:
• Hypertrophy = adaptive tissue enlargement (benign)
• Tumor = neoplastic growth (malignant or benign neoplasm codes)
• Hydronephrosis = fluid dilation (obstruction codes)
 
NO LATERALITY:
• N28.81 doesn't specify right vs left
• Document which kidney in clinical notes
• Same code used regardless of laterality
 
COMPENSATORY HYPERTROPHY:
• Single kidney enlarges to maintain function
• Typically increases 40-50% in size
• Maintains 70-80% of bilateral kidney function
• Generally well-tolerated with proper care
 
COMMON PAIRINGS:
• Z90.5 (Acquired absence of kidney - post-nephrectomy)
• Q60.0 (Renal agenesis - congenital absence)
• Q60.3 (Renal hypoplasia - congenital small kidney)
• I10 (Hypertension - monitor with single kidney)
 
NOT HCC:
• No direct HCC mapping
• Used for diagnosis and monitoring documentation
 
MONITORING NEEDED:
• BP control (risk of hypertension)
• Renal function (annual Cr, eGFR)
• Proteinuria screening (hyperfiltration risk)
• Kidney protection (avoid nephrotoxins)
 
BOTTOM LINE:
N28.81 = kidney hypertrophy, typically compensatory when 
opposite kidney absent/dysfunctional. Benign adaptive 
enlargement, NOT tumor. Document size and underlying cause.

Last Updated: February 9, 2026
For coding reference only - always verify against the current ICD-10-CM, official guidelines, payer policies, and facility rules.
Key concept: N28.81 is for hypertrophy (enlargement) of the kidney, most commonly seen as compensatory hypertrophy when one kidney enlarges to maintain adequate renal function due to absence or dysfunction of the contralateral kidney. This represents adaptive, benign enlargement - NOT tumor, NOT hydronephrosis. The code does not specify laterality (right vs left), though clinical documentation should specify which kidney is hypertrophied. Commonly paired with Z90.5 (post-nephrectomy), Q60.0 (congenital agenesis), or Q60.3 (congenital hypoplasia) to document the underlying cause.
Monitoring is essential as single kidneys are at increased risk for hypertension, proteinuria, and progressive CKD.

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N28.81