4.1 Acute Kidney Injury (AKI) & Chronic Kidney Disease (CKD)

4.1 Acute Kidney Injury (AKI) & Chronic Kidney Disease (CKD)

Quick Summary: Renal disorders represent high-volume inpatient admissions and critical secondary comorbidities across medical and surgical MS-DRGs. On the AHIMA Certified Coding Specialist (CCS) examination, candidates must master the distinct classification of Acute Kidney Injury (category N17) versus Chronic Kidney Disease (category N18), navigate the assumed causal relationships in hypertensive renal and cardiorenal disease (categories I12 and I13), correctly capture dialysis and kidney transplant status, and apply rigorous clinical validation criteria (KDIGO/RIFLE) to withstand payer recovery audits.

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Acute Kidney Injury (AKI / Category N17)

Acute Kidney Injury (AKI)—formerly termed acute renal failure (ARF)—is characterized by an abrupt, rapid decline in renal filtration function occurring over hours to days, resulting in the retention of nitrogenous waste products (urea and creatinine) and dysregulation of extracellular volume and electrolytes.

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|                   ACUTE KIDNEY INJURY (N17) SPECTRUM                    |
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| N17.0: AKI with Acute Tubular Necrosis (ATN)                            |
| - Intrinsic ischemic, toxic, or nephrotoxic tubular epithelial injury   |
| - Includes: Acute tubular necrosis, renal tubular necrosis, ATN NOS     |
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| N17.1: AKI with Acute Cortical Necrosis                                 |
| - Ischemic necrosis of renal cortex (sepsis, obstetric shock, DIC)      |
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| N17.2: AKI with Acute Medullary Necrosis                                |
| - Necrosis of renal pyramids/papillae (sickle cell, analgesic abuse)    |
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| N17.8: Other Acute Kidney Injury                                        |
| - Other specified acute renal impairment patterns                       |
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| N17.9: Acute Kidney Injury, Unspecified                                 |
| - Acute renal failure NOS, acute kidney injury NOS (High-Value CC)      |
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1. Etiological Classification & ICD-10-CM Coding

• Prerenal Azotemia vs. Acute Renal Failure: Prerenal azotemia is caused by transient renal hypoperfusion (e.g., dehydration, hemorrhage, congestive heart failure, sepsis-induced vasodilation) without structural parenchymal damage. In ICD-10-CM, prerenal azotemia is coded to R79.89 (Other specified abnormal findings of blood chemistry) unless the physician documents acute kidney injury or acute renal failure (N17.9), in which case N17.9 is assigned.
• Acute Tubular Necrosis (ATN / N17.0): Represents sustained intrinsic renal damage resulting from prolonged ischemia (hypovolemic shock, sepsis, cross-clamping during aortic aneurysm repair) or direct nephrotoxic exposure (iodinated radiocontrast media, aminoglycosides, amphotericin B, cisplatin). When documented as "acute kidney injury with ATN" or "acute tubular necrosis," assign N17.0.
• Postrenal AKI (Obstructive Uropathy): Results from bilateral urinary tract outflow obstruction (or unilateral obstruction in a solitary kidney) caused by benign prostatic hyperplasia (N40.1 + N13.8), bilateral ureteral calculi (N20.1), or retroperitoneal malignancy. Code both N17.9 and the underlying obstructive condition (e.g., N13.8 Other obstructive and reflux uropathy).
• Nephrotoxic AKI & Adverse Effects: If AKI is caused by the therapeutic administration of a prescribed drug (e.g., IV contrast or NSAID), sequence N17.0 or N17.9 first, followed by the appropriate adverse effect code from Chapter 19 (e.g., T50.8X5A Adverse effect of diagnostic agents, initial encounter).

2. Clinical Validation & Diagnostic Criteria (KDIGO vs. RIFLE)

CMS severity depends on subtype and grouper version: in MS-DRG v43.1, N17.0–N17.2 are MCCs, while N17.8 and N17.9 are CCs, subject to exclusions. Coders assign the documented provider diagnosis under Guideline I.B.14; separate clinical-validation or CDI review may compare the record with recognized criteria such as KDIGO or RIFLE without independently diagnosing the patient:

[!CAUTION] Audit Denial Risk — Transient Creatinine Elevation: When a provider documents "AKI" in a dehydrated patient whose serum creatinine elevates from 1.0 to 1.3 mg/dL and normalizes within 12 hours after a single 500 mL IV fluid bolus without monitoring or renal consultation, auditors frequently issue clinical validation denials arguing the condition was transient prerenal azotemia rather than true AKI. Coders should look for sustained elevation, active fluid management, serial lab monitoring, or nephrology consultation.

Chronic Kidney Disease (CKD / Category N18) & Dialysis Status

Chronic Kidney Disease (CKD) represents progressive, irreversible loss of renal nephron architecture and excretory function over months to years. ICD-10-CM classifies CKD based on the clinical stages established by the National Kidney Foundation's Kidney Disease Outcomes Quality Initiative (KDOQI).

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|                        CKD STAGING (CATEGORY N18)                       |
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| N18.1: CKD, Stage 1 (Normal or High GFR >= 90 mL/min with kidney damage)|
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| N18.2: CKD, Stage 2 (Mild GFR Reduction: 60 - 89 mL/min)                |
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| N18.30: CKD, Stage 3 Unspecified (GFR: 30 - 59 mL/min)                  |
| - N18.31: CKD, Stage 3a (Mild-to-Moderate: GFR 45 - 59 mL/min)          |
| - N18.32: CKD, Stage 3b (Moderate-to-Severe: GFR 30 - 44 mL/min)        |
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| N18.4: CKD, Stage 4 (Severe GFR Reduction: 15 - 29 mL/min) [CC]         |
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| N18.5: CKD, Stage 5 (Kidney Failure: GFR < 15 mL/min, non-ESRD) [CC]    |
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| N18.6: End-Stage Renal Disease (ESRD / Chronic Dialysis Dependent) [CC] |
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| N18.9: Chronic Kidney Disease, Unspecified                              |
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1. CKD Staging Rules & End-Stage Renal Disease (ESRD / N18.6)

• GFR Staging Specificity: When the provider documents both the CKD stage and the underlying GFR measurement, the stage documented by the provider directs code selection. If there is a discrepancy between the stage and the GFR, the provider's documented stage governs code assignment.
• End-Stage Renal Disease (N18.6): Code N18.6 is assigned when the provider documents ESRD or when a patient with stage 5 CKD requires chronic, ongoing maintenance renal replacement therapy (hemodialysis or peritoneal dialysis).
• Mandatory Dialysis Status Code: Per ICD-10-CM Tabular List instructions under code N18.6, the coder must: "Use additional code to identify dialysis status (Z99.2)." Code Z99.2 (Dependence on renal dialysis) must always accompany N18.6 when the patient is on chronic maintenance dialysis.
• Acute Dialysis vs. Chronic Dependence: Code Z99.2 is reserved strictly for patients on chronic maintenance dialysis. Do not assign Z99.2 for temporary or emergency hemodialysis administered for acute kidney injury or drug toxicity.

2. CKD in Kidney Transplant Recipients

When a patient who has undergone a kidney transplant continues to suffer from chronic kidney disease:

1. Routine Post-Transplant CKD: Assign code Z94.0 (Kidney transplant status) and the appropriate CKD stage code (N18.1–N18.5, N18.9). Note that transplant patients may still have residual CKD due to baseline nephrosclerosis, calcineurin inhibitor toxicity, or chronic allograft wear without acute rejection.
2. Transplant Complication / Allograft Failure: If the provider documents acute or chronic kidney transplant failure, rejection, or allograft nephropathy, assign category T86.1- (Complications of transplanted kidney, e.g., T86.12 Kidney transplant failure or T86.11 Kidney transplant rejection) as the principal/primary code, followed by the specific CKD stage code (N18.4, N18.5, or N18.6). Do not assign Z94.0 when a T86.1- complication code is assigned for the same organ.

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Mandatory Combination Codes: Hypertensive Renal & Cardiorenal Disease

ICD-10-CM Section I.C.9.a establishes official presumed causal relationships connecting hypertension, heart failure, and chronic kidney disease.

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|            HYPERTENSIVE RENAL & CARDIORENAL COMBINATION CODES           |
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| HYPERTENSIVE CHRONIC KIDNEY DISEASE (CATEGORY I12)                      |
| - I12.0: Hypertensive CKD with Stage 5 CKD or ESRD                      |
|          (Requires secondary code N18.5 or N18.6 + Z99.2)               |
| - I12.9: Hypertensive CKD with Stage 1 - Stage 4 CKD, or Unspecified    |
|          (Requires secondary code N18.1 - N18.4, or N18.9)              |
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| HYPERTENSIVE HEART & CHRONIC KIDNEY DISEASE (CATEGORY I13)              |
| - I13.0: Hypertensive Heart & CKD with HF and Stage 1 - 4 CKD / Unspec  |
| - I13.10: Hypertensive Heart & CKD without HF, with Stage 1 - 4 / Unspec|
| - I13.11: Hypertensive Heart & CKD without HF, with Stage 5 CKD / ESRD  |
| - I13.2: Hypertensive Heart & CKD with HF and Stage 5 CKD / ESRD        |
|          (Requires secondary I50.- code + N18.5/N18.6 code + Z99.2)     |
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1. Hypertensive CKD (Category I12)

• Assumed Linkage Guideline (Section I.C.9.a.2): The classification presumes a cause-and-effect relationship between hypertension and chronic kidney disease. When both conditions are documented, the coder must assign a combination code from category I12, even if the clinician did not explicitly state "hypertensive nephropathy" or "CKD due to hypertension." The only exception is if the provider explicitly documents that the CKD is completely unrelated to hypertension (e.g., CKD due to polycystic kidney disease or glomerulonephritis).
• Mandatory Secondary Code: Category I12 requires an additional secondary code from category N18 to identify the specific stage of CKD.

2. Hypertensive Heart and Chronic Kidney Disease (Category I13)

• Triple Combination Requirement (Section I.C.9.a.3): When a patient has hypertension, heart disease/heart failure, and chronic kidney disease, category I13 is mandatory. Category I13 takes precedence over both category I11 (Hypertensive heart disease) and category I12 (Hypertensive CKD).
• Required Multi-Code Sequencing:
  1. Primary/Combination Code: Category I13 code (e.g., I13.2 Hypertensive heart and chronic kidney disease with heart failure and with stage 5 chronic kidney disease, or end stage renal disease).
  2. Heart Failure Code: Specific code from category I50 (e.g., I50.23 Acute on chronic systolic heart failure).
  3. CKD Stage Code: Specific code from category N18 (e.g., N18.6 End stage renal disease).
  4. Dialysis Status: Z99.2 (Dependence on renal dialysis) if applicable.

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Complex Syndromic Renal Presentations & Clinical Sequencing

1. Acute Pulmonary Edema in ESRD (Missed Dialysis)

When a patient with ESRD presents to the emergency department in acute respiratory distress with pulmonary edema following missed outpatient hemodialysis sessions:

• Sequencing Logic:
  • If the pulmonary edema is documented as acute heart failure in a hypertensive ESRD patient: Select the principal diagnosis after study under the applicable heart-failure and hypertensive CKD rules, then report the supported I13.2, specific I50.-, N18.6, and Z99.2 codes. Do not infer noncompliance merely from a missed session; if the provider documents it and the reason is known, add the current reason-specific code (for example, Z91.158 for another documented reason).
  • If the pulmonary edema is documented purely as acute fluid overload/hypervolemia secondary to missed dialysis without heart failure: Select the principal diagnosis after study from the provider-supported condition that occasioned the admission, which may be E87.70 (Fluid overload, unspecified) or J81.0 (Acute pulmonary edema) depending on the record, followed by N18.6 and Z99.2. Add a current reason-specific noncompliance code only when the provider documents noncompliance and its reason.

2. Hepatorenal Syndrome (K76.7)

• Clinical Mechanism: Development of acute renal failure in patients with advanced chronic liver disease, cirrhosis, or acute liver failure in the absence of an identifiable intrinsic renal cause, driven by intense renal cortical vasoconstriction from splanchnic arterial vasodilation.
• Coding & Sequencing Rule: Assign K76.7 for documented hepatorenal syndrome and follow its Code First instruction by sequencing the documented underlying liver disease before it. That relative order does not automatically establish the principal diagnosis; apply the UHDDS definition and all encounter-specific guidelines. Verify current CC/MCC status and exclusions in the applicable grouper.

3. Cardiorenal Syndrome

• Clinical Definition: Spectrum of disorders involving bidirectional acute or chronic dysfunction of both the heart and kidneys (Types 1 through 5).
• Coding Standard: When documented as "cardiorenal syndrome," assign the appropriate hypertensive heart and CKD combo code (I13.-) along with the specific heart failure (I50.-) and kidney disease codes (N17.- for acute cardiorenal decompensation, N18.- for chronic cardiorenal disease).