13.3 AKI Criteria, Tubular Injury, and CKD Staging

Acute kidney injury, tubular injury, and chronic kidney disease

Renal review asks three different questions: Has kidney function changed acutely? What mechanism does the clinician identify? What chronic disease was already present? These questions affect care, documentation, and coding in different ways. A creatinine-based AKI stage does not identify its cause, and neither a laboratory value nor a dialysis order independently supplies every diagnosis in the code set.

Source status matters. At this guide's October 23, 2026 review, KDIGO's AKI page identifies its 2026 AKI/AKD update as a public-review draft being prepared for publication and still links the final 2012 guideline. The criteria below are explicitly the final KDIGO 2012 criteria, not adoption of a draft as a final standard. The CKD discussion uses the final 2024 guideline; the separate medication-focused update under public review is not substituted for it.

Recognize the AKI criteria and time windows

KDIGO 2012 defines AKI by any of these findings:

• Serum creatinine increases by at least 0.3 mg/dL within 48 hours.
• Serum creatinine reaches at least 1.5 times baseline, with the increase known or presumed to have occurred within the preceding seven days.
• Urine volume is below 0.5 mL/kg/hour for six hours.

Creatinine and urine output provide alternative routes; both are not required. Check measurement times, units, baseline reliability, body weight used for urine-output calculations, and whether urine collection is complete. A copied “low urine output” statement without a measured interval is not interchangeable with six hours of documented output.

Staging after AKI is identified

Use the highest qualifying stage when routes differ. At twelve hours of urine output below 0.5 mL/kg/hour, the stage 2 threshold is met. The stage 3 absolute-creatinine route requires the acute change specified in the AKI definition; stable chronic creatinine of 4.2 mg/dL does not automatically become stage 3 AKI. Similarly, existing maintenance dialysis is not “new initiation of renal replacement therapy” for an acute event.

For patients younger than 18, the 2012 table includes a decrease in eGFR below 35 mL/min/1.73 m² as a stage 3 route. Apply age-appropriate clinical interpretation and the guideline's qualifications. These criteria assist the clinician; CDI must not independently assign a diagnosis merely because an EHR calculator flags a stage.

Work the numbers and preserve the timeline

A verified creatinine of 1.0 mg/dL on Monday rises to 2.4 mg/dL on Tuesday. The increase is 1.4 mg/dL and the ratio is 2.4. The creatinine route supports stage 2 under the specified framework. If urine output is 0.4 mL/kg/hour throughout twelve hours, that independently reaches stage 2. Stage 3 does not follow merely because the creatinine exceeds 2.0 mg/dL.

When the only available comparison is an old value, the required acute timing needs clinical assessment rather than an invented normal baseline. The 2026 query brief permits relevant prior records in a current clinical scenario. Use them with their actual dates and limitations; an old normal value is not proof that all change occurred during the last seven days. Uncertainty can support asking whether the process is acute, chronic, or acute on chronic.

Separate syndrome from mechanism

AKI can involve reduced kidney perfusion, intrinsic kidney disease or injury, obstruction, or several mechanisms together. A response to volume restoration can inform the clinician's assessment but does not automatically negate AKI. The KDIGO discussion specifically cautions against calling an acute reduction in kidney function benign merely because tubular concentration is preserved.

Acute tubular necrosis (ATN) is a clinical syndrome associated with tubular injury, often considered in ischemic or toxic contexts. Its name does not mean that every clinical case requires biopsy-proven widespread cell death. The 2012 guideline explains the mismatch between the term and the limited morphologic changes sometimes found in human disease. A fixed requirement for 7–21 days of oliguria or a mandatory three-phase course is inappropriate.

Urine sediment, exposure history, perfusion, serial kidney function, and response to treatment may inform the differential. Muddy brown granular casts can strongly support tubular injury. Fractional sodium excretion is contextual evidence, not a binary ATN classifier. In a prospective inpatient study, low fractional sodium excretion occurred in many patients with muddy brown casts; a value below 1% therefore did not exclude tubular injury. Do not use a single sodium-excretion, urea-excretion, BUN/creatinine, or osmolality cutoff as a universal rule-in/rule-out test.

If the record describes persistent AKI after shock but only lists “azotemia,” ask the involved provider to clarify the renal condition and, where supported, its mechanism. Present the actual findings and credible alternatives. Do not ask for ATN because it carries an MCC, or imply that a slower creatinine recovery proves necrosis. When terminology such as tubular injury leaves the intended coded condition unclear, resolve that specific ambiguity rather than silently substituting a preferred diagnosis.

CKD is more than a single eGFR

KDIGO 2024 defines CKD by kidney structure or function abnormalities with health implications present for at least three months. It classifies CKD by cause, GFR category, and albuminuria category (CGA). The GFR categories are:

G1 or G2 alone does not establish CKD without evidence of kidney damage. Chronicity can be supported by prior results, imaging, pathology, history, or repeat assessment; one abnormal eGFR or albumin/creatinine ratio does not establish it. A low eGFR during an acute illness may reflect an acute process. CDI evaluates the history and provider assessment rather than deriving a chronic stage from that isolated result.

CKD stage is a provider diagnosis, outside the routine nonprovider documentation exceptions in I.B.14. If stage 3 is documented without a supported subdivision, N18.30 is available. Query when there is a meaningful, supported unresolved question; do not force a 3a/3b answer solely because a laboratory system prints an estimate.

ESRD, dialysis, and transplant status

FY 2027 I.C.14.a directs N18.6 when the provider documents ESRD. If a CKD stage and ESRD are both documented, assign N18.6 only. The Tabular List also includes CKD requiring chronic dialysis under N18.6 and directs an additional dialysis-status code. An acute dialysis episode for AKI does not, by itself, establish chronic dialysis dependence or ESRD.

A transplant recipient can still have CKD. Documented CKD after a transplant is not automatically a transplant complication and does not mean every recipient should retain ESRD coding. Apply the documented CKD stage and kidney-transplant status, with separate complication guidance when failure, rejection, or another transplant complication is established. Query a genuinely unclear complication relationship.

Apply the coding relationships and severity rules

When AKI and CKD are established, both N17.- and the appropriate N18.- code may be needed. Principal diagnosis depends on the circumstances of admission and applicable conventions; an acute abnormal result does not automatically make AKI principal.

Hypertension and CKD are presumed related under I.C.9.a.2 unless the provider indicates that the CKD is not related to hypertension. Another documented CKD cause alone does not establish that exception. Use I12 with the stage code when applicable; qualifying heart involvement with hypertension and CKD can require I13 and additional heart-failure/stage codes. Follow N18's code-first instructions for associated diabetic or hypertensive CKD rather than treating the CKD stage as the entire code set.

The FY 2027 Index maps azotemia to R79.89, with a different route when it means uremia. Do not automatically replace it with R39.89 or dehydration. N17.9 includes nontraumatic AKI, while N17.0 describes acute kidney failure with tubular necrosis. In final MS-DRG v44, N17.9 is a CC and N17.0 an MCC; N18.4 and N18.5 are CCs, while N18.6 is an MCC. These secondary-diagnosis classifications remain subject to exclusions and complete grouping. A clinical AKI stage is not itself a CC/MCC category, and a renal code does not guarantee an individual risk-model or payment result.

Sources: KDIGO AKI guideline status; final KDIGO 2012 AKI guideline, 1.1 and 2.1; KDIGO CKD guideline status; final KDIGO 2024 CKD guideline, tables 1–3 and 1.1.3; prospective urine-sediment/FENa study; FY 2027 ICD-10-CM guidelines; FY 2027 Index and Tabular List; final MS-DRG v44 Appendix C; 2026 query brief, X.