How Do I Code Acute Kidney Injury (AKI)? N17.9, Acute Tubular Necrosis, and AKI on Top of Chronic Kidney Disease

Coders searching “AKI” meet four different homes in the classification, and the provider’s exact wording decides which applies. Category N17, Acute kidney failure, is where nontraumatic acute kidney injury lives: N17.9, Acute kidney failure, unspecified, carries the inclusion term Acute kidney injury (nontraumatic). The category splits by what has happened to the kidney tissue — N17.0 with tubular necrosis (inclusion terms: acute tubular necrosis, renal tubular necrosis, tubular necrosis NOS), N17.1 with acute cortical necrosis, N17.2 with medullary (papillary) necrosis, N17.8 other — and N17.9 is the code when the record says only AKI.

The category carries two notes that shape the claim. Code also associated underlying condition — the sepsis, dehydration, obstruction or drug the provider names as the cause is a separate code. And an Excludes1 for posttraumatic renal failure (T79.5, Traumatic anuria, whose inclusion terms are crush syndrome and renal failure following crushing); N17.9 adds an Excludes2 for traumatic kidney injury (S37.0-). A kidney that failed because it was injured is coded from the injury chapter.

The two neighbours that are not N17:

• N28.9, Disorder of kidney and ureter, unspecified, carries the inclusion terms Renal insufficiency (acute), Renal disease (acute) NOS and Nephropathy NOS. A provider who writes “acute renal insufficiency” and nothing more has documented N28.9, not acute kidney failure. The two are separated by a word, and N19’s Excludes1 lists them as different things.
• N19, Unspecified kidney failure (inclusion term: Uremia NOS), is kidney failure with no statement of acute or chronic. Its Excludes1 note names N17.-, N18.-, N28.9 and R39.2, so it is not reported beside them.

Finally, anuria and oliguria (R34) are symptom codes: when the provider documents reduced urine output but no kidney diagnosis, the symptom is what is coded. O90.4, Postpartum acute kidney failure, is the obstetric form and carries its own Excludes1 for R34.

Acute kidney injury and chronic kidney disease are different conditions in different categories, and nothing at N17 or N18 excludes the other. The Official Guidelines state the rule twice, in the hypertension section, where the question arises most often:

Official source

“If a patient has hypertensive chronic kidney disease and acute renal failure, the acute renal failure should also be coded. Sequence according to the circumstances of the admission/encounter.”

Official source

“For patients with both acute renal failure and chronic kidney disease, the acute renal failure should also be coded. Sequence according to the circumstances of the admission/encounter.”

So a patient admitted with AKI who also has stage 3b CKD takes the N17 code and N18.32; if the CKD is hypertensive, the I12 combination code (or I13 with heart disease) leads that pair and the N18 stage follows, exactly as the hypertension guide describes, and the N17 code is added. Which of the acute and chronic codes goes first is a question about the encounter — what occasioned the admission — not a fixed rule. Note that “acute on chronic” here is not the Section I.B.8 situation of one condition with acute and chronic Index subentries; AKI and CKD are two diagnoses, each with its own code, and neither replaces the other.

1. Read the provider’s exact term. “Acute kidney injury,” “acute kidney failure,” “acute renal failure” → category N17. “Acute renal insufficiency,” “renal insufficiency” → N28.9. Reduced urine output alone → R34. Failure with no acute/chronic statement → N19.
2. Pick the N17 code by the documented tissue finding: tubular necrosis N17.0, cortical N17.1, medullary N17.2, otherwise N17.9.
3. Code the associated underlying condition the provider names — N17’s Code Also note.
4. If CKD is documented, add the N18 stage code (N18.6 if ESRD), and the I12/I13 combination code when the CKD is hypertensive. Sequence acute and chronic by the circumstances of the encounter.
5. If the kidney failure follows trauma or crush injury, code from the injury chapter (S37.0-, T79.5) — N17’s notes send it there.
6. Postpartum acute kidney failure is coded in subcategory O90.4 (O90.49, or O90.41 for hepatorenal syndrome following labor and delivery), not N17.

1. “AKI, prerenal, secondary to dehydration; creatinine normalised with fluids.”N17.9, plus the dehydration code the Code Also note asks for. No necrosis is documented, so N17.9 rather than N17.0.
2. “Acute tubular necrosis following contrast exposure.”N17.0 — ATN is its inclusion term — with the documented cause coded as N17’s Code Also note directs.
3. “Hypertension; CKD stage 3b; admitted with acute kidney injury.” I12.9 with N18.32 for the hypertensive CKD, and N17.9 for the AKI, sequenced by what occasioned the admission — the Guidelines’ own I.C.9.a.2 case.
4. “Acute renal insufficiency” with no other kidney diagnosis.N28.9, whose inclusion terms name exactly that phrase. If the clinical picture reads like AKI, the coder queries; the code does not change until the documentation does.

• Coding “acute renal insufficiency” to N17 — the tabular puts that phrase at N28.9.
• Dropping the N18 stage code because the admission was for AKI. Both conditions are coded; only the order depends on the encounter.
• Assigning N17.0 for AKI without a documented tubular necrosis.
• Reporting N19 with N17 or N18 — N19’s Excludes1 names both.
• Coding traumatic or crush-related kidney failure to N17 instead of S37.0- or T79.5.
• Leaving off the associated underlying condition N17’s Code Also note asks for.

• The provider’s term: acute kidney injury or failure (N17), renal insufficiency (N28.9), failure not stated as acute or chronic (N19), or a symptom only (R34).
• Tubular, cortical or medullary necrosis, which selects N17.0, N17.1 or N17.2.
• Chronic kidney disease and its stage, or ESRD, which adds the N18 code.
• Hypertension, which brings I12 or I13 in with the stage code.
• The documented cause of the AKI, for the Code Also note.
• Trauma or crush injury as the mechanism.