N12: Tubulo-interstitial nephritis, not specified as acute or chronic

The ICD-10-CM code for pyelonephritis when the provider has documented the kidney infection but has not characterized the episode as acute or as chronic.

Billable

FY2027

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At a glance

Code Family

N12: Tubulo-interstitial nephritis, not specified as acute or chronic.

Condition

Pyelonephritis

Family Specificity

N12 is a three character code with no children. There is no decimal to add and no fifth character to look for. Specificity is gained by moving to another code in the N10–N16 block, not by extending this one.

HCC Mapping (v28)

None. N12 does not appear in the CMS-HCC v28 category code lists. The only renal categories in v28 are HCC 326 through HCC 329, which carry chronic kidney disease staging codes.

DRG Grouping

MS-DRG 689 with MCC or MS-DRG 690 without MCC, the kidney and urinary tract infection pair in MDC 11 under the FY2027 MS-DRG v44 grouper.

Billable

Yes

Which N12 code do I assign?

N12 is a three character code with no children, which means there is no decimal to add and no specificity ladder inside it. The split that matters runs on acuity across the N10–N16 block: N10 for an acute episode, category N11 for a chronic one, and N12 only when the note characterizes neither. The descriptor reads not specified as acute or chronic. N12 therefore records a gap in the documentation rather than a distinct clinical entity.

When documentation says…

Code

Acute pyelonephritis

N10

Acute pyelonephritis. The provider characterized the episode as acute. Acute pyelitis, acute infectious interstitial nephritis and acute tubulo-interstitial nephritis are among the inclusion terms printed at N10 in the FY2027 tabular. N10 carries a use additional code instruction for the infectious agent from B95–B97.

Chronic pyelonephritis

N11.9

Chronic tubulo-interstitial nephritis, unspecified. Chronic pyelonephritis NOS, chronic pyelitis NOS and chronic interstitial nephritis NOS are the inclusion terms printed at N11.9. Assign it when the note says chronic and names neither reflux nor obstruction. Category N11 carries the use additional code instruction for the infectious agent from B95–B97.

Chronic pyelonephritis with ureteral obstruction or stricture

N11.1

Chronic obstructive pyelonephritis. The inclusion terms at N11.1 cover chronic pyelonephritis associated with obstruction of the ureter, stricture of the ureter, stricture of the pelviureteric junction, a crossing vessel or kinking of the ureter. Where the chronic pyelonephritis is documented as reflux associated instead, the code is N11.0. N11.1 carries Excludes1 notes for calculous pyelonephritis at N20.9 and for the obstructive uropathy codes in category N13.

Pyelonephritis, acuity not stated

N12

Tubulo-interstitial nephritis, not specified as acute or chronic. Pyelonephritis NOS, pyelitis NOS and interstitial nephritis NOS are the inclusion terms printed at N12. Assign it only where the record names pyelonephritis and does not characterize the episode as acute or as chronic. Where the provider did characterize the episode, N12 is a documentation failure rather than a coding choice.

Urinary tract infection, no site named

N39.0

Urinary tract infection, site not specified. N39.0 is correct only while the site is absent from the record. The Excludes1 note at N39.0 bars it from being reported with a urinary tract infection of specified site for the same infection. Pyelonephritis names the kidney. Once pyelonephritis is documented the site is specified and N39.0 no longer applies.

Calculous pyelonephritis

N20.9

Urinary calculus, unspecified. Calculous pyelonephritis is a printed inclusion term at category N20. The Excludes1 note at N12 names calculous pyelonephritis and gives N20.9, which bars N12 for this fact pattern. Where the stone has produced hydronephrosis and the kidney is infected, the code is N13.6 for pyonephrosis instead.

Pyelonephritis in a pregnant patient

O23.00

Infections of kidney in pregnancy, unspecified trimester. Pyelonephritis in pregnancy is a printed inclusion term at O23.0. The trimester documented in the note selects among O23.00 through O23.03. Chapter 15 codes take sequencing priority under Guideline I.C.15.a. Category O23 carries its own use additional code instruction for the organism from categories B95 and B96.

Tabular notes

INCLUSION TERMS

Pyelonephritis NOS, pyelitis NOS and interstitial nephritis NOS are printed as inclusion terms at N12 in the FY2027 tabular. The word pyelonephritis with no qualifier therefore resolves to N12 rather than to N10. The N10–N16 block header carries pyelonephritis as an Includes term for the block as a whole. The acute form, the chronic form and the uncharacterized form therefore sit in one block. N12 carries no Includes note of its own.

EXCLUDES1

N12 carries one Excludes1 note: calculous pyelonephritis (N20.9). Excludes1 means the two codes are never reported together for the same condition. Where a stone is documented as the cause of the pyelonephritis, N20.9 replaces N12 rather than accompanying it. Calculous pyelonephritis is a printed inclusion term at category N20, which is what that Excludes1 note points to. The N10–N16 block header carries a second Excludes1 that N12 inherits: pyeloureteritis cystica (N28.85). Category N11 carries the same calculous pyelonephritis Excludes1 at N11.1.

EXCLUDES2

N12 carries no Excludes2 note of its own. Chapter 14 carries one at the chapter level and N12 inherits it. That note excludes, among others, complications of pregnancy, childbirth and the puerperium (O00–O9A), certain infectious and parasitic diseases (A00–B99), certain conditions originating in the perinatal period (P04–P96) and symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00–R94). Excludes2 permits dual coding where both conditions are genuinely present and clinically appropriate. The last clause is what places bacteriuria (R82.71) and pyuria (R82.81) outside the infection codes. The pregnancy clause governs pyelonephritis in a pregnant patient. Guideline I.C.15.a gives chapter 15 codes sequencing priority, which places that infection in the O23.0- codes rather than at N12.

USE ADDITIONAL

N12 carries no use additional code instruction in the FY2027 tabular. This separates it from its neighbors. N10 and category N11 each carry use additional code (B95–B97), to identify infectious agent. N13.6 and N39.0 carry the same instruction. The absence of the instruction at N12 does not bar the organism code. Where the provider documents the organism, the B95–B97 code remains reportable as a secondary code for a disease classified elsewhere. Escherichia coli unspecified is B96.20. Klebsiella pneumoniae is B96.1, Proteus is B96.4 and Pseudomonas is B96.5. The organism code sequences after the infection code and is never a principal diagnosis.

FY2027: EFFECTIVE 1 OCTOBER 2026

N12 was valid in the FY2026 code set and is unchanged in the FY2027 update. The FY2027 codes and tabular addenda make no change in the N10–N16 block. N12 has no child codes and remains billable at three characters.

Operational & financial impact

Why specificity matters beyond the chart: risk adjustment, DRG assignment and denial exposure all follow from which code is selected.

CMS-HCC v28 (PY2024+)

None. N12 does not appear in the CMS-HCC v28 category code lists.

CMS-HCC v28 carries four renal categories. They are HCC 326 for chronic kidney disease stage 5, HCC 327 for stage 4, HCC 328 for stage 3b, and HCC 329 for stage 3 other than 3b. Their published code lists carry the chronic kidney disease staging codes from category N18 together with I12.0, I13.11 and I13.2.

CMS-HCC v28 carries no category for pyelonephritis, for tubulo-interstitial nephritis or for urinary tract infection. Stepping from N12 up to N10 or to a code in category N11 does not change that. None of the three codes risk adjusts.

CMS-HCC v28 is at full weight for payment year 2026.

Where the encounter also documents a staged chronic kidney disease, that code risk adjusts on its own. Published v28 code lists place N18.32 in HCC 328 and place N18.30 and N18.31 in HCC 329.

MS-DRG v44 (FY2027)

N12 sits on the principal diagnosis logic list for the kidney and urinary tract infection pair. That pair is MS-DRG 689 with MCC and MS-DRG 690 without MCC, in MDC 11.

The pair splits on whether an MCC is present among the other reported diagnoses. There is no with CC tier between the two.

N10, N11.0, N11.8, N11.9 and N13.6 sit on the same principal diagnosis list. N11.1, chronic obstructive pyelonephritis, groups instead to the urinary stones pair, MS-DRG 693 or 694. Moving between N10, N11.9 and N12 does not by itself move the MS-DRG. What it moves is the clinical record and the exposure on audit.

A stone documented as the cause moves the case off that list. N20.9 sits on the principal diagnosis logic list for the urinary stones pair: MS-DRG 693 with MCC or MS-DRG 694 without MCC.

Where the record supports pyonephrosis, the principal diagnosis becomes N13.6. N13.6 sits on the same MS-DRG 689 and 690 logic list. The diagnosis change alone therefore does not move the DRG. An operating room procedure performed to relieve the obstruction does move the case to a surgical MS-DRG in MDC 11.

Medicare and commercial payers

N12 is a valid diagnosis code for Medicare Part A and Part B claims.

There is no National Coverage Determination specific to pyelonephritis diagnosis coding.

WHERE SPECIFICITY IS LOST

N12 records that the note did not say acute or chronic. There is no more specific code inside the family to step up to. The step up is to N10 or to a code in category N11. Each of those requires the provider to characterize the episode.

Reporting N12 when the note documents an acute episode understates the encounter. The correct code is N10.

Reporting N12 alongside N39.0 for one infection reports the same infection twice. The Excludes1 note at N39.0 bars a site specified urinary tract infection from being reported with it for the same infection.

Reporting N39.0 when the note documents pyelonephritis discards the upper tract. Pyelonephritis names the kidney, which makes the site specified.

Reporting N12 where a stone is documented as the cause is an Excludes1 violation. The Excludes1 note at N12 gives N20.9 for calculous pyelonephritis.

Reporting N12 for a pregnant patient sets aside Guideline I.C.15.a. Pyelonephritis in pregnancy belongs in the O23.0- codes, selected by trimester.

Omitting the organism code discards the organism the culture named. N12 carries no use additional instruction. The B95–B97 code remains reportable when the provider documents the organism.

SEVERITY DESIGNATION (MS-DRG V44.0)

N12 is a CC. It appears in Appendix G, diagnoses defined as complications or comorbidities, in the ICD-10 MS-DRG Definitions Manual Version 44.0. As a secondary diagnosis it can move a case to a with-CC tier where the DRG pair offers one.

Documentation checklist to support N12

Chart elements that support code selection and reduce specificity-driven denials.

A provider diagnosis of pyelonephritis, pyelitis or interstitial nephritis stated in the note, rather than flank pain, fever and pyuria alone.

Whether the provider characterized the episode as acute or as chronic. An acute characterization moves the encounter to N10. A chronic characterization moves it to a code in category N11.

Absence of any stone documented as the cause. A causal stone moves the encounter to N20.9 under the Excludes1 note at N12.

Whether obstruction, a ureteral stricture or vesicoureteral reflux is documented, which routes a chronic episode to N11.1 or N11.0 and an obstructed infected kidney to N13.6.

Imaging findings for hydronephrosis, which separate an uncomplicated upper tract infection from pyonephrosis at N13.6.

Culture results with the organism named by the provider, to support the B95–B97 secondary code.

Pregnancy status and the trimester, since pyelonephritis in a pregnant patient is coded from the O23.0- codes under Guideline I.C.15.a.

Whether the provider documents sepsis. When sepsis and the pyelonephritis are the reason for admission, Guideline I.C.1.d.4 sequences the systemic infection first and N12 moves to a secondary position.

For outpatient encounters, a confirmed diagnosis only. A suspected or rule out pyelonephritis is not coded in the outpatient setting. An uncertain diagnosis documented at discharge may be coded as confirmed on the inpatient side under Section II.H of the Official Guidelines.

FAQ

What is the ICD-10-CM code for pyelonephritis?

It depends on the acuity the note records. N10 is acute pyelonephritis. N11.9 is chronic tubulo-interstitial nephritis, unspecified, which carries chronic pyelonephritis NOS as an inclusion term. N12 is pyelonephritis that the provider did not characterize either way. Pyelonephritis NOS is a printed inclusion term at N12. N12 is billable at three characters and has no children. Readers arriving expecting a decimal will not find one.

Can N12 be used in the outpatient setting?

Yes. N12 can be used in the outpatient setting on a confirmed diagnosis. Outpatient coding does not permit a suspected, probable or rule out pyelonephritis to be coded as confirmed. An uncertain diagnosis documented at discharge may be coded as confirmed on the inpatient side under Section II.H of the Official Guidelines. The acuity question is the same in both settings. N12 is correct in neither setting once the provider has characterized the episode.

When do I use N12 instead of N39.0?

Use N39.0 only while no site appears in the record. Pyelonephritis names the kidney, which specifies the site. The Excludes1 note at N39.0 bars it from being reported with a urinary tract infection of specified site. N39.0 and N12 are therefore not reported together for the same infection. The opposite mistake is also a coding error. Reporting N39.0 when the note says pyelonephritis discards the upper tract finding.

Does N12 map to an HCC under CMS-HCC v28?

No. N12 does not appear in the CMS-HCC v28 category code lists. The only renal categories in v28 are HCC 326 through HCC 329. Their code lists carry chronic kidney disease staging codes. A staged chronic kidney disease documented at the same encounter does risk adjust, through its own code rather than through N12.

What does the note have to say for N10 instead of N12?

The provider has to characterize the episode. The word acute supports N10, as do the printed inclusion terms acute pyelitis and acute tubulo-interstitial nephritis. A description of fever, flank pain and pyuria does not by itself support N10. The coder cannot infer acuity from the clinical picture. Where the presentation reads acute and the note does not say so, the correct action is a provider query.

How is N12 sequenced when sepsis is also documented?

The systemic infection is sequenced first. Guideline I.C.1.d directs that a code for the underlying systemic infection is assigned first where the reason for admission is sepsis with a localized infection. N12 is then reported as a secondary diagnosis. The term urosepsis is not synonymous with sepsis and has no default code in the Alphabetic Index. A provider who uses it has to be queried.

How does an obstructing stone change the code?

A stone changes the code in one of two ways, depending on what the record documents. A stone documented as the cause of the pyelonephritis goes to N20.9. The Excludes1 note at N12 names calculous pyelonephritis and gives that code. A stone that has produced hydronephrosis in an infected kidney goes to N13.6 for pyonephrosis, which carries a use additional code instruction for the organism. The financial effect differs between the two. N20.9 sits on the MS-DRG 693 and 694 logic list for urinary stones. N12 and N13.6 sit on the MS-DRG 689 and 690 list for kidney and urinary tract infections.

What changed for N12 in FY2027?

N12 is unchanged in the FY2027 update, effective 1 October 2026. It was valid in the FY2026 code set. The FY2027 codes and tabular addenda make no change in the N10–N16 block.

Related codes

Codes in the same family as N12. Only codes with a published page render as links.

Sibling codes in the N12 family

N10: Acute pyelonephritis.
N11.0: Nonobstructive reflux-associated chronic pyelonephritis.
N11.1: Chronic obstructive pyelonephritis.
N11.8: Other chronic tubulo-interstitial nephritis.
N11.9: Chronic tubulo-interstitial nephritis, unspecified.
N13.6: Pyonephrosis.
N14.19: Nephropathy induced by other drugs, medicaments and biological substances.
N15.1: Renal and perinephric abscess.
N15.9: Renal tubulo-interstitial disease, unspecified.
N16: Renal tubulo-interstitial disorders in diseases classified elsewhere.

Additional codes to consider

Codes outside the N12 family considered alongside N12.

Companion: code alongside when documented

B96.20: Unspecified Escherichia coli [E. coli] as the cause of diseases classified elsewhere.
B96.1: Klebsiella pneumoniae [K. pneumoniae] as the cause of diseases classified elsewhere.
B96.4: Proteus (mirabilis) (morganii) as the cause of diseases classified elsewhere.
B96.5: Pseudomonas (aeruginosa) (mallei) (pseudomallei) as the cause of diseases classified elsewhere.
B95.61: Methicillin susceptible Staphylococcus aureus infection as the cause of diseases classified elsewhere.
B95.62: Methicillin resistant Staphylococcus aureus infection as the cause of diseases classified elsewhere.
A41.9: Sepsis, unspecified organism, sequenced first under Guideline I.C.1.d where sepsis is documented.

Consider instead

N10: Acute pyelonephritis, where the provider characterized the episode as acute.
N11.9: Chronic tubulo-interstitial nephritis, unspecified, where the provider characterized the episode as chronic.
N11.1: Chronic obstructive pyelonephritis, where chronic pyelonephritis is documented with ureteral obstruction or stricture.
N11.0: Nonobstructive reflux-associated chronic pyelonephritis, where chronic pyelonephritis is documented as reflux associated.
N20.9: Urinary calculus, unspecified, the code the Excludes1 note at N12 gives for calculous pyelonephritis.
N13.6: Pyonephrosis, for an obstructed and infected kidney.
N13.9: Obstructive and reflux uropathy, unspecified, where obstruction is documented without a stated infection.
N39.0: Urinary tract infection, site not specified, where the record names no site.
N15.1: Renal and perinephric abscess, where an abscess is documented.
N15.9: Renal tubulo-interstitial disease, unspecified, which carries infection of kidney NOS as an inclusion term and an Excludes1 against N39.0.
N16: Renal tubulo-interstitial disorders in diseases classified elsewhere, where the pyelonephritis is documented as due to an underlying disease named in its Code first note.
N28.85: Pyeloureteritis cystica, named in the Excludes1 note at the N10–N16 block header.
O23.00: Infections of kidney in pregnancy, unspecified trimester, for pyelonephritis in a pregnant patient.
O86.21: Infection of kidney following delivery, for the puerperium.
R82.71: Bacteriuria, where bacteria are found on urinalysis without a provider diagnosis of infection.
R82.81: Pyuria, where pyuria is documented without a provider diagnosis of infection.

Sources

All claims on this page derive from government and professional-standards sources.

National Center for Health Statistics. (2026). International classification of diseases, tenth revision, clinical modification (ICD-10-CM) FY2027 files. https://www.cdc.gov/nchs/icd/icd-10-cm/files.html
Centers for Medicare & Medicaid Services and National Center for Health Statistics. (2026). ICD-10-CM official guidelines for coding and reporting FY2027. https://ftp.cdc.gov/pub/Health_Statistics/NCHS/Publications/ICD10CM/2027/ICD-10-CM-October-1-2026-FY27-Guidelines.pdf
Centers for Medicare & Medicaid Services. (2025). 2026 model software/ICD-10 mappings. https://www.cms.gov/medicare/payment/medicare-advantage-rates-statistics/risk-adjustment/2026-model-software-icd-10-mappings
Centers for Medicare & Medicaid Services. (2026). MS-DRG classifications and software, version 44. https://www.cms.gov/medicare/payment/prospective-payment-systems/acute-inpatient-pps/ms-drg-classifications-and-software