H16.321 ICD-10-CM Code: Diffuse interstitial keratitis, right eye

Coder workflow for H16.321

MedCoder structured workflow — derived from this code’s own official record

Before you code H16.321

1. Laterality is coded in this family. Confirm the side documented — right, left, or bilateral — and select the matching code (this page’s code: right). The unspecified-side code applies only when the record states no side. Laterality is assigned from the documented side; where a bilateral code exists and both sides are documented, it is used instead of two unilateral codes (Guidelines I.B.13).

   ReviewH16.322 · left, H16.323 · bilateral, H16.329 · unspecified

   Guide: Laterality coding →

Choose the right path

1. Is the side documented?
   Yes → Select the code for the documented side (or the bilateral code when both sides are documented and one exists).
   No → Use the unspecified-side code only when the record states no side; a query is the alternative.

   ReviewH16.322 · left, H16.323 · bilateral, H16.329 · unspecified

Consider H16.321. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement

Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).

Laterality

Right, left or bilateral as documented; unspecified only when the record states no side (Guidelines I.B.13).

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Don’t confuse H16.321 with…

MedCoder analysis — pairs from this code’s official notes, distinctions written by MedCoder

The codes most likely to be picked instead of H16.321 — named in its own official notes, or sharing its laterality or encounter family — with what separates them and which documentation moves the case. Each links to the full comparison.

• H16.329— Specificity relationship (Review)

  Overlapping diagnoses in the same category.

  Overlapping codes in the same category — the documentation decides which is the more specific fit.

  What moves you there: A record that documents no more than “Diffuse interstitial keratitis, unspecified eye” takes H16.329; specificity in the record moves it back here.

  Full comparison: H16.321 vs H16.329 →

• H16.322— Laterality relationship (Review)

  Same condition; these differ only in which side is documented.

  The same condition on the other side; the documented side decides, and a bilateral code, where one exists, replaces the pair when both sides are documented.

  What moves you there: A documented left side moves the case to H16.322.

  Full comparison: H16.321 vs H16.322 →

• H16.323— Laterality relationship (Review)

  Same condition; these differ only in which side is documented.

  The same condition on the other side; the documented side decides, and a bilateral code, where one exists, replaces the pair when both sides are documented.

  What moves you there: Documentation of both sides moves the case to H16.323.

  Full comparison: H16.321 vs H16.323 →

Relationships are read from this code's own official notes, its laterality siblings and its encounter family; the wording of each distinction is MedCoder editorial, and the documentation named in each “what moves you there” line is the official note phrase or sibling title itself. Source: CMS ICD-10-CM tabular instructional notes; ICD-10-CM Official Guidelines, Section I.A.12.a.

Verify Before Coding

• No Medicare Code Editor or MS-DRG Definitions Manual restrictions apply to this code.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

FY2026 MS-DRG: not on the CMS CC/MCC list — as a secondary diagnosis this code does not change MS-DRG severity for that release.

Named in the grouper logic of 2 MS-DRGs: DRG 124 (MDC 02), DRG 125 (MDC 02).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Clinical classification (AHRQ CCSR):EYE001 — Cornea and external disease (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Related Codes

Same condition, opposite side

Same category and title, differing only in which side of the body is affected.

H16.322 — Diffuse interstitial keratitis, left eye (left)Compare H16.321 vs H16.322 →, H16.323 — Diffuse interstitial keratitis, bilateral (bilateral)Compare H16.321 vs H16.323 →, H16.329 — Diffuse interstitial keratitis, unspecified eye (unspecified)Compare H16.321 vs H16.329 →

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Cornea and external disease).

+809 more

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main terms “Cogan's syndrome”, “Keratitis”; these codes share that main term but sit in a different category of the Tabular List.

A18.52 — Tuberculous keratitis (tuberculous), A30.9 — Leprosy, unspecified (punctata, leprosa), A50.31 — Late congenital syphilitic interstitial keratitis (syphilitic), A54.33 — Gonococcal keratitis (gonococcal), A71.1 — Active stage of trachoma (trachomatous), B00.52 — Herpesviral keratitis (disciform), B01.81 — Varicella keratitis (disciform, varicella), B02.33 — Zoster keratitis (in, zoster), B05.81 — Measles keratitis and keratoconjunctivitis (in, measles), B09 — Unspecified viral infection characterized by skin and mucous membrane lesions (in, exanthema), B30.0 — Keratoconjunctivitis due to adenovirus (in, adenovirus), B60.13 — Keratoconjunctivitis due to Acanthamoeba (in, acanthamebiasis), B94.0 — Sequelae of trachoma (trachomatous, sequelae), E50.4 — Vitamin A deficiency with keratomalacia (xerotic, vitamin A deficiency), H51.8 — Other specified disorders of binocular movement (oculomotor apraxia), L71.8 — Other rosacea (rosacea)

Contextual Map

Every relationship of H16.321 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Hierarchy

• H00-H59 — Chapter 7: Diseases of the Eye and Adnexa (H00-H59) (H00-H59)[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• H15-H22 — Disorders of sclera, cornea, iris and ciliary body[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026

Clinical classification (CCSR)

• EYE001 — Cornea and external disease[CCSR]— AHRQ CCSR for ICD-10-CM Diagnoses (HCUP)

MS-DRG Grouper

• DRG 124 — OTHER DISORDERS OF THE EYE WITH MCC OR THROMBOLYTIC AGENT[MS-DRG]: “OTHER DISORDERS OF THE EYE WITH MCC OR THROMBOLYTIC AGENT (MDC 02)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 125 — OTHER DISORDERS OF THE EYE WITHOUT MCC[MS-DRG]: “OTHER DISORDERS OF THE EYE WITHOUT MCC (MDC 02)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026

MDC crossing

• MDC 02 — Diseases and Disorders of the Eye[MDC crossing]: “Diseases and Disorders of the Eye — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs. 1,200 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026

Nearest codes (40)

• H16 — Keratitis[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• H16.262 — Vernal keratoconjunctivitis, with limbar and corneal involvement, left eye[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• H16.263 — Vernal keratoconjunctivitis, with limbar and corneal involvement, bilateral[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• H16.269 — Vernal keratoconjunctivitis, with limbar and corneal involvement, unspecified eye[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• H16.29 — Other keratoconjunctivitis[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• H16.291 — Other keratoconjunctivitis, right eye[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• H16.292 — Other keratoconjunctivitis, left eye[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• H16.293 — Other keratoconjunctivitis, bilateral[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• and 32 more

Change history

• FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data

CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page

Claim edits Official source data

CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page

Inpatient payment groups Official source data

CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v43 Release, file and checksum · Publisher’s page

Change history and date-of-service validity Official source data

CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page

Comparisons, relationships and the contextual map MedCoder structured relationship

Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears

Summary and FAQ answers MedCoder explanation

Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Nearest Codes in This Family

Official ICD-10-CM classifications closest to H16.321 in its code family, with their registry titles.

• H16.311 — Corneal abscess, right eye
• H16.312 — Corneal abscess, left eye
• H16.313 — Corneal abscess, bilateral
• H16.319 — Corneal abscess, unspecified eye
• H16.32 — Diffuse interstitial keratitis
• H16.322 — Diffuse interstitial keratitis, left eye
• H16.323 — Diffuse interstitial keratitis, bilateral
• H16.329 — Diffuse interstitial keratitis, unspecified eye
• H16.33 — Sclerosing keratitis
• H16.331 — Sclerosing keratitis, right eye

View all codes in the H16 family