2026 ICD-10-CM Diagnosis Code M21.42Flat foot [pes planus] (acquired), left foot

ICD-10-CM Codes›M00–M99›M20-M25›M21

ICD-10-CM M21.42
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

M21.42 is a billable ICD-10-CM diagnosis code for flat foot [pes planus] (acquired), left foot. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 564 through 566. Coders also document this condition as acquired bilateral pes planus. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Acquired foot deformities.

Code Identity

ICD-10-CM Code
M21.42
Billable Status
Yes — Valid for Submission
Code Describes
Flat foot [pes planus] (acquired), left foot
Short Description
Flat foot [pes planus] (acquired), left foot
Same as the full description in the CMS dataset.
Parent Code
M21.4
Flat foot [pes planus] (acquired)

Code Classification

ChapterM00–M99Diseases of the musculoskeletal system and connective tissue
SectionM20-M25Other joint disorders
CategoryM21Other acquired deformities of limbs
This CodeM21.42Flat foot [pes planus] (acquired), left foot

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acquired bilateral pes planus
  • Acquired pes planus of left foot
  • Acquired pes planus of right foot

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR MUS021
Acquired foot deformities
Default principal diagnosis: inpatient Yes · outpatient Yes

Patient EducationClinical

Foot Injuries and Disorders

Each of your feet has 26 bones, 33 joints, and more than 100 tendons, muscles, and ligaments. No wonder a lot of things can go wrong. Here are a few common problems:

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert M21.42 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
734 Flat foot
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About M21.42Overview

Is M21.42 (Flat foot [pes planus] (acquired)) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report flat foot [pes planus] (acquired), left foot on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does M21.42 group to?

When flat foot [pes planus] (acquired), left foot is the principal diagnosis on an inpatient stay, it groups to MS-DRG 564, 565, 566, with relative weights from 0.7493 to 1.5436 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of M21.42?

Under the General Equivalence Mappings, flat foot [pes planus] (acquired), left foot converts to ICD-9-CM 734 (flat foot). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.