2026 ICD-10-CM Diagnosis Code B27.91Infectious mononucleosis, unspecified with polyneuropathy

ICD-10-CM Codes›A00–B99›B25-B34›B27

ICD-10-CM B27.91
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

B27.91 is a billable ICD-10-CM diagnosis code for infectious mononucleosis, unspecified with polyneuropathy. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 865 through 866. Coders also document this condition as polyneuropathy due to infectious mononucleosis. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other nervous system disorders (neither hereditary nor degenerative), Polyneuropathies, and Viral infection.

Code Identity

ICD-10-CM Code
B27.91
Billable Status
Yes — Valid for Submission
Code Describes
Infectious mononucleosis, unspecified with polyneuropathy
Short Description
Infectious mononucleosis, unspecified with polyneuropathy
Same as the full description in the CMS dataset.
Parent Code
B27.9
Infectious mononucleosis, unspecified

Code Classification

ChapterA00–B99Certain infectious and parasitic diseases
SectionB25-B34Other viral diseases
CategoryB27Infectious mononucleosis
This CodeB27.91Infectious mononucleosis, unspecified with polyneuropathy

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Polyneuropathy due to infectious mononucleosis

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Mononucleosis, infectious
      • with
        • polyneuropathy
    • Polyneuropathy(peripheral)
      • in (due to)
        • infectious
          • mononucleosis

Clinical ClassificationClinical

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

CCSR NVS020
Other nervous system disorders (neither hereditary nor degenerative)
Default principal diagnosis: inpatient No · outpatient No
CCSR NVS015
Polyneuropathies
Default principal diagnosis: inpatient No · outpatient No
CCSR INF008
Viral infection
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Fatigue Syndrome, Chronic

    a syndrome characterized by persistent or recurrent fatigue, diffuse musculoskeletal pain, sleep disturbances, and subjective cognitive impairment of 6 months duration or longer. symptoms are not caused by ongoing exertion; are not relieved by rest; and result in a substantial reduction of previous levels of occupational, educational, social, or personal activities. minor alterations of immune, neuroendocrine, and autonomic function may be associated with this syndrome. there is also considerable overlap between this condition and fibromyalgia. (from semin neurol 1998;18(2):237-42; ann intern med 1994 dec 15;121(12): 953-9)

  • Herpesvirus 4, Human

    the type species of lymphocryptovirus, subfamily gammaherpesvirinae, infecting b-cells in humans. it is thought to be the causative agent of infectious mononucleosis and is strongly associated with oral hairy leukoplakia (leukoplakia, hairy;), burkitt lymphoma; and other malignancies.

  • Infectious Mononucleosis

    a common, acute infection usually caused by the epstein-barr virus (herpesvirus 4, human). there is an increase in mononuclear white blood cells and other atypical lymphocytes, generalized lymphadenopathy, splenomegaly, and occasionally hepatomegaly with hepatitis.

  • Fibromyalgia

    a common nonarticular rheumatic syndrome characterized by myalgia and multiple points of focal muscle tenderness to palpation (trigger points). muscle pain is typically aggravated by inactivity or exposure to cold. this condition is often associated with general symptoms, such as sleep disturbances, fatigue, stiffness, headaches, and occasionally depression. there is significant overlap between fibromyalgia and the chronic fatigue syndrome (fatigue syndrome, chronic). fibromyalgia may arise as a primary or secondary disease process. it is most frequent in females aged 20 to 50 years. (from adams et al., principles of neurology, 6th ed, p1494-95)

Patient EducationClinical

Infectious Mononucleosis

Infectious mononucleosis (mono) is a disease caused by viruses.The most common cause is the Epstein-Barr virus (EBV). Mono is contagious, which means it can spread from person to person. It is common among teenagers and young adults, especially college students.

The full article covers:

  • What is infectious mononucleosis (mono)?
  • What causes infectious mononucleosis (mono)?
  • What are the symptoms of infectious mononucleosis (mono)?
  • How is infectious mononucleosis (mono) diagnosed?
  • What are the treatments for infectious mononucleosis (mono)?
  • Can infectious mononucleosis (mono) be prevented?

Read the full article at MedlinePlus

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

Convert B27.91 to ICD-9-CMHistory

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

ICD-9-CM
075 Infectious mononucleosis
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 B27.91Overview

Is B27.91 (Infectious mononucleosis, unspecified) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report infectious mononucleosis, unspecified with polyneuropathy on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does B27.91 group to?

When infectious mononucleosis, unspecified with polyneuropathy is the principal diagnosis on an inpatient stay, it groups to MS-DRG 865, 866, with relative weights from 0.8696 to 1.4983 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of B27.91?

Under the General Equivalence Mappings, infectious mononucleosis, unspecified with polyneuropathy converts to ICD-9-CM 075 (infectious mononucleosis). 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.