ICD-10 Code B27.91 – Infectious mononucleosis, unspecified with polyneuropathy (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code B27.91 – Infectious mononucleosis, unspecified with polyneuropathy
What it is
This code identifies infectious mononucleosis when the record also documents polyneuropathy. It means the patient has mono with nerve involvement affecting multiple peripheral nerves.
Clinical signs
Typical mono findings may include fever, sore throat, fatigue, and lymphadenopathy. Polyneuropathy can present with numbness, tingling, weakness, or reduced reflexes; clinical features vary, so refer to documentation.
When to use this code
Use B27.91 when the provider documents infectious mononucleosis and specifically links it to polyneuropathy. The neurologic complication must be part of the diagnosis, not a separate suspected symptom. If the chart only says mono or only mentions neuropathy without linkage, check documentation.
Do not use for
Do not assign this code for uncomplicated infectious mononucleosis. Also avoid it when neuropathy is due to another cause, or when the record does not clearly support polyneuropathy.
Coding tip
Look for explicit provider wording connecting mononucleosis with polyneuropathy before coding B27.91.