ICD-10 Code B27.9 – Infectious mononucleosis, unspecified (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code B27.9 – Infectious mononucleosis, unspecified
What it is
B27.9 identifies infectious mononucleosis when the provider documents mono or EBV-related illness without naming a specific subtype. Use it for a nonspecific diagnosis of the condition.
Clinical signs
Typical findings include fever, sore throat, fatigue, and enlarged lymph nodes. Clinical features vary; refer to documentation for splenomegaly, atypical lymphocytosis, or other confirmed findings.
When to use this code
Use this code when the record clearly states infectious mononucleosis but does not specify another ICD-10-CM subtype. It also fits cases documented as unspecified mono after evaluation. If the note names a different organism or a more specific form, code that instead.
Do not use for
Do not use B27.9 when the provider documents a specific infectious mononucleosis type, such as due to another named virus. Check documentation if the chart only mentions “viral illness” or “pharyngitis.”
Coding tip
Code the most specific mononucleosis type documented; if the subtype is not stated, B27.9 is appropriate.