ICD-10 Code B27.99 – Infectious mononucleosis, unspecified with other complication (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code B27.99 – Infectious mononucleosis, unspecified with other complication
What it is
B27.99 identifies infectious mononucleosis when the type is not specified and the record documents another complication. Use it for mono caused by Epstein-Barr virus when the chart does not name a more specific mononucleosis code.
Clinical signs
Typical findings include fever, sore throat, fatigue, and lymphadenopathy, with possible splenomegaly. Clinical features vary; refer to documentation for the specific complication that supports this code.
When to use this code
Use B27.99 when the provider documents infectious mononucleosis and an additional complication, but does not specify a more exact mononucleosis subtype. It is appropriate when the complication is clearly recorded in the encounter note. If the record is vague, check documentation before assigning the code.
Do not use for
Do not use this code for uncomplicated infectious mononucleosis or when the chart specifies a different mononucleosis code. Do not assume the complication; if it is not documented, check documentation.
Coding tip
Link the mononucleosis diagnosis to the documented complication in the provider note before coding B27.99.