ICD-10 Code B27 – Infectious mononucleosis (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code B27 – Infectious mononucleosis
What it is
B27 identifies infectious mononucleosis, a viral illness most often linked to Epstein-Barr virus. It is used when the diagnosis is documented and the condition is the focus of care.
Clinical signs
Typical findings include fever, sore throat, fatigue, and swollen lymph nodes. Clinical features vary; refer to documentation when the presentation is atypical or the cause is not specified.
When to use this code
Use B27 when the provider documents infectious mononucleosis or mono as the diagnosis. It also fits encounters where the condition is confirmed and is driving evaluation, counseling, or follow-up. If the record does not clearly identify infectious mononucleosis, check documentation.
Do not use for
Do not use this code for nonspecific viral pharyngitis, fatigue alone, or suspected mono without confirmation. Check documentation if another infection is named instead.
Coding tip
Code B27 only when the chart supports infectious mononucleosis, and code any documented associated manifestations separately if instructed.