Medical Coding Mononucleosis

Mononucleosis (Infectious Mononucleosis) – Coding Tips & Guidelines

Mononucleosis, commonly referred to as “mono”, is a viral infection most often caused by the Epstein–Barr virus (EBV). It is frequently seen in adolescents and young adults and is characterized by fatigue, fever, sore throat, and lymphadenopathy.

For medical coders, accurate reporting requires careful review of the physician documentation, correct ICD-10-CM code selection, and attention to related manifestations or complications.

Understanding Mononucleosis

Infectious mononucleosis is primarily transmitted through saliva, which is why it is sometimes referred to as the “kissing disease.” The infection can cause systemic symptoms and may involve multiple body systems.

Common clinical features include:

• Fever

• Severe fatigue

• Sore throat (pharyngitis)

• Swollen lymph nodes

• Enlarged spleen (splenomegaly)

• Enlarged liver (hepatomegaly)

Because the presentation can vary significantly, coders must rely on the provider’s documented diagnosis, not just symptoms.

ICD-10-CM Codes for Mononucleosis

The primary ICD-10-CM category for infectious mononucleosis is B27 – Infectious mononucleosis.

Common code options include:

B27.0 – Gammaherpesviral mononucleosis
Used when documentation specifies Epstein–Barr virus (EBV) infection.

B27.1 – Cytomegaloviral mononucleosis
Used when the infection is caused by cytomegalovirus (CMV).

B27.8 – Other infectious mononucleosis
Used for other specified viral causes.

B27.9 – Infectious mononucleosis, unspecified
Used when the provider documents mononucleosis but does not specify the viral cause.

Condition ICD-10-CM Codes
Infectious mononucleosis
B27 (infectious mononucleosis)
B27.0 (gammaherpesviral mononucleosis…)
– B27.00 (without complication)
– B27.01 (with polyneuropathy)
– B27.02 (with meningitis)
– B27.09 (with other complications)
B27.1 (cytomegaloviral mononucleosis…)
– B27.10 (without complication)
– B27.11 (with polyneuropathy)
– B27.12 (with meningitis)
– B27.19 (with other complications)
B27.8 (other infectious mononucleosis…)
– B27.80 (without complication)
– B27.81 (with polyneuropathy)
– B27.82 (with meningitis)
– B27.99 (with other complications)
B27.9 (infectious mononucleosis, unspecified…)
– B27.90 (without complication)
– B27.91 (with polyneuropathy)
– B27.92 (with meningitis)
– B27.89 (with other complications)

Coding Tip

When the documentation states “Epstein–Barr virus mononucleosis”, code B27.0, not B27.9.

Symptoms vs. Confirmed Diagnosis

If the provider has confirmed infectious mononucleosis, do not code the associated symptoms separately unless they are unrelated or clinically significant beyond the diagnosis.

Common symptoms that are usually not coded separately include:

• Fever

• Fatigue

• Sore throat

• Lymphadenopathy

These symptoms are considered integral to the disease process.

Medical Coding Signs & Symptoms in Diseases 

Coding Complications of Mononucleosis

In some cases, mononucleosis may cause complications that should be coded separately if documented.

Possible complications include:

Splenomegaly

• ICD-10-CM Code: R16.1

Hepatitis associated with EBV infection

• May require additional liver disease codes if documented.

Airway obstruction due to tonsillar swelling

Thrombocytopenia

Neurological complications (rare)

Always review documentation carefully to determine whether the complication is separately reportable.

Laboratory Testing and Diagnosis

Mononucleosis is typically diagnosed using laboratory tests such as:

Monospot test (heterophile antibody test)
Common screening test for mono.

EBV antibody panel

From a coding perspective, laboratory confirmation supports the diagnosis, but coders should only code what the provider documents, not the lab result alone.

Medical Coding Labs 

Medical Necessity and Documentation

Proper documentation is essential for accurate coding and reimbursement. Providers should clearly document:

• Confirmed diagnosis of infectious mononucleosis

• Viral cause if known (EBV, CMV, etc.)

• Associated complications

• Any additional conditions requiring treatment

Incomplete documentation may result in unspecified coding, which provides less clinical detail.

Medical Coding MDM 

Coding Guidelines to Remember

When coding mononucleosis:

• Always code the confirmed diagnosis rather than symptoms.

• Select the most specific ICD-10-CM code available.

• Review documentation for viral cause (EBV, CMV).

• Report complications separately if documented.

• Do not assign diagnosis codes based solely on lab results without provider confirmation.

Final Coding Tip

Mononucleosis may appear straightforward, but accurate coding depends on careful review of documentation and proper code specificity. Whenever possible, coders should avoid unspecified codes and instead capture the exact viral cause and any complications documented in the record.

B27.90   Infectious mononucleosis, unspecified without complication