Opportunistic infections are serious infections that usually occur in patients with weakened immune systems. These infections are common in individuals with conditions such as HIV/AIDS, cancer, autoimmune diseases, organ transplants, or patients receiving chemotherapy or immunosuppressive therapy.
From a medical coding perspective, opportunistic infection coding can be challenging because symptoms are often vague and documentation may involve multiple organ systems, infectious organisms, and complex clinical presentations.
Accurate ICD-10-CM coding depends heavily on provider documentation that clearly identifies:
- The type of infection
- The causative organism
- The affected body system
- Whether the infection is localized or disseminated
This article explains the major categories of opportunistic infections and provides coding guidance for medical coders.
What Are Opportunistic Infections?
Opportunistic infections occur when the immune system becomes too weak to fight infections that normally would not cause severe illness in healthy individuals.
Common Symptoms of Opportunistic Infections
Patients may present with:
- Fever
- Night sweats
- Weight loss
- Headaches
- Fatigue
- Swollen lymph nodes
- Joint pain
- Loss of appetite
- Respiratory symptoms
- Neurological symptoms
Because these symptoms are nonspecific, providers often rely on:
- Blood tests
- Stool studies
- Culture and sensitivity reports
- Imaging studies
- Biopsies
- Physical examinations
For coders, reviewing diagnostic findings is essential to assign the correct ICD-10-CM code.
ICD-10-CM Coding for Opportunistic Infections
Most opportunistic infections are classified in:
ICD-10-CM Chapter 1: Certain Infectious and Parasitic Diseases (A00–B99)
These codes are organized by:
- Viral infections
- Bacterial infections
- Fungal infections
- Parasitic infections
Viral Opportunistic Infection ICD-10-CM Codes
2019" data-end="2107">Viral opportunistic infections are commonly seen in severely immunocompromised patients.
Cytomegalovirus (CMV) Disease Coding
CMV disease is classified under:
Category B25.- Cytomegaloviral Disease
Documentation should identify the affected organ system.
Common CMV ICD-10-CM Codes
- B25.0 – CMV pneumonitis
- B25.1 – CMV hepatitis
- B25.2 – CMV pancreatitis
- B25.8 – Other CMV diseases
Important Coding Tip
Do not report B25.- codes with:
- P35.1 – Congenital CMV infection
- B27.1- – CMV mononucleosis
These conditions are classified separately in ICD-10-CM.
Bacterial Opportunistic Infection Coding
Bacterial opportunistic infections require documentation specifying:
- Localized vs disseminated infection
- Identified mycobacterial species
- Affected organ system
Pulmonary Mycobacterial Infection
ICD-10-CM Code:
- A31.0 – Pulmonary mycobacterial infection
This includes infections caused by:
- Mycobacterium avium
- Mycobacterium intracellulare
- Mycobacterium kansasii
Disseminated Mycobacterium Avium Complex (DMAC)
ICD-10-CM Code:
- A31.2 – Disseminated mycobacterium avium-intracellulare complex
This condition is common in advanced HIV/AIDS patients and may involve:
- Liver
- Bone marrow
- Spleen
- Lymphatic system
MAC sepsis is also classified under A31.2.
Fungal Opportunistic Infection ICD-10-CM Coding
Fungal infections are among the most severe opportunistic infections in immunocompromised patients.
ICD-10-CM fungal infection codes are usually organized by:
- Organism type
- Anatomical site
- Severity
- Dissemination status
Candidiasis ICD-10-CM Codes
Category B37.- Candidiasis
Common Candidiasis Codes
- B37.0 – Candidal stomatitis
- B37.1 – Pulmonary candidiasis
- B37.2 – Candidiasis of skin and nail
- B37.5 – Candidal meningitis
- B37.6 – Candidal endocarditis
- B37.7 – Candidal sepsis
Documentation should clearly identify:
- Body site
- Type of candidiasis
- Systemic involvement
Aspergillosis ICD-10-CM Coding
Category B44.- Aspergillosis
Common ICD-10-CM codes include:
- B44.0 – Invasive pulmonary aspergillosis
- B44.1 – Other pulmonary aspergillosis
- B44.7 – Disseminated aspergillosis
- B44.81 – Allergic bronchopulmonary aspergillosis
Cryptococcosis ICD-10-CM Codes
Category B45.- Cryptococcosis
Common Codes:
- B45.0 – Pulmonary cryptococcosis
- B45.1 – Cerebral cryptococcosis
- B45.2 – Cutaneous cryptococcosis
- B45.7 – Disseminated cryptococcosis
This infection is frequently seen in advanced HIV disease.
Mucormycosis ICD-10-CM Coding
Category B46.- Mucormycosis (Zygomycosis)
Common Codes:
- B46.0 – Pulmonary mucormycosis
- B46.1 – Rhinocerebral mucormycosis
- B46.2 – Gastrointestinal mucormycosis
- B46.3 – Cutaneous mucormycosis
- B46.4 – Disseminated mucormycosis
This aggressive fungal infection is commonly associated with:
- Diabetes mellitus
- Cancer
- Organ transplantation
Penicillosis ICD-10-CM Code
Unlike other fungal infections, penicillosis has a single ICD-10-CM code.
ICD-10-CM Code:
- B48.4 – Penicillosis
This infection is commonly seen in:
- Southeast Asia
- Southern China
- Northeastern India
Parasitic Opportunistic Infection Coding
Parasitic infections may use:
- Single ICD-10-CM codes
- Site-specific codes
depending on the condition.
Cryptosporidiosis ICD-10-CM Code
ICD-10-CM Code:
- A07.2 – Cryptosporidiosis
Common symptoms include:
- Diarrhea
- Dehydration
- Abdominal cramping
- Weight loss
This condition does not have site-specific ICD-10-CM subdivisions.
Toxoplasmosis ICD-10-CM Coding
Category B58.- Toxoplasmosis
Common ICD-10-CM Codes:
- B58.0- – Toxoplasma oculopathy
- B58.1 – Toxoplasma hepatitis
- B58.2 – Toxoplasma meningoencephalitis
- B58.3 – Pulmonary toxoplasmosis
- B58.81 – Toxoplasma myocarditis
- B58.82 – Toxoplasma myositis
- B58.83 – Toxoplasma tubulo-interstitial nephropathy
Important Coding Reminder
Do not assign B58.- codes with:
- P37.1 – Congenital toxoplasmosis
B58.- codes apply only to acquired infections after birth.
Documentation Tips for Coding Opportunistic Infections
To support accurate ICD-10-CM coding, providers should document:
- Confirmed diagnosis
- Specific organism
- Body system involved
- Localized vs disseminated infection
- Associated manifestations
- HIV/AIDS or immunocompromised status
- Laboratory confirmation
Clear documentation improves:
- Coding accuracy
- Medical necessity support
- Reimbursement
- Clinical reporting quality
Final Thoughts on Opportunistic Infection Coding
Coding opportunistic infections requires strong knowledge of infectious disease terminology, ICD-10-CM guidelines, and clinical documentation standards. Since many infections affect multiple organ systems and involve highly specific organisms, coders must carefully review the medical record to assign the most accurate codes.
Understanding how ICD-10-CM organizes viral, bacterial, fungal, and parasitic infections can help coders
