Medical Coding Infectious Diseases

Key ICD-10-CM Guidelines for Infectious Diseases

Code the Confirmed Diagnosis

• Only code confirmed infections when documented by the provider
• Terms like “suspected” or “rule out” are not coded as confirmed in outpatient settings

Identify the Causative Organism

• Many conditions require an additional code to identify the organism
• Example:
  • Pneumonia due to Streptococcus → code both condition + organism (if not included)

Use Combination Codes When Available

• ICD-10-CM often provides combination codes that include:
  • Disease + organism
  • Disease + manifestation

Example:

• J10.0 → Influenza with pneumonia (no additional code needed)

Understand “Code First” and “Use Additional Code” Notes

• These instructional notes are critical:
  • Code First → underlying condition must be sequenced first
  • Use Additional Code → add organism or manifestation code

ICD-10-CM Coding Conventions 

Sepsis Coding Rules (High Priority Topic)

Sepsis coding is heavily tested on the CPC exam.

Key rules:

• Code underlying systemic infection first (A40–A41)
• Add codes for:
  • Severe sepsis (R65.2-)
  • Organ dysfunction (if documented)
• If septic shock is present → assign R65.21

Medical Coding Sepsis 

Acute vs. Chronic Infections

• Always code acute conditions first when both acute and chronic are documented
• Follow guideline: “acute and chronic” → code both, acute first

Acute vs Chronic in Medical Coding 

Medical Coding for Chronic Conditions 

HIV Coding (Another CPC Favorite)

• B20 → confirmed HIV disease
• Z21 → asymptomatic HIV
• Once diagnosed with B20, the patient is always coded as B20 in future encounters

Medical Coding HIV/AIDS 

Antibiotic Resistance

• Use Z16 codes to capture drug resistance when documented
• Important for risk adjustment and clinical accuracy

Practical Coding Tips for Infectious Diseases

Always Code From Documentation — Not Assumptions

• Never assume bacterial vs viral
• The provider must document it clearly

Watch for Manifestations

• Example:
  • Diabetes with infection
  • Postprocedural infections
• These often require multiple codes or sequencing rules

What Is a Manifestation Code in Medical Coding 

Know When Symptoms Are NOT Coded

• Do not code symptoms when:
  • They are integral to the diagnosis
• Example:
  • Fever with sepsis → do not code fever separately

Medical Coding Signs & Symptoms in Diseases 

Pay Attention to Laterality and Site

• Skin infections, abscesses, cellulitis → require specific site coding

Read the Entire Note

• Infectious disease details are often buried in:
  • Lab results
  • Assessment/Plan
  • Culture reports

Common Infectious Disease Coding Scenarios

Scenario 1: Cellulitis of Right Lower Leg

• ICD-10-CM: L03.115
• If organism identified → add additional code

Scenario 2: UTI Due to E. coli

• N39.0 → UTI
• B96.20 → E. coli as cause

Scenario 3: Sepsis Due to Pneumonia

• A41.9 → Sepsis
• J18.9 → Pneumonia
• Sequence depends on documentation and guidelines

CPC Exam Tips for Infectious Disease Coding

Focus on the “Real Ask”

• Is the question asking for:
  • The infection?
  • The organism?
  • The complication?

Follow the Coding Process

Your step-by-step workflow:

1. Identify the diagnosis
2. Look up in the Alphabetic Index
3. Verify in the Tabular List
4. Apply all notes (Includes, Excludes, Code First, etc.)

Watch for Tricky Wording

• “With” → implies a relationship in ICD-10-CM
• “Due to” → indicates causation

Eliminate Wrong Answers Strategically

• Remove options that:
  • Don’t match the site
  • Ignore sequencing rules
  • Miss organism coding

Time Management Strategy

• Infectious disease questions can be detailed
• If it’s long → flag it and return later

Memorize High-Yield Topics

Focus on:

• Sepsis
• HIV
• Pneumonia
• UTIs
• Postprocedural infections

Coding Clarified Final Takeaway for CPC Students

Infectious disease coding is all about precision and guideline mastery. If you:

• Follow sequencing rules
• Identify organisms correctly
• Apply “code first” and “use additional code” notes
• Avoid assumptions

You will not only pass the CPC exam—you’ll code confidently in real-world scenarios.

College of American Pathologists