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
• In outpatient settings, terms like “suspected” or “rule out” do not get coded as confirmed

Identify the Causative Organism

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

Use Combination Codes When Available

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

For 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 → sequence the underlying condition first
  • Use Additional Code → add organism or manifestation code

ICD-10-CM Coding Conventions 

Sepsis Coding Rules (High Priority Topic)

The CPC exam tests sepsis coding heavily, so learn these rules well.

Key rules:

• Code underlying systemic infection first (A40–A41)
• Then 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
• When a provider diagnoses B20, always code the patient as B20 in future encounters

Medical Coding HIV/AIDS 

Antibiotic Resistance

• Use Z16 codes to capture drug resistance when documented
• This matters because it supports risk adjustment and clinical accuracy

Practical Coding Tips for Infectious Diseases

Always Code From Documentation — Not Assumptions

• Never assume bacterial vs viral
• Instead, the provider must document it clearly

Watch for Manifestations

• For example:
  • Diabetes with infection
  • Postprocedural infections
• These also 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
• For 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

• Key infectious disease details often hide 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
• However, the 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. Then look up the term in the Alphabetic Index
3. After that, verify the code in the Tabular List
4. Finally, 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 often include many details
• If it’s long → flag it and return later

Memorize High-Yield Topics

In particular, focus on:

• Sepsis
• HIV
• Pneumonia
• UTIs
• Postprocedural infections

Coding Clarified Final Takeaway for CPC Students

In short, 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 

Ready to become a certified medical coder? Our online CPC certification training prepares you for the AAPC CPC exam with a credentialed instructor. Many students train at little or no cost through medical coding scholarships and WIOA grants available in 30+ states.