ICD-10-CM Basics: What Every CPC Candidate Must Know

TL;DR — ICD-10-CM codes are 3 to 7 alphanumeric characters. Always go Alphabetic Index → Tabular List and read every instructional note. Know Excludes1 vs Excludes2, the "with" convention, and the 7th-character extensions for injuries. Expect 15–20 direct questions on the CPC.

ICD-10-CM is the foundation of the CPC exam. Roughly 15–20 questions test your ability to assign the correct diagnosis code — and many CPT/E/M questions also depend on choosing the right diagnosis first.

#Code structure

ICD-10-CM codes are 3 to 7 alphanumeric characters:

E11.9      → Type 2 diabetes mellitus without complications
S52.521A   → Torus fracture of lower end of right radius, initial encounter

• First character: always a letter
• Characters 2–3: numeric (the category)
• Characters 4–6: etiology, anatomic site, severity
• Character 7: extension (often A, D, S for injuries)

If a 7th character is required and the code is fewer than 6 characters, use the placeholder X.

#The conventions that trip people up

#Excludes1 vs Excludes2

• Excludes1 — the two conditions can never be coded together
• Excludes2 — both can be coded together when both are present

#"With" and "And"

• "With" — assumes a causal relationship in the alphabetic index (e.g., diabetes with CKD)
• "And" in a code title — means and/or

#"Code first" / "Use additional code"

Sequencing matters. The instructional note tells you which condition is primary.

#High-yield chapters

Spend extra time on:

1. Chapter 1 — Infectious diseases (sepsis sequencing!)
2. Chapter 4 — Endocrine (diabetes combination codes)
3. Chapter 9 — Circulatory (hypertension, heart failure)
4. Chapter 15 — Pregnancy (trimester rules)
5. Chapter 19 — Injury (7th character extensions)
6. Chapter 21 — Z codes (status, history, screening)

When you see an ICD-10-CM question:

1. Read the scenario twice
2. Identify the main term (not the anatomy)
3. Look it up in the Alphabetic Index first
4. Verify in the Tabular List
5. Check all instructional notes at category, subcategory, and code level

Skipping the tabular verification is the #1 cause of wrong answers. Don't do it.

Once these basics click, ICD-10-CM stops feeling like a maze and starts feeling like a well-indexed reference. For the bigger study picture, see /blog/how-to-prepare-for-cpc.