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📘 ICD-10-CM: Fast, Clear, Complete Crash Course

Your teacher for FY 2025 Guidelines + 2026 Addenda Updates

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🧱 PART 1 — What IS ICD-10-CM? (The Foundation)

⚠️ Rule #1: Never assign a code from the Alphabetic Index alone. Always verify in the Tabular List.

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🔤 PART 2 — Code Structure (How Codes Are Built)

  S72.141D
  │ │ │ │└─ 7th character = encounter type (D = subsequent)
  │ │ │└─── 6th character
  │ │ └──── 5th character = more detail
  │ └────── 4th character
  └──────── Category (3 characters) = S72 = femur fracture

• Codes are 3 to 7 characters long
• Always code to the highest level of specificity available
• A code that requires a 7th character is invalid without it
• If a code is only 5 characters but needs a 7th, fill the 6th spot with placeholder "X"

• T36.0 = category
• X = placeholder (no 6th character exists)
• 1 = 7th character (accidental)
• A = encounter (initial)

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🔑 PART 3 — The Big Abbreviations & Symbols (Memorize These)

Excludes1 vs Excludes2 - The Critical Distinction

• Excludes1: The conditions are mutually exclusive - code ONLY one. Example: congenital vs. acquired form of the same condition.
  • Exception: if the two conditions are clearly unrelated to each other, you can code both even with an Excludes1 note.
• Excludes2: The excluded condition is separate but can coexist. You MAY use both codes. Example: D12.6 (benign colon neoplasm) now has Excludes2 (not Excludes1) for inflammatory polyp - they can coexist.

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🔗 PART 4 — The Instructional Notes (Sequencing Rules)

The Etiology/Manifestation Rule (Big One)

Underlying cause ALWAYS goes first. Manifestation ALWAYS goes second.

1. First: G20.- (Parkinson's disease = the etiology)
2. Second: F02.80 (Dementia in diseases classified elsewhere = the manifestation)

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📋 PART 5 — The "With" and "And" Rules

• Example: "Tuberculosis of bones and joints" covers bones alone, joints alone, OR both.

• If the Tabular List or Alphabetic Index links two conditions with "with," code them as related even without explicit provider documentation - UNLESS documentation clearly says they're unrelated.
• Example: Diabetic patient with CKD - the "with" in the classification presumes the CKD is diabetic nephropathy.

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🏗️ PART 6 — General Coding Rules You Must Know

Acute vs. Chronic

• Same condition documented as BOTH acute and chronic? Code BOTH. Acute goes first.

Combination Codes

• One code that captures two diagnoses, or a diagnosis + complication. Use it when available - don't split it into separate codes unless it lacks specificity.

Sequela (Late Effects)

• Definition: Residual condition after the acute phase has ended. No time limit.
• Sequence: Residual condition FIRST, sequela/cause code SECOND.
• Never use the acute phase code together with a sequela code.

Signs & Symptoms

• Code signs/symptoms only when no definitive diagnosis has been established.
• If a symptom is integral to a disease process, do NOT code it separately.
• If it's NOT integral (unusual or additional), DO code it.

Laterality

• Always specify left/right when codes offer it.
• Bilateral? Use the bilateral code (not two separate codes) - even when treating one side at a time.
• If one side is treated and cured, switch to the remaining unilateral code.

Impending/Threatened Conditions

• If the condition did occur → code it as confirmed.
• If it did NOT occur → check the Index for "impending"/"threatened" subterms; if not listed, code the existing underlying condition.

Borderline Diagnosis

• Code as confirmed - unless the classification has a specific "borderline" entry (e.g., borderline diabetes).

Same Code Reported Twice

• Never report the same ICD-10-CM code more than once per encounter.

Who Can Document What

• BMI, pressure ulcer stage, coma scale, NIH stroke scale, blood alcohol level, social determinants of health (SDOH), laterality, underimmunization status.
• The diagnosis itself (obesity, stroke, etc.) still requires the provider.

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🗂️ PART 7 — The Four Sections of the Guidelines

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🏥 PART 8 — Section II: Principal Diagnosis (Inpatient)

Principal Diagnosis = The condition established after study to be chiefly responsible for the admission.

• Two interrelated conditions equally meeting the definition? Either may be sequenced first.
• Symptom + established diagnosis? Use the diagnosis, not the symptom.
• Uncertain diagnosis ("probable," "suspected," "likely," "questionable," "possible," "rule out," "compatible with," "consistent with")? → Code it as confirmed (inpatient only!).
• Original treatment plan not carried out? The condition that caused admission = principal diagnosis (even if treatment didn't happen).
• Complication from surgery? The complication = principal diagnosis.
• Admission from outpatient surgery? If complication occurred → complication is principal; if no complication → reason for the outpatient surgery is principal.
• Rehabilitation admission? The condition being rehabilitated = principal diagnosis. If that condition is resolved, use aftercare code.

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📊 PART 9 — Section III: Additional Diagnoses (Inpatient)

• Required clinical evaluation, OR
• Required therapeutic treatment, OR
• Required diagnostic procedures, OR
• Extended the hospital stay, OR
• Increased nursing care/monitoring

• Resolved conditions from prior admissions (no current impact)
• Abnormal findings UNLESS the provider indicates clinical significance

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🏪 PART 10 — Section IV: Outpatient Coding

• Surgery encounter? The reason for surgery = first-listed diagnosis (even if post-op problems occur).
• Chronic diseases treated on ongoing basis? Code every encounter.
• Code all coexisting conditions that are treated or affect management.
• Pre-op evaluation? Code the reason for surgery + any findings.

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🆕 PART 11 — 2026 Addenda: What Changed (ICD-10CM-Update-1.pdf)

New Codes Added (Big Ones):

Excludes1 → Excludes2 Conversions (Important Pattern):

• D12.6 - benign colon neoplasm: inflammatory polyp changed from Excludes1 → Excludes2 (can now code both)
• D53.8 - nutritional anemias: nutritional deficiencies without anemia changed to Excludes2
• D68.312 - antiphospholipid antibody: antiphospholipid syndrome changed to Excludes2

Revisions (Formatting/Spelling Fixes):

• C7B.04: "Mesentary" corrected to "Mesentery"
• E. coli notation standardized: E.coli → E. coli (space added)

Additions to Existing Codes:

• B18 (Chronic viral hepatitis): added "Use additional code, if applicable, for ascites (R18.8)"
• D48.11 (Desmoid tumor): "Aggressive fibromatosis" added as inclusion term

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⚡ PART 12 — Quick-Reference Rules (Test Prep Cheat Sheet)

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🧠 PART 13 — Memory Anchors (Learn It Once, Keep It Forever)

• 1 = One code only. "One at a time!"
• 2 = Two codes possible. "Two can play!"

• The root (etiology) always comes first
• The symptom/branch (manifestation) comes second

• NEC = you have detail, just no specific code for it → "Other specified"
• NOS = you DON'T have enough detail → "Unspecified"

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