NEOPLASM TABLE:
  Determine: primary, secondary, in situ, benign, uncertain, unspecified
  C codes: malignant primary neoplasms
  C77-C79: secondary (metastatic) neoplasms
  D00-D09: carcinoma in situ
  D10-D36: benign neoplasms

  Coding sequence for cancer:
  1. Primary site (if known and being treated)
  2. Secondary sites
  3. Comorbidities affecting care
  4. Z codes for follow-up if cancer excised

OBSTETRIC CODING (O codes):
  7th character = trimester or delivery
    1 = first trimester (less than 14 weeks)
    2 = second trimester (14-27 weeks)
    3 = third trimester (28+ weeks)
    4 = postpartum / delivery
  Always code O80 Normal delivery when applicable
  Principal dx must be O code when delivery occurs

EXTERNAL CAUSE CODES (V/W/X/Y):
  Never principal diagnosis — always supplementary
  Activity code (Y93): what patient was doing
  Place of occurrence (Y92): where injury happened
  Status (Y99): work/leisure/military

POISONING vs ADVERSE EFFECT vs UNDERDOSING:
  Poisoning: overdose or wrong substance — T codes, sequence first
  Adverse effect: correct drug, correct dose, bad reaction — T code last
  Underdosing: taking less than prescribed — T code, complications second
  Combination codes available for drug + manifestation

HIV CODING:
  Z21: asymptomatic HIV infection (positive test, no symptoms)
  B20: HIV disease (symptomatic) — sequence first
  Never use inconclusive test result codes for confirmed HIV
  Confirmed by provider statement, not lab alone

SEQUENCING HIERARCHY:
  1. Etiology/manifestation convention (code etiology first)
  2. Two or more diagnoses equally meeting definition of principal
  3. Two or more comparative/contrasting diagnoses
  4. Original treatment plan not carried out
  5. Complications of surgery and other treatment

SIGNS AND SYMPTOMS:
  Code when no definitive diagnosis established
  Do not code if sign/symptom integral to confirmed diagnosis
  R codes acceptable as principal dx in outpatient setting

CHRONIC CONDITIONS:
  Always code if documented and affect patient care
  No limit on number of codes per encounter
  Conditions documented as "history of" — use Z codes if resolved

UNCERTAINTY WORDS (inpatient only):
  "Probable," "suspected," "likely," "possible" — CODE as if confirmed
  (Outpatient: code signs/symptoms, not uncertain diagnoses)