── ICD-10-CM CODE STRUCTURE ──────────────────────────────
Format: XXX.XXXX
  First 3 characters: Category (e.g., J06 = Acute upper respiratory infections)
  Characters 4-7: Additional specificity
  Decimal after 3rd character

── CODING PROCESS ────────────────────────────────────────
1. Identify the main term in documentation
2. Look up in Alphabetic Index (Volume 2)
3. Verify in Tabular List (Volume 1) - NEVER code from index alone
4. Review any instructions (includes, excludes, notes)
5. Select highest level of specificity
6. Sequence correctly (principal/primary diagnosis first)

── PLACEHOLDER X ─────────────────────────────────────────
X is used as a placeholder to allow future expansion
Example: T36.0X1A = Poisoning by penicillins, accidental, initial encounter
The X in position 6 is a placeholder

── 7th CHARACTER EXTENSIONS ─────────────────────────────
For injuries and external causes:
A = Initial encounter (active treatment)
D = Subsequent encounter (routine care after active phase)
S = Sequela (late effect/complication)

For fractures:
A = Initial encounter, closed fracture
B = Initial encounter, open fracture type I/II
G = Subsequent encounter, delayed healing
K = Subsequent encounter, nonunion
P = Subsequent encounter, malunion
S = Sequela

── EXCLUDES NOTES ────────────────────────────────────────
Excludes1: Code should NEVER be used with the excluded code
  (mutually exclusive conditions)
Excludes2: Code for excluded condition may be added IF
  both conditions exist simultaneously (not included here)

── COMMON CODE CATEGORIES ────────────────────────────────
A00-B99: Infectious and parasitic diseases
C00-D49: Neoplasms
E00-E89: Endocrine, nutritional, metabolic
F01-F99: Mental, behavioral, neurodevelopmental
G00-G99: Nervous system
I00-I99: Circulatory system
J00-J99: Respiratory system
K00-K95: Digestive system
M00-M99: Musculoskeletal/connective tissue
N00-N99: Genitourinary system
O00-O9A: Pregnancy, childbirth
S00-T88: Injuries, poisoning, external causes
Z00-Z99: Factors influencing health status
V00-Y99: External causes of morbidity

── SEQUENCING RULES ──────────────────────────────────────
Outpatient first-listed = reason for visit after study
Inpatient principal = reason after study + workup
Secondary = additional conditions affecting care
External cause codes = never principal/first-listed
Manifestation codes = never principal/first-listed