---
name: icd10-coding-discipline
description: "Assigns ICD-10-CM diagnosis codes to the highest documented specificity — honoring laterality, the required 7th character for injuries, combination codes, principal-diagnosis sequencing, and Z-codes for status/history — and never codes an unconfirmed diagnosis as confirmed. Use when a clinical note, encounter summary, or chart is handed over and someone asks for the ICD-10-CM diagnosis code(s) or how a condition should be coded. Do NOT use for CPT/HCPCS procedure coding, drug or treatment decisions, claim appeals, or clinical documentation improvement queries that aren't about code selection."
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  decimalai-version-id: "2c090e38-2e14-4f84-aae9-33a0f2b01fc8"
  decimalai-version: "1"
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  decimalai-name: "icd10-coding-discipline"
  decimalai-format: "1"
  decimalai-invocation: "model"
  decimalai-category: "science"
  decimalai-source: "https://app.decimal.ai/s/icd10-coding-discipline@1/SKILL.md"
---
# Assign ICD-10-CM to the highest documented specificity

Given a documented clinical encounter, select the diagnosis code(s) that capture exactly what the
provider wrote — no less specific, no more inferred. The failure mode this fixes: emitting a
plausible-looking code that drops the required last character, ignores a documented side, splits one
condition into two codes, lists the wrong diagnosis first, uses an active-disease code for a resolved
one, or treats a "rule out" as if it were confirmed. Apply the rules below in order; when a fact
pattern sits on the edge, consult the chapter catalog in `references/chapter-conventions.md`.

## The selection rules

1. **Code to the highest documented specificity.** If the note documents a detail that a more
   specific code captures, use that code — never fall back to an "unspecified" variant when the
   documentation supports a precise one. Specificity is driven by what is written, not by what is
   easiest to find.

2. **Honor laterality.** When the condition is on a paired site (limb, eye, ear, kidney, lung,
   breast, ovary) and the note states which side, pick the right / left / bilateral code. Reach for
   an unspecified-side code only when the side genuinely is not documented.

3. **Add the required last character on injuries and other 7th-character categories.** Fractures,
   other injuries, burns, poisonings, and several external-cause and obstetric categories require a
   final character that names the encounter type: `A` for the active-treatment (initial) phase, `D`
   for the routine-healing / recovery (subsequent) phase, and `S` for a sequela — a lasting effect
   of a prior injury coded alongside the residual condition. If the base code is shorter than seven
   characters, pad the empty positions with the placeholder `X` so the encounter character lands in
   the seventh slot. A code in one of these categories without its final character is incomplete.

4. **Prefer a combination code over multiple codes.** When one code captures both a condition and
   its manifestation, complication, or a paired condition (e.g. diabetes with a named complication,
   or hypertension documented with related chronic kidney disease), assign that single combination
   code instead of two separate ones. Split into multiple codes only when no combination code exists.

5. **Sequence the principal / first-listed diagnosis correctly.** List first the condition chiefly
   responsible for the encounter (the reason for the admission or visit), then the secondary
   conditions. When a code carries a "code first" / etiology-then-manifestation instruction, sequence
   the underlying cause before the manifestation it produced.

6. **Use Z-codes for status, history, screening, and aftercare — not the active-disease code.** A
   condition that is resolved and only being monitored, a personal or family history, an
   asymptomatic screening visit, or routine aftercare gets the appropriate Z-code. Reserve the
   active-disease code for a condition that is currently present and being treated.

7. **Never code an unconfirmed diagnosis as confirmed.** In the outpatient / physician setting, a
   diagnosis qualified as "probable," "suspected," "likely," "questionable," "rule out," or
   "consistent with" is not established — code the documented sign, symptom, or reason for the visit
   instead of the uncertain condition.

8. **Code only what the provider documented.** Do not assign a diagnosis the note does not state,
   however strongly a medication, lab value, or history hints at it. If it isn't documented as a
   diagnosis, it isn't coded.

## Output

State the code(s) in sequence order, and for each say briefly why that code (the specificity,
side, encounter character, combination, or Z-code choice) — so the selection is auditable. When the
documentation is too thin to reach the specificity a code demands, say what additional documentation
would be needed rather than inventing detail.

## Deeper lookup

`references/chapter-conventions.md` — a chapter-by-chapter catalog of the special conventions
(7th-character injury rules, Z-code families, combination-code pairs, etiology/manifestation
sequencing, neoplasm-table and obstetric specifics) that the rules above point to. It is a lookup
reference, not the decision rules; the rules live here in the body.