3.3 ICD-10-CM conventions, specificity & sequencing

Conventions come first

Conventions are the rules, symbols, and instructional notes built directly into ICD-10-CM. They are not optional: the Official Guidelines state that conventions take precedence over the guidelines themselves and must be followed. Recognizing them is one of the most heavily tested skills on the CBCS exam.

Includes and Excludes notes

• Includes notes appear under a category to further define, or give examples of, its content.
• Excludes1 — "NOT CODED HERE." This means the two conditions are mutually exclusive and can never be reported together, because a patient cannot have both forms at once (for example, a congenital versus an acquired form of the same condition).
• Excludes2 — "NOT INCLUDED HERE." This means the excluded condition is not part of the code above it, but a patient may have both — so you may report both codes together when both are documented.

Distinguishing Excludes1 from Excludes2 is one of the most frequently tested conventions on the exam.

"Code first" and "use additional code"

These paired notes signal mandatory sequencing and etiology/manifestation relationships.

• Code first directs you to sequence the underlying cause (etiology) before the manifestation.
• Use additional code directs you to add a second code for a related condition or manifestation, sequenced afterward.
• Manifestation codes described as "in diseases classified elsewhere" are never sequenced first; in the Index they appear in brackets after the etiology code.

NEC and NOS

• NEC — Not Elsewhere Classifiable means "other specified." The provider documented a specific condition, but ICD-10-CM has no unique code for it.
• NOS — Not Otherwise Specified means "unspecified." The documentation lacks the detail needed to assign a more specific code.

The word "with"

The word "with" or "in" in the Alphabetic Index or Tabular List means the two conditions are presumed to be linked whenever both are present, even if the provider did not explicitly connect them — unless the documentation states the conditions are unrelated. The classic example is diabetes "with" chronic kidney disease, coded as a diabetic complication. In the Index, "with" terms are listed immediately after the main term, not in alphabetical order.

Punctuation and cross-references

Several smaller conventions round out the set. Brackets [ ] enclose synonyms, alternative wording, or — in the Alphabetic Index — the manifestation code that must be sequenced second. Parentheses ( ) enclose nonessential modifiers that do not affect code assignment. A colon in the Tabular List follows an incomplete term that needs one of the modifiers listed beneath it. The word "and" in a code title actually means "and/or." A "see" cross-reference is a mandatory instruction to look under a different term, while "see also" is optional guidance the coder may follow if the first entry does not fit. When the Index shows a single default code beside a main term, that default is assigned only when the documentation offers no further detail.

Coding to the highest specificity

Coders must assign codes to the highest level of specificity — the full number of characters available and the most precise code the documentation supports. An unspecified code is acceptable only when the record genuinely lacks the detail for a more specific code; it is never a shortcut for lazy abstraction. A code that is missing a required character, a required seventh character, or a laterality digit is invalid and will be rejected by claims-editing software. When documentation is ambiguous or incomplete, the correct action is to query the provider, not to guess at the detail.

Sequencing: first-listed and principal diagnosis

Order matters. The condition chiefly responsible for the encounter is listed first.

• Principal diagnosis (inpatient): the condition, established after study, that was chiefly responsible for the patient's admission to the hospital.
• First-listed diagnosis (outpatient / physician office): the main reason for the encounter, listed first, followed by any coexisting conditions that were treated or that affected care.
• Report each diagnosis to the highest degree of certainty. In the outpatient setting, do not code "probable," "suspected," or "rule out" conditions as if confirmed — code the documented signs and symptoms instead. (Inpatient rules differ and permit coding uncertain diagnoses documented at discharge.)

The signs and symptoms rule

• When a definitive diagnosis has been established, code the diagnosis and not the signs and symptoms that are integral to it. For instance, do not separately code "abdominal pain" when the confirmed diagnosis is acute appendicitis.
• Do code signs and symptoms when no definitive diagnosis has been established, or when a symptom is not routinely associated with the confirmed condition.
• Codes from Chapter 18 (R00–R99, Symptoms, Signs, and Abnormal Findings) are used when no more specific diagnosis can be made.