What do “code first” and “use additional code” mean in ICD-10-CM?

In the FY2027 U.S. ICD-10-CM guidelines, “code first” and “use additional code” are instructions about the order of codes when the classification calls for more than one. In the familiar etiology/manifestation pattern, the underlying condition is sequenced first and the manifestation follows. A “use additional code” note appears with the etiology; a “code first” note appears with the manifestation. The FY2027 official guidelines, section I.A.13, describe this relationship.

The notes guide a lookup. They do not, by themselves, diagnose a condition or decide every code on a claim.

An etiology is the underlying cause in this classification pattern; a manifestation is the effect it produces in another body system. The official FY2027 guidelines place the etiology before the manifestation when that convention applies. The two notes often point at the same ordering from opposite sides of the pair: “use additional code” at the underlying-condition entry, and “code first” at the manifestation entry.

The guidelines add a useful Index check: when the Alphabetic Index displays an etiology followed by a manifestation code in brackets, the bracketed code is sequenced second. Many manifestation titles include “in diseases classified elsewhere”; the guidelines say those particular manifestation codes cannot be first-listed or principal diagnoses. Do not extend that title rule to every entry that merely contains a “code first” note. Read the actual CDC/NCHS FY2027 Index and Tabular files for the entry in front of you.

FY2027 section I.A.13 explicitly says these notes also serve as sequencing rules for certain codes outside the etiology/manifestation combination. Section I.B.7 explains that a “use additional code” note can call for a secondary code to describe a known detail even when the entry is not part of that pair. It says the secondary code should be added if known. For some non-manifestation entries with “code first,” a known, present underlying condition is sequenced first.

That distinction matters when the record lacks the detail a second code would identify. Do not fill the gap from an assumption or from a search snippet. Check the exact note, the relevant guideline, and the documentation. If the situation is unclear, a qualified coding professional should resolve the assignment under the applicable workflow. This guide explains how to navigate the reference; it does not assign codes to a patient.

1. Set the release. For an applicable FY2027 encounter, use the October 1, 2026 release; check the date and setting first. The CDC/NCHS release page gives the effective period, and our date-boundary guide explains the changeover.
2. Start from the documented term. Follow the FY2027 Alphabetic Index, including any bracketed manifestation entry.
3. Open the full hierarchy. Verify the candidate in the versioned FY2027 reference. Read the code, parent category, inherited notes, and any referenced code family.
4. Check what is actually known. Establish whether the underlying condition or additional detail named by the instruction is documented and applicable. Do not create a second diagnosis merely to satisfy a phrase on a page.
5. Apply the official sequencing rule. Compare the full entry with FY2027 sections I.A.13 and I.B.7 and the setting-specific guidance before finalizing an assignment.

The CDC/NCHS browser overview likewise describes moving between the Index, Tabular List, fiscal-year update, and instructional information. Our Index-to-Tabular guide gives the broader lookup method.

“Use additional code” does not mean every search result supplies a mandatory second code in every circumstance. “Code first” does not mean an unidentified underlying condition can be invented. The official instructions are tied to the specific code family and the information known for the encounter. Also distinguish these notes from “code also”: the FY2027 guidelines say a “code also” note may call for two codes but does not itself specify their order. The note actually present in the Tabular List controls the next check.

A manifestation code titled “in diseases classified elsewhere” has a stronger first-listed restriction under section I.A.13. Even then, a page about conventions cannot determine a principal diagnosis, payer result, or the complete sequence for a real encounter. Keep the full official guideline and the source entry with the work item.

When sharing a code or training note, include the classification, FY2027 release, and link to the complete entry so the recipient can see its instructions. The FY2027 source library points back to the official files. ICD Code Hub is also available on iPhone, Android, and Mac for reopening the reference away from your browser; confirm the release and full note on any device before using a saved code.