Is first-listed the same as principal diagnosis?

Not for outpatient reporting. Principal diagnosis is an inpatient concept built on the Uniform Hospital Discharge Data Set definition, which does not apply to outpatient services and office visits.

What if the clinician documents rule out pneumonia?

Do not code the uncertain diagnosis. Report the signs, symptoms, or abnormal test results that describe what was known at the encounter.

Can the first-listed code be a Z code?

Yes. Encounters for circumstances other than a disease or injury, such as preventive visits, use Z codes. The code for the reason for the encounter is still reported first.

What if two conditions seem equally responsible?

The guidelines treat complete documentation as a shared responsibility between the provider and the coder. When the record does not support a confident first-listed choice, the missing piece is documentation, not coding judgment.

ICD-10-CM guide

First-Listed Diagnosis: The Outpatient Rule and Common Mistakes

What first-listed means in outpatient coding, how it differs from principal diagnosis, and the ordering mistakes that cost accuracy.

Reviewed 2026-09-16 | 3 min read

First-listed diagnosis sounds like a formatting rule. It is a decision rule. On an outpatient report, the first code describes why the patient was seen, and everything after it follows from that choice. Get the first line wrong and the rest of the report looks wrong, even when every code is valid.

What the term means

In the outpatient setting, first-listed diagnosis is used in place of principal diagnosis. The Uniform Hospital Discharge Data Set definition of principal diagnosis does not apply to hospital-based outpatient services or to provider-based office visits.

The first-listed diagnosis is the diagnosis, condition, problem, or other reason shown in the record to be chiefly responsible for the services provided. It may be a confirmed diagnosis, or it may be a symptom when no definitive diagnosis has been established.

It is not a severity ranking

Coders sometimes order codes by how serious a condition sounds. That is not the rule. A chronic condition managed on an ongoing basis can be first-listed when it drove the visit, and an alarming incidental finding that required no workup may not belong on the report at all.

Chronic conditions treated on an ongoing basis may be coded and reported as many times as the patient receives treatment for them. That repetition does not by itself make them first-listed.

It can be a symptom

When the provider has not confirmed a related definitive diagnosis by the end of the encounter, reporting the symptom is correct. Signs and symptoms are acceptable for reporting purposes when a related definitive diagnosis has not been established.

The encounter type changes the answer

Outpatient surgery: code the reason for the surgery as first-listed, even when the procedure is not performed because of a contraindication.
Observation stay: assign the medical condition as first-listed. When surgery leads to observation, report the reason for the surgery first and the complications as secondary diagnoses.
Diagnostic services only: sequence first the condition chiefly responsible for the services provided. Other diagnoses, including chronic conditions, may follow as additional diagnoses.

Common mistakes

Coding a probable, suspected, or rule out diagnosis as if it were confirmed. Do not code it at all, and report the known level of certainty instead.
Reporting a symptom that is routinely associated with a confirmed diagnosis. The definitive diagnosis goes first, and the routine symptom is normally not added.
Ordering by severity or by the longest history rather than by the purpose of the encounter.
Keeping conditions on the report that were treated previously and no longer exist.

Questions coders ask

Is first-listed the same as principal diagnosis?: Not for outpatient reporting. Principal diagnosis is an inpatient concept built on the Uniform Hospital Discharge Data Set definition, which does not apply to outpatient services and office visits.
What if the clinician documents rule out pneumonia?: Do not code the uncertain diagnosis. Report the signs, symptoms, or abnormal test results that describe what was known at the encounter.
Can the first-listed code be a Z code?: Yes. Encounters for circumstances other than a disease or injury, such as preventive visits, use Z codes. The code for the reason for the encounter is still reported first.
What if two conditions seem equally responsible?: The guidelines treat complete documentation as a shared responsibility between the provider and the coder. When the record does not support a confident first-listed choice, the missing piece is documentation, not coding judgment.

Practice these rules

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Sources

ICD-10-CM Official Guidelines for Coding and Reporting FY 2027
Section IV introduction, UHDDS definition and outpatient scope
ICD-10-CM Official Guidelines for Coding and Reporting FY 2027
Section IV.A, Selection of first-listed condition
ICD-10-CM Official Guidelines for Coding and Reporting FY 2027
Section IV.A.1 and IV.A.2, outpatient surgery and observation
ICD-10-CM Official Guidelines for Coding and Reporting FY 2027
Section IV.D, Codes that describe symptoms and signs
ICD-10-CM Official Guidelines for Coding and Reporting FY 2027
Section IV.H, Uncertain diagnosis
ICD-10-CM Official Guidelines for Coding and Reporting FY 2027
Section IV.J, Code all documented conditions that coexist
ICD-10-CM Official Guidelines for Coding and Reporting FY 2027
Section IV.K, Patients receiving diagnostic services only
ICD-10-CM Official Guidelines for Coding and Reporting FY 2027
Section I.C.18.b, Use of a symptom code with a definitive diagnosis code