When a patient has multiple diagnoses at a single encounter, which one goes first on the claim? The answer depends entirely on the setting — inpatient hospital or outpatient. The terms used are different, the selection rules are different, and mixing them up is one of the most common errors beginner coders make. This article clears up the confusion once and for all.
Two Settings, Two Different Terms
The ICD-10-CM Official Guidelines for Coding and Reporting are divided into separate sections for inpatient and outpatient coding — and for good reason. The rules governing which diagnosis is reported first differ significantly between the two settings. Understanding which term applies in which setting is the foundation of this topic.
🏥 Inpatient — Principal Diagnosis
Used when a patient is admitted to an acute care hospital and occupies a bed overnight. The principal diagnosis is selected after study — meaning after all workup and treatment during the stay is complete.
🏢 Outpatient — First-Listed Diagnosis
Used in physician offices, clinics, emergency departments, same-day surgery, and outpatient settings. The first-listed diagnosis is the main reason for the visit — determined at the time of the encounter.
Principal Diagnosis — Inpatient Rule
The Uniform Hospital Discharge Data Set (UHDDS) defines the principal diagnosis as the condition established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care. Three words in this definition are critical: after study.
This means the principal diagnosis is not necessarily what the admitting physician wrote on the admission order. It is the condition determined — after all testing, imaging, consultations, and treatment during the hospitalization — to be the primary reason the patient needed to be admitted. The admitting diagnosis and the principal diagnosis may be the same, but they are often different once the workup is complete.
Inpatient Principal Diagnosis Selection Rules
- If two or more diagnoses equally meet the criteria for principal diagnosis, either may be sequenced first — unless the guidelines or payer-specific instructions provide direction
- When the principal diagnosis is still uncertain at discharge, code the condition to the highest degree of certainty — suspected, probable, or likely diagnoses may be coded as if confirmed in the inpatient setting
- Symptoms that are integral to a confirmed diagnosis are not coded separately — code the confirmed diagnosis
- Complications of surgery or procedures — if they require extended hospitalization — may become the principal diagnosis
First-Listed Diagnosis — Outpatient Rule
In the outpatient setting, the term first-listed diagnosis replaces principal diagnosis. The first-listed diagnosis is the main condition treated or investigated during the encounter. For outpatient visits, code the condition to the highest degree of certainty — but do NOT code uncertain diagnoses as confirmed.
Outpatient First-Listed Diagnosis Selection Rules
- Code the condition, disease, or injury being managed, treated, or investigated as the first-listed diagnosis
- If the visit is for a sign or symptom and no definitive diagnosis has been established, code the sign or symptom as the first-listed diagnosis
- Do NOT code probable, suspected, rule-out, or questionable diagnoses as confirmed — code the signs and symptoms instead
- For chronic conditions being managed on an ongoing basis, code the chronic condition even if not the focus of the current visit
- For pre-operative evaluations, code the reason for the surgery as the first-listed diagnosis
- For encounters for test results only — and no treatment is provided — code the sign or symptom that prompted the test
Side-by-Side Comparison
| Feature | Principal Diagnosis (Inpatient) | First-Listed Diagnosis (Outpatient) |
|---|---|---|
| Setting | Acute inpatient hospital admission | Physician office, clinic, ED, outpatient surgery |
| Timing of selection | After study — determined at discharge | At the time of the encounter |
| Uncertain diagnoses | Code as if confirmed (probable, likely, suspected) | Never code as confirmed — code signs/symptoms instead |
| Governing guidelines | UHDDS definition + Section II of ICD-10-CM guidelines | Section IV of ICD-10-CM guidelines |
| Admitting diagnosis | Reported separately from principal diagnosis | Not applicable |
Additional Diagnoses and Comorbidities
Beyond the first-listed or principal diagnosis, additional diagnoses are reported when they affect patient care during the encounter. In the inpatient setting, additional diagnoses are called secondary diagnoses and include comorbidities, complications, and other conditions that require clinical evaluation, therapeutic treatment, diagnostic procedures, extended hospital stay, or nursing care. In the outpatient setting, additional diagnoses are coded when they are treated, affect treatment decisions, or are chronic conditions managed alongside the primary condition.