ICD-10-CM has two distinct sets of guidelines depending on the care setting — one for outpatient encounters and one for inpatient admissions. These guidelines differ in meaningful ways and applying the wrong set of rules to the wrong setting is one of the most consistently tested error types on the CPC exam. This article makes the differences crystal clear so you always know which rules to apply.
Where the Guidelines Come From
The ICD-10-CM Official Guidelines for Coding and Reporting are published annually by CMS and the NCHS (National Center for Health Statistics). The guidelines are organized into four sections. Section I contains general coding guidelines that apply to all settings. Section II covers inpatient hospital guidelines. Section III covers reporting of additional diagnoses in the inpatient setting. Section IV covers outpatient coding guidelines. On the CPC exam, Sections II and IV are most heavily tested because of their key differences.
Side-by-Side Comparison
🏥 Inpatient Guidelines (Section II)
- Term used: Principal diagnosis
- Selected after study — at time of discharge
- Uncertain diagnoses (probable, suspected, likely) coded AS IF confirmed
- Signs/symptoms integral to confirmed diagnosis not coded separately
- Admitting diagnosis reported separately (UB-04 form)
- Governed by UHDDS definition
- Used for acute inpatient hospital stays
🏢 Outpatient Guidelines (Section IV)
- Term used: First-listed diagnosis
- Selected at time of the encounter
- Uncertain diagnoses NEVER coded as confirmed — code signs/symptoms
- Signs/symptoms coded when no definitive diagnosis established
- No admitting diagnosis concept
- Applies to all outpatient and physician office settings
- Used for office visits, clinics, ED, same-day surgery, home health
The Uncertain Diagnosis Rule — The Most Tested Difference
The single most important difference between inpatient and outpatient guidelines — and the one tested most frequently on the CPC exam — is how uncertain diagnoses are handled.
Inpatient — Uncertain Diagnoses Coded as Confirmed
In the inpatient setting, if the physician documents a condition as “probable,” “suspected,” “likely,” “questionable,” “possible,” or “still to be ruled out” at the time of discharge — that condition may be coded as if it were confirmed. The rationale is that the patient has undergone extensive workup during the admission and the physician’s clinical judgment at discharge represents their best assessment.
Words that trigger this rule in the inpatient setting include: probable, suspected, likely, questionable, possible, consistent with, compatible with, and indicative of.
Outpatient — Uncertain Diagnoses NEVER Coded as Confirmed
In the outpatient setting, the opposite rule applies. You never code a probable, suspected, or rule-out diagnosis as confirmed. Instead you code the sign or symptom that brought the patient in for care. The physician has not had the opportunity to complete a full workup and their uncertainty must be reflected in the code assignment.
Additional Diagnoses — Inpatient vs Outpatient
The rules for what gets coded as an additional diagnosis also differ by setting:
| Situation | Inpatient | Outpatient |
|---|---|---|
| Chronic conditions | Code if monitored, treated, or affects care | Code if documented and affects management |
| Conditions that develop during stay | Code as additional diagnoses | Code if present and treated during encounter |
| Abnormal test findings | Code only if clinical significance is documented | Code if no definitive diagnosis established |
| Integral symptoms | Do not code separately when diagnosis confirmed | Do not code separately when diagnosis confirmed |
| Uncertain conditions | Code as confirmed if documented at discharge | Code signs/symptoms — never as confirmed |
Settings That Are Always Outpatient
It is important to know which care settings use outpatient guidelines regardless of how the visit is described. These are always outpatient settings governed by Section IV:
- Physician office visits and group practice clinics
- Hospital outpatient departments
- Emergency department encounters (even when the patient is later admitted)
- Same-day surgery (ambulatory surgery centers)
- Observation status stays (billed as outpatient even if overnight)
- Home health encounters
- Telehealth visits