Selection of principal diagnosis — Section II
The UHDDS definition, and the eleven rules that decide which of several conditions is sequenced first on an inpatient record.
The circumstances of inpatient admission always govern the selection of the principal diagnosis. The UHDDS defines it as that condition established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care. Two things outrank these guidelines: the coding conventions in the Tabular List and the Alphabetic Index.
The UHDDS definitions are used by hospitals to report inpatient data elements in a standardised manner, and can be found in the Federal Register of 31 July 1985 (Vol. 50, No. 147), pp. 31038–40. Their application has since been expanded to all non-outpatient settings — acute care, short term, long term care and psychiatric hospitals, home health agencies, rehabilitation facilities, nursing homes — and they also apply to hospice services at all levels of care.
The eleven principal diagnosis rules#
| Rule | What to do | |
|---|---|---|
| A | Symptoms, signs and ill-defined conditions | NEVER Chapter 18 codes are not used as principal diagnosis when a related definitive diagnosis has been established |
| B | Two or more interrelated conditions, each potentially meeting the definition | Either condition may be sequenced first — unless the circumstances of the admission, the therapy provided, the Tabular List or the Alphabetic Index indicate otherwise. Interrelated means diseases in the same chapter, or manifestations characteristically associated with a certain disease |
| C | Two or more diagnoses that equally meet the definition | In this unusual instance, where the circumstances of admission, the diagnostic workup and/or therapy provided, the Index, the Tabular List and other guidelines give no sequencing direction, any one of them may be sequenced first |
| D | Two or more comparative or contrasting conditions documented as “either/or” | They are coded as if the diagnoses were confirmed and sequenced according to the circumstances of the admission. If no further determination can be made, either may be sequenced first |
| E | A symptom followed by contrasting or comparative diagnoses | EXCEPTION Guideline deleted effective October 1, 2014 |
| F | Original treatment plan not carried out | Sequence as principal diagnosis the condition which, after study, occasioned the admission — even though treatment may not have been carried out due to unforeseen circumstances |
| G | Complications of surgery and other medical care | The complication code is the principal diagnosis. If the complication is classified to T80–T88 and that code lacks the necessary specificity, assign an additional code for the specific complication |
| H | Uncertain diagnosis | A diagnosis qualified at discharge as probable, suspected, likely, questionable, possible, still to be ruled out, compatible with, consistent with, or other similar terms, is coded as if it existed or was established. EXCEPTION Applicable only to inpatient admissions to short-term, acute, long-term care and psychiatric hospitals |
| I.1 | Admission following medical observation | Where a patient is admitted to observation for a medical condition which worsens or does not improve, and is subsequently admitted as an inpatient of the same hospital for that same condition, the principal diagnosis is the medical condition which led to the hospital admission |
| I.2 | Admission following post-operative observation | Where a patient is admitted to observation to monitor a condition or complication developing after outpatient surgery, and is subsequently admitted as an inpatient of the same hospital, apply the UHDDS definition of principal diagnosis |
| J | Admission from outpatient surgery | Reason for the inpatient admission is a complication → the complication is the principal diagnosis. No complication or other condition documented as the reason → the reason for the outpatient surgery is the principal diagnosis. Reason is another condition unrelated to the surgery → the unrelated condition is the principal diagnosis |
| K | Admissions or encounters for rehabilitation | Sequence first the code for the condition for which the service is being performed. See the table below for the three variants |
Rehabilitation admissions — three variants#
| Situation | First-listed / principal | Example |
|---|---|---|
| The condition for which rehabilitation is provided is still present | The code for that condition | Rehabilitation for right-sided dominant hemiplegia following a cerebral infarction → I69.351 |
| The condition is no longer present, and the rehabilitation does not follow an injury | The appropriate aftercare code | Severe degenerative osteoarthritis of the hip, hip replacement performed, current admission for rehabilitation → Z47.1 aftercare following joint replacement surgery |
| Rehabilitation following active treatment of an injury | The injury code with the appropriate 7th character for subsequent encounter | Rehabilitation post hip replacement for a right intertrochanteric femur fracture → S72.141D |
Where the uncertain-diagnosis rule does and does not apply#
| Setting | Uncertain diagnosis | Guideline |
|---|---|---|
| Inpatient — short-term, acute, long-term care and psychiatric hospitals | ALWAYS Coded as if it existed or was established, based on the diagnostic workup, arrangements for further workup or observation, and the initial therapeutic approach | Section II.H and Section III.C |
| Hospital outpatient and provider office | NEVER Not coded. Code the condition to the highest degree of certainty for that encounter — symptoms, signs, abnormal test results, or other reason for the visit | Section IV.H |
| EXCEPTION HIV, Zika virus and COVID-19 | NEVER Never coded as confirmed on an uncertain statement, in any setting — three explicit exceptions to Section II.H | Section I.C.1.a.1, I.C.1.f.1, I.C.1.g.1.a |
EXCEPTION Influenza J09 and J10 | NEVER Only confirmed cases — another exception to Section II.H. Suspected, possible or probable → J11 | Section I.C.10.c |
Practice questions#
Q1A patient is admitted for a scheduled cholecystectomy for symptomatic cholelithiasis. The surgery is cancelled because of an equipment failure in theatre and the patient is discharged the next day. What is the principal diagnosis?
- AA code from
Z53for a procedure not carried out - BThe cholelithiasis
- CThe equipment failure
- DNo principal diagnosis may be assigned
Show answer & rationale
Correct answer: B. The cholelithiasis
Rationale. Section II.F states that where the original treatment plan is not carried out, the principal diagnosis is the condition which, after study, occasioned the admission to the hospital, even though treatment may not have been carried out due to unforeseen circumstances. A Z53 code may be reported additionally to explain the circumstance, but it does not displace the condition that occasioned the admission.
Q2A patient is admitted to observation for chest pain, which worsens, and is then admitted as an inpatient of the same hospital for that same condition. What is the principal diagnosis?
- AAn observation Z code
- BThe medical condition which led to the hospital admission
- CThe chest pain symptom code in every case
- DWhichever condition produces the higher-weighted DRG
Show answer & rationale
Correct answer: B. The medical condition which led to the hospital admission
Rationale. Section II.I.1 states that when a patient is admitted to an observation unit for a medical condition which either worsens or does not improve, and is subsequently admitted as an inpatient of the same hospital for that same medical condition, the principal diagnosis is the medical condition which led to the hospital admission. What that condition is depends on what was established after study.
Q3A patient has outpatient surgery at a hospital and is admitted as an inpatient at the same hospital for a condition entirely unrelated to the surgery. What is the principal diagnosis?
- AThe reason for the outpatient surgery
- BThe unrelated condition
- CA complication code
- DEither, at the coder's discretion
Show answer & rationale
Correct answer: B. The unrelated condition
Rationale. Section II.J sets out three outcomes for admission from outpatient surgery: if the reason for the inpatient admission is a complication, the complication is the principal diagnosis; if no complication or other condition is documented as the reason, the reason for the outpatient surgery is the principal diagnosis; and if the reason is another condition unrelated to the surgery, the unrelated condition is the principal diagnosis.
Q4A discharge summary documents “pneumonia versus pulmonary embolism” with no further determination. This is an acute inpatient admission. How is this coded?
- ACode the symptoms only
- BCode both as if confirmed; either may be sequenced first
- CCode only the pneumonia, as the more likely diagnosis
- DQuery the provider and assign nothing until the query is answered
Show answer & rationale
Correct answer: B. Code both as if confirmed; either may be sequenced first
Rationale. Section II.D states that in those rare instances when two or more contrasting or comparative diagnoses are documented as either/or, or similar terminology, they are coded as if the diagnoses were confirmed and are sequenced according to the circumstances of the admission — and that if no further determination can be made as to which should be principal, either diagnosis may be sequenced first.
Q5A patient is admitted for rehabilitation following active treatment of a displaced intertrochanteric fracture of the right femur, treated with hip replacement. What is the principal diagnosis?
- A
Z47.1aftercare following joint replacement surgery - B
S72.141D, the injury code with the 7th character for subsequent encounter - C
Z51.89 - DThe osteoarthritis code
Show answer & rationale
Correct answer: B. S72.141D, the injury code with the 7th character for subsequent encounter
Rationale. Section II.K states that for rehabilitation services following active treatment of an injury, the injury code with the appropriate seventh character for subsequent encounter is assigned as the first-listed or principal diagnosis, and uses this exact example: rehabilitation post hip replacement for a right intertrochanteric femur fracture is reported with S72.141D. The aftercare code Z47.1 applies only where the rehabilitation does not follow an injury.
Scenarios#
A patient is admitted with both acute systolic heart failure and atrial fibrillation with rapid ventricular response. Both were present on admission, both were treated, and the record does not identify one as chiefly responsible.
Show coded answer & rationale
| Seq | Code | Description |
|---|---|---|
| 1 | Either | the acute systolic heart failure code, or the atrial fibrillation code, as principal diagnosis |
| 2 | The other | as a secondary diagnosis |
Rationale. Section II.B states that when there are two or more interrelated conditions — such as diseases in the same ICD-10-CM chapter — each potentially meeting the definition of principal diagnosis, either condition may be sequenced first, unless the circumstances of the admission, the therapy provided, the Tabular List or the Alphabetic Index indicate otherwise. Read the Tabular List notes before finalising: an instructional note would override this guideline under Section I.A.
Guideline: Section II.B; Section I.A
A patient discharged after a knee arthroplasty is readmitted eight days later with a periprosthetic joint infection documented by the surgeon as a complication of the arthroplasty.
Show coded answer & rationale
| Seq | Code | Description |
|---|---|---|
| 1 | T84.5- | Infection and inflammatory reaction due to internal joint prosthesis, with the appropriate 7th character — principal diagnosis |
| 2 | Organism | an additional code identifying the infectious agent |
| 3 | Specificity | an additional code for the specific complication where the T code lacks necessary specificity |
Rationale. Section II.G states that when the admission is for treatment of a complication resulting from surgery or other medical care, the complication code is sequenced as the principal diagnosis, and that if the complication is classified to the T80-T88 series and lacks the necessary specificity, an additional code for the specific complication should be assigned. Section I.B.16 requires the provider's documentation of the relationship, which is present here. Section I.C.19.a clarifies that for complication codes, active treatment refers to treatment for the condition described by the code even though it relates to an earlier precipitating problem.
Guideline: Section II.G, I.B.16, I.C.19.a
A patient is admitted with severe abdominal pain. After study the provider establishes acute diverticulitis of the colon with abscess as the cause.
Show coded answer & rationale
| Seq | Code | Description |
|---|---|---|
| 1 | K57.2- | Diverticulitis of large intestine with perforation and abscess — or the applicable code; principal diagnosis |
Rationale. Section II.A states that codes for symptoms, signs and ill-defined conditions from Chapter 18 are not to be used as the principal diagnosis when a related definitive diagnosis has been established. Section I.B.5 adds that signs and symptoms routinely associated with a disease process are not assigned as additional codes, so the abdominal pain is not reported at all. The UHDDS definition supports the same answer: the diverticulitis is the condition established after study to be chiefly responsible for occasioning the admission.
Guideline: Section II introduction, II.A; Section I.B.5
Primary sources#
- ICD-10-CM Official Guidelines for Coding and Reporting, FY 2027CMS — the source document for every page on this site (effective 1 Oct 2026)
- NCHS ICD-10-CM files — Tabular List, Index, POA exempt listOfficial code files, addenda and the list of codes exempt from POA reporting
- ICD-10 Coordination and Maintenance CommitteeWhere code proposals are debated; agendas, summaries and meeting materials
- CMS ICD-10 homeCode sets, transition guidance and Medicare coding policy
- AHA Coding ClinicOfficial coding advice from the AHA Central Office — subscription required
- HHS OIG Work PlanActive audit topics — useful for prioritising internal coding audits
- MEDESUN Medical Coding AcademyTraining, credential preparation and audit services by the author