Section II. Selection of Principal Diagnosis

The circumstances of inpatient admission always govern the selection of principal diagnosis. The principal diagnosis is defined in the Uniform Hospital Discharge Data Set (UHDDS) as "that condition established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care."

The UHDDS definitions are used by hospitals to report inpatient data elements in a standardized manner. These data elements and their definitions can be found in the July 31, 1985, Federal Register (Vol. 50, No, 147), pp. 31038-40.

Since that time the application of the UHDDS definitions has been expanded to include all non-outpatient settings (acute care, short term, long term care and psychiatric hospitals; home health agencies; rehab facilities; nursing homes, etc). The UHDDS definitions also apply to hospice services (all levels of care).

In determining principal diagnosis, coding conventions in the ICD-10-CM, the Tabular List and Alphabetic Index take precedence over these official coding guidelines.

(See Section I.A., Conventions for the ICD-10-CM)

The importance of consistent, complete documentation in the medical record cannot be overemphasized. Without such documentation the application of all coding guidelines is a difficult, if not impossible, task.

A. Codes for symptoms, signs, and ill-defined conditions

Codes for symptoms, signs, and ill-defined conditions from Chapter 18 are not to be used as principal diagnosis when a related definitive diagnosis has been established.

B. Two or more interrelated conditions, each potentially meeting the definition for principal diagnosis.

When there are two or more interrelated conditions (such as diseases in the same ICD-10-CM chapter or manifestations characteristically associated with a certain disease) 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.

C. Two or more diagnoses that equally meet the definition for principal diagnosis

In the unusual instance when two or more diagnoses equally meet the criteria for principal diagnosis as determined by the circumstances of admission, diagnostic work-up and/or therapy provided, and the Alphabetic Index, Tabular List, or another coding guidelines does not provide sequencing direction, any one of the diagnoses may be sequenced first.

D. Two or more comparative or contrasting conditions

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 the diagnoses are sequenced according to the circumstances of the admission. If no further determination can be made as to which diagnosis should be principal, either diagnosis may be sequenced first.

E. A symptom(s) followed by contrasting/comparative diagnoses


F. Original treatment plan not carried out

Sequence as the principal diagnosis the condition, which after study occasioned the admission to the hospital, even though treatment may not have been carried out due to unforeseen circumstances.

G. Complications of surgery and other medical care

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. If the complication is classified to the T80-T88 series and the code lacks the necessary specificity in describing the complication, an additional code for the specific complication should be assigned.

H. Uncertain Diagnosis

If the diagnosis documented at the time of discharge is qualified as "probable", "suspected", "likely", "questionable", "possible", or "still to be ruled out", or other similar terms indicating uncertainty, code the condition as if it existed or was established. The bases for these guidelines are the diagnostic workup, arrangements for further workup or observation, and initial therapeutic approach that correspond most closely with the established diagnosis.

Note: This guideline is applicable only to inpatient admissions to short-term, acute, long-term care and psychiatric hospitals.

I. Admission from Observation Unit

1. Admission Following Medical Observation

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 this same medical condition, the principal diagnosis would be the medical condition which led to the hospital admission.

2. Admission Following Post-Operative Observation

When a patient is admitted to an observation unit to monitor a condition (or complication) that develops following outpatient surgery, and then is subsequently admitted as an inpatient of the same hospital, hospitals should apply the Uniform Hospital Discharge Data Set (UHDDS) definition of principal diagnosis as "that condition established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care."

J. Admission from Outpatient Surgery

When a patient receives surgery in the hospital's outpatient surgery department and is subsequently admitted for continuing inpatient care at the same hospital, the following guidelines should be followed in selecting the principal diagnosis for the inpatient admission:

K. Admissions/Encounters for Rehabilitation

When the purpose for the admission/encounter is rehabilitation, sequence first the code for the condition for which the service is being performed. For example, for an admission/encounter for rehabilitation for right-sided dominant hemiplegia following a cerebrovascular infarction, report code I69.351, Hemiplegia and hemiparesis following cerebral infarction affecting right dominant side, as the first-listed or principal diagnosis. If the condition for which the rehabilitation service is no longer present, report the appropriate aftercare code as the first-listed or principal diagnosis. For example, if a patient with severe degenerative osteoarthritis of the hip, underwent hip replacement and the current encounter/admission is for rehabilitation, report code Z47.1, Aftercare following joint replacement surgery, as the first-listed or principal diagnosis.

See Section I.C.21.c.7, Factors influencing health states and contact with health services, Aftercare.