A Review of Principal Diagnosis Assignment

Official ICD-9-CM coding guidelines indicate that 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.” Most inpatient coders have this rule ingrained and are aware of it when reviewing medical records to determine which diagnosis to sequence in the principal position. Those candidates taking the CCS credential examination should carefully review the principal diagnosis coding conventions, which include the following official guidelines [Section II: Selection of Principal Diagnosis]:

A. Codes for symptoms, signs and ill-defined conditions
Codes for symptoms, signs and ill-defined conditions from Chapter 16 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-9-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 workup and/or therapy provided, and the Alphabetic Index, Tabular List, or another coding guideline 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
When a symptom(s) is followed by contrasting/comparative diagnoses, the symptom code is sequenced first. All the contrasting/comparative diagnoses should be coded as additional 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 996-999 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,” code the condition as if it existed or was established. The basis for this guideline is 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 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 that 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:

  • If the reason for the inpatient admission is a complication, assign the complication as the principal diagnosis.
  • If no complication, or other condition, is documented as the reason for the inpatient admission, assign the reason for the outpatient surgery as the principal diagnosis.
  • If the reason for the inpatient admission is another condition unrelated to the surgery, assign the unrelated condition as the principal diagnosis.

The vast majority of cases can be coded based upon one or another of the scenarios listed above. But what about those cases in which a patient presents with vague symptoms and there may be documentation of a condition that is “evolving?” Although these cases may require an additional query to the physician for clarification, it’s helpful for the coder to understand the common types of clinical conditions in which the disease process may have existed on admission but wasn’t clear or apparent to the physician at that point in time.

The most common example of an evolving diagnosis is that of an evolving myocardial infarction. Sometimes this condition causes right ventricular failure, which may progress to congestive heart failure (CHF). The patient may be admitted with CHF but may also have evidence of an evolving or impending AMI. This is another reason that initial lab values may not tell the entire story; those first drawn in the ED may not be abnormal enough to indicate a definite MI but those cardiac enzyme values may rise precipitously soon thereafter, depending upon the patient and treatment. Additionally, patients with diabetes may not experience chest pain so may present with the CHF as a complication of the MI rather than with complaints of chest pain. In this case, if the acute MI is determined to have been evolving since the time of admission, it is sequenced as the principal diagnosis.

Another common example of an evolving type of diagnosis may involve pneumonia, particularly in the elderly Medicare population, which may also have other superimposed respiratory conditions such as COPD or asthmatic bronchitis. A patient may present to the ED with acute shortness of breath and be admitted, seemingly due to an exacerbation of the underlying respiratory condition. A chest X-ray may initially be interpreted as negative for acute infiltrates but after admission and minor hydration or other treatment, the pneumonia may clearly appear. The physician should be queried in this case to ascertain whether or not they felt that the pneumonia was evolving and could have been present on admission. If so, the pneumonia may be sequenced as the principal diagnosis.

The part of the UHDDS definition of principal diagnosis that should be kept in mind is “after study.” If a specific disease process is determined to be present 2 or 3 days after admission and the patient was initially admitted with a failed organ or other symptom generally associated with that condition, be sure to query the physician to determine whether or not there was a connection. It would be easy to follow black and white rules and never consider questioning whether a condition was present if the physician has not clearly documented it at the time of admission. But the advanced coder understands that in the real clinical world today, sometimes the picture is somewhat blurred at the time of admission, and that it’s only after some additional time and work-up that the picture is focused and the true picture of the admission becomes clear. Coders must work with the medical staff so the documentation reflecting this focusing process is clear in the medical record and can be appropriately translated into the coding language.

Test your knowledge with the following questions:

1. A 72 -year-old patient was seen in the ED with acute shortness of breath. Because he had a significant history and showed symptoms of CHF, he was started on Lasix. He also had some chest pain and because of suspicion of an impending infarction was taken to the cardiac cath lab where a PTCA was performed. He was found to have an “evolving acute inferior myocardial infarction” that day. The appropriate diagnosis codes for this admission are:

a. 428.0, 410.41

b. 410.91, 428.0

c. 428.0, 786.50

d. 410.41, 428.0

2. A 69-year-old male patient with longstanding COPD was seen in his physician office with increased shortness of breath. He was sent to the hospital ED, where he had a chest X-ray showing chronic changes but no new infiltrate. He was found to be somewhat dehydrated as well and was admitted with the admission order of: “COPD acute exacerbation and dehydration.” The patient had IV fluids administered and was treated by respiratory therapy for his COPD. On the second day of the admission a follow-up chest X-ray revealed a right lower lobe infiltrate. The physician was queried and documented that she felt that the patient had pneumonia at the time of admission. The appropriate diagnosis codes for this admission are:

a. 486, 496, 276.51

b. 496, 276.51

c. 486, 491.21, 276.51

d. 491.21, 486, 276.51

This month’s column has been prepared by Cheryl D’Amato, RHIT, CCS, director of HIM, hospital solutions, and Melinda Stegman, MBA, CCS, clinical technical editor, OptumInsight (formerly Ingenix).

Coding Clinic is published quarterly by the American Hospital Association.
CPT is a registered trademark of the American Medical Association.


1. d. The patient had an evolving acute inferior MI at the time of admission. The CHF was present on admission but was the precursor condition related to the acute MI.

2. c. Since the physician documented that the pneumonia was present on admission, sequence it (486) as the principal diagnosis. The patient was experiencing an acute exacerbation of his COPD so add code 491.21 and also code 276.51 for the dehydration.

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