Health Information Management

Q&A: Primary, principal, and secondary diagnoses

CDI Strategies, December 23, 2015

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Q: Sometimes I confuse the secondary diagnosis for the primary diagnosis. Do you have any tips for me to enable me to discern better?

A: This question touches on several concepts which are at the core of CDI practices. Consider the following term:

  1. Primary diagnosis
  2. Principal diagnosis
  3. Secondary diagnosis

Let’s take each of these individually.

The primary diagnosis is often confused with the principal diagnosis. In the inpatient setting, the primary diagnosis is the most serious and/or resource-intensive during the hospitalization or the inpatient encounter. Typically, the primary diagnosis and the principal diagnosis are the same diagnosis, but this is not necessarily always so.

Principal diagnosis is defined as the condition, after study, which occasioned the admission to the hospital, according to the ICD-10-CM Official Guidelines for Coding and Reporting. We must remember that the principal diagnosis is not necessarily what brought the patient to the emergency room, but rather, what occasioned the admission.

For example, a patient might present to the emergency room because he is dehydrated and is admitted for gastroenteritis. Gastroenteritis is the principal diagnosis in this instance. Many people define it as the diagnosis that “bought the bed,” or the diagnosis that led the physician to decide to admit the patient. A good question for CDI specialists to ask when examining the record is: “What is the diagnosis that was significant enough to require inpatient care?”

The physician doesn’t have to state the condition in the history and physical (H&P) in order for the coder to be able to use it as the principal diagnosis. However, the presenting symptomology that necessitated admission must be linked to the final diagnosis by the physician. Coders cannot infer a cause-and-effect relationship, according to the AHA’s Coding Clinic, Second Quarter 1984, pp. 9–10. It is the condition “after study” meaning we may not identify the definitive diagnosis until after the work up is complete.

Next, let’s look at an example of when these two would differ. A patient is admitted for a total knee replacement for osteoarthritis. The patient is brought to pre-operative holding area to prepare for surgery and suffers a ST-segment elevation myocardial infarction (STEMI) before the surgery begins. Instead of going to the operating room for the knee replacement, the patient goes to the cath lab for a stent placement.

The first question is what was the diagnosis that occasioned the admission? What was the principal diagnosis? The answer would be the osteoarthritis. This is the diagnosis that brought the patient to the hospital and the diagnosis which occasioned the need for the inpatient bed.

The second question would be what is the diagnosis that led to the majority of resource use? What is the primary diagnosis? In this scenario, it would be the acute myocardial infarction, the STEMI. But we cannot use the STEMI as the principal diagnosis because it was not the “condition that occasioned the admission.”

Now, the Uniform Hospital Discharge Data Set (UHDDS) defines a secondary diagnosis or “other diagnosis” as = conditions that coexist at the time of admission, or develop subsequently, and that affect the patient care during the current episode. I often describe these diagnoses as the patient’s “baggage,” or the diagnoses they bring along with them that must be considered when treating the principal diagnosis.

For example, our patient admitted with the principal diagnosis of osteoarthritis with the planned total knee replacement also has a history of Type 2 diabetes, chronic obstructive pulmonary disease, and coronary artery disease. These diagnoses were present prior to admission, but were not the reason for admission. They would be coded as secondary diagnoses because they will require treatment and monitoring during the patient stay.

We must also consider those diagnoses that develop subsequently, and will affect the patient care for the current episode of admission. In our example, that would be the acute STEMI. It developed after admission, so it would be a secondary diagnosis.

To be considered a secondary diagnosis the condition must require additional:

  • Clinical evaluation or
  • Therapeutic treatment or
  • Diagnostic studies or
  • An extended length of stay or
  • Increased nursing care and/or monitoring

Identifying the principal and the secondary diagnosis can be confusing when you have a patient who is admitted with two or more acute issues present such as a patient admitted with an aspiration pneumonia and acute cerebrovascular accident (CVA). In this case, there are specific coding guidelines to assist you. I would suggest you review the 2016 Official Guidelines for Coding and Reporting. There are a number of guidelines that describe how to determine the principal diagnosis.

In this scenario of an acute aspiration and an acute CVA both being present on admission, it may be difficult to discern which should be the principal diagnosis. They both would likely lead to an inpatient admission, and would meet medical necessity. If it is thought both equally could lead to an admission, the Official Guidelines for Coding and Reporting tell us that either can be chosen as the principal diagnosis. If, in review of the record, it is not clear if the conditions equally contributed to the admission, or you wish confirmation, you should query the provider as to the diagnosis that led to the admission.

Editor’s Note: Laurie L. Prescott, RN, MSN, CCDS, CDIP, AHIMA Approved ICD-10-CM/PCS Trainer, and CDI Education Director at HCPro in Danvers, Massachusetts, answered this question. Contact her at For information regarding CDI Boot Camps visit


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