Identifying the Admitting, Principal, Primary, and Secondary Diagnoses

by  Aimee L. Wilcox, CPMA, CCS-P, CST, MA, MT
Apr 25th, 2023

Knowing how to differentiate the admitting, principal, primary, and secondary diagnoses for reporting and sequencing purposes can be intimidating and confusing. The following are some commonly asked questions related to reporting diagnoses in the facility setting. For example:

• What is the difference between the principal diagnosis and the primary, or admitting diagnosis?
• Why is it so important to identify a principal diagnosis?
• Can the principal diagnosis and primary diagnosis be the same diagnosis?
• Do we use the same terminology for diagnoses in the office or other outpatient setting?

Sequencing diagnoses is important for every place of service and healthcare organization who wishes to ensure accurate claim processing and avoid denials. Understanding the ICD-10-CM Official Coding Guidelines for Coding and Reporting is the first step in successfully sequencing codes for reporting. Additionally, another key factor is understanding the definitions of and how to identify the following types of diagnoses:

• Admitting
• Principal
• Primary
• Secondary

Let’s take a look at each of these, individually, and how they are identified and defined.

Admitting Diagnosis: The sign symptom, condition, injury, or disease that was the reason the patient sought medical care is considered the admitting diagnosis (even if the patient isn't actually admitted to the hospital). This is the problem that caused the patient to seek medical care, likely in the emergency department (ED).  

Principal Diagnosis: As published in the ICD-10-CM Official Guidelines for Coding and Reporting, and according to the Uniform Hospital Discharge Data Set (UHDDS), the principal diagnosis is defined as, “that condition established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care.”

For example, while chest pain may have brought the patient to the ED, after examination and testing, the patient was diagnosed with an acute myocardial infarction (MI) requiring hospital admission. In this case, the acute MI would be the principal diagnosis. 

Primary Diagnosis: In the inpatient setting, the primary diagnosis is the diagnosis that is the most severe or resource intensive (uses the most hospital resources) during the patient’s stay. In some patients, this may be the same as the principal diagnosis, but in others it will be different. 

For example, a patient who presented to the ED with an acute, yet severe nosebleed (epistaxis) which had become too difficult to manage at home, so she presented to the emergency department. Upon arrival, examination, and obtaining a thorough patient history, it was noted the patient had been discharged from a recent hospital stay on coumadin but had not had any follow-up or testing to ensure adequate coagulation and now her blood had become so thin she was at risk of a nasal hemorrhage. While in the ED, she had an episode of bloody diarrhea, and additional testing revealed rupture of a prior gastric ulcer, increasing her risk of death, and requiring additional resources for testing and treatment of a high risk condition. In this setting the primary diagnosis would likely be the bleeding gastric ulcer, which required many more resources than the acute epistaxis. 

Secondary Diagnosis: The secondary diagnosis or diagnoses, can be compared to side dishes to the main course or primary diagnosis. Using the same example, the patient who presented with an acute episode of severe epistaxis, followed by a bleeding gastric ulcer all caused by unmonitored anticoagulant use (prescribed due to a recent deep vein thrombosis), may also have additional, secondary diagnosis such as hypertension or type 2 diabetes unrelated to the current encounter. These additional diagnoses would be considered the “side dishes” or what some might refer to as “patient baggage.” 

For a secondary diagnosis to make the list of reportable diagnoses, the medial record must include documentation to support that a secondary diagnosis has been either monitored, evaluated, assessed, or treated (MEAT) in any one of the following ways, during the encounter or hospital stay: 

• A medical evaluation includes evaluation of the condition

Example: Patient admitted for altered mental status with fall but has a secondary diagnosis of leg ulcers. The provider examines the status of the leg ulcers to ensure they haven’t reopened or become infected after the fall

• Diagnostic testing, studies, or imaging were performed related to the secondary diagnosis

Example: Diabetes type 2 with A1c being tested during encounter or stay

• Prescription or administration of a therapeutic intervention or treatment

Example: Patient’s leg ulcer has re-opened and needs attention, including debridement and new dressings

• Escalated hospital care/extended length of stay caused by a secondary diagnosis

Example: On top of severe epistaxis, due to unregulated Coumadin use, the patient was also noted to have dangerously high blood glucose levels warranting a temporary change in diabetic medications and another day of admission to ensure adequate control of her diabetes (secondary condition) before discharge. 

• Increased monitoring or nursing care

Example: Following fusion of her cervical C4-5, the patient was noted to have difficulty breathing when she fell asleep with the SpO2 alarm constantly going off. To ensure closer monitoring of this issue, we have moved the patient to the Critical Care Unit and requested pulmonary and respiratory therapy consults. 

Sometimes a patient is admitted with multiple, acute conditions and coders must determine which will be listed as the principal diagnosis, especially if either could lead to an inpatient admission. Luckily, the Official ICD-10-CM Coding Guidelines provide additional guidance for these circumstances. If, however, after a complete review of the medical record you are still unsure which diagnosis should be listed as the principal diagnosis, it is recommended you query the provider for the diagnosis that led to the admission. 

About Aimee L. Wilcox, CPMA, CCS-P, CST, MA, MT

Aimee L. Wilcox is a medical coding, billing, and auditing consultant, author, and educator with more than 30 years of clinical and administrative experience in healthcare, coding, billing, and auditing. Medicine, including coding and billing, is a constantly changing field full of challenges and learning and she loves both. Aimee believes there are talented medical professionals who, with proper training and excellent information, can continue to practice the art of healing while feeling secure in their billing and reimbursement for such care.