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Hint: Use coding conventions as your North Star.

While selecting a principal diagnosis sounds easy in theory, the reality is patients sometimes have evolving or complicated medical conditions. While coders reporting on the conditions of hospitalized patients may have the most complex coding predicaments, knowing how to correctly follow ICD-10-CM guidelines is a skill any diagnosis coder requires.

Find out some tips for navigating principal diagnosis for hospital coding from Heather Greene, MBA, RHIA, CDEI, CIC, RCMS, CDIP, CPC, CPMA, CDEO, CRC, which she shared during her presentation 鈥淭he Principles of the Principal Diagnosis鈥� at AAPC鈥檚 2025 HEALTHCON. Although her focus was on reporting for hospitalized patients, any diagnosis coder should find her explanations of the ICD-10-CM guidelines helpful.

Stick to the Definition and Follow These Steps

In the ICD-10-CM coding system, principal diagnosis has an explicit definition, and coders need to understand and remember it to ensure accurate reporting. Greene provided this definition of principal diagnosis: 鈥淭hat condition established after study to be chiefly responsible for the admission of the patient to the hospital for care.鈥�

She said there are two important things to remember when coding principal diagnosis in non-outpatient settings: A principal diagnosis is not necessarily the primary diagnosis or the condition that required the most resources.

Although coders always know that documentation is crucial, complicated or complex conditions with multiple diagnoses are especially reliant on comprehensive and accurate documentation.

Greene said coders should remember this order of precedence when navigating the ICD-10-CM coding book and guidelines: coding conventions, then Tabular List, then Alphabetic Index.

Detangle Interrelated Diagnoses Like This

Medically complex patients may be admitted to a hospital with several interrelated conditions that coders have to sort through when reporting diagnoses. These situations may involve coding for signs and symptoms or ill-defined conditions, and coders may find multiple interrelated conditions potentially qualifying as a principal diagnosis.

Greene said three situations may arise with interrelated conditions, and they can be tough for coders to figure out:

• Two or more interrelated conditions, each potentially meeting the definition for principal diagnosis
• Two or more diagnoses that equally meet the definition for principal diagnosis
• Two or more comparative or contrasting conditions

Situations like this can get even more complicated depending on the patient鈥檚 age and condition. For example, if a Medicare beneficiary enters a hospital from a nursing home, they need to stay at least three days to be able to return to the nursing home and continue receiving the care they need, Greene said.

In such situations, a patient may have, for example, chronic pulmonary obstructive disease (COPD) and pneumonia, and a coder can look at the documentation and then go back to the coding conventions to try and figure out which diagnosis should be the principal 鈥� all while keeping diagnosis-related groups (DRGs) in the back of their minds. Physicians may not realize that a patient鈥檚 DRG is going a certain way, Greene said, and savvy navigation of that can really affect the patient.

When you have two or more diagnoses that equally meet the definition for principal diagnosis, fall back on the industry standard of a coder being trusted to make the call, Greene said.

鈥淚f we have two conditions that meet the definition of principal diagnosis, the coder can choose the higher-weighted diagnosis. We can do that because we鈥檙e paid a lump sum. And so, when you choose like this, if you run into this, it鈥檚 a really good idea to make sure you have solid documentation that supports the higher-weighted one and really does meet it. You may look more into what was done, what was treated more, and where did the money come from? That might be more helpful in having a conversation with the payer and getting the care reimbursed,鈥� Greene said.

Comparative contrasting conditions aren鈥檛 as common anymore, Greene said, because documentation has really improved, but sometimes if you have an against medical advice (AMA) patient or an unhoused patient, then you get short stays where the treatment plan isn鈥檛 carried out. But in these situations, you still want to focus on the condition that sends the patient to the hospital.

Stay Focused on Reason for Admittance

Complications can always arise. For example, if a patient comes into the hospital for acute kidney failure but has a myocardial infarction (MI) while there, the hospital is going to spend a lot of time and resources treating the MI 鈥� maybe even more than treating the kidney[s]. But acute kidney failure is still what brought the patient to the hospital and would still be principal diagnosis, even if it wasn鈥檛 treated with the same intensity.

Payers see other facility-originated incidents or conditions the same way. If a patient suffers a fall or develops a pressure injury/ulcer, payers generally don鈥檛 believe they should have to pay for that care. 鈥淚f you鈥檙e not taking care of the patient, and a pressure ulcer develops and requires a lot of attention, then the facility covers that. That鈥檚 our responsibility as a facility, to make sure the patient鈥檚 OK,鈥� she explained.

Choosing a principal diagnosis can get tricky if a patient is at a hospital for surgery but then experiences a situation that requires admittance.

For example, if a patient comes in for knee surgery but experiences acute blood loss and is admitted, then the acute blood loss becomes the principal diagnosis.

Think about what changed the status, Greene said. 鈥淎t what point did the physician decide that the patient required this level of care? And that鈥檚 what becomes the principal diagnosis.鈥�

Use this advice as a North Star, and look for next month鈥檚 RCI article on ICD-10-CM coding to test your knowledge of principal diagnosis via several scenarios.

Rachel Dorrell, MA, MS, CPC-A, CPPM, Development Editor, 星空入口