Secondary Diagnosis Coding

One of the most common mistakes that medical coders make is incorrect or missing secondary diagnoses. Incorrect or missing secondary diagnosis codes can result in incorrect reimbursement have a direct financial impact, and affect the overall quality stats for the facility.

Secondary diagnoses are “conditions that coexist at the time of admission, that develop subsequently, or that affect the treatment received and/or length of stay.” Please note that diagnoses that relate to an earlier episode which have no bearing on the current inpatient admission should not be coded.

Some examples of secondary diagnoses can include chronic conditions like diabetes and hypertension, or complications like infection following surgery. Secondary diagnoses can also be coexisting conditions such as asthma in a patient with pneumonia and other relevant health issues. For example, a secondary diagnosis of diabetes may require its own set of treatments alongside those for a principal diagnosis of chronic kidney disease. Understanding and capturing secondary diagnoses is crucial to getting the full picture of the patient and to develop an effective treatment plan that addresses all the patient’s health issues.

Coincidentally, we are seeing secondary diagnosis coding trending in courses taken in the past several months on Here's the top 3:

Secondary Diagnosis Coding in the Outpatient setting

Secondary diagnosis codes are important in the outpatient setting. It is important to report all secondary diagnosis codes, history, and status codes (when documented appropriately) to confirm medical necessity. Some payors will deny tests done in the outpatient setting if the medical necessity is not met. Many times, medical necessity is determined by the ICD-10-CM codes reported on the outpatient claim. For example, if an EKG is done on a patient in an encounter for outpatient fracture repair, and the chronic atrial fibrillation that is documented in the patient’s record as reason for the EKG is not coded as a secondary diagnosis by the coder, the EKG charge/reimbursement could be denied by the payor. For further guidance on coding diagnoses on outpatient encounters, read this blog post:

In addition to an uptick in courses related to secondary diagnosis coding taken in HIAlearn, HIA’s review staff is seeing an uptick in review requests focused on secondary diagnosis coding from our clients. It’s safe to say secondary diagnosis coding is a hot topic at HIA these days.

Secondary Diagnosis Review Services

In 2023, HIA reviewed over 300,000 secondary diagnosis codes, resulting in 14,370 additions and 7,357 deletions. Furthermore, out of all the DRG Reasons for Change in 2023, 30% were related to a CC/MCC Addition, Deletion or Revision, which would be considered a secondary diagnosis change. These stats tell us that there are some coders that do not fully understand the guidelines. These changes can affect the DRG or quality stats on the inpatient side and, as we said before, it can also impact medical necessity on the outpatient side.

HIA’s comprehensive review services, for both the inpatient and outpatient setting, involves validating all secondary/diagnosis codes documented in the record, not just those which affect reimbursement. The goal is to ensure complete and accurate coding that reflects a thorough description of the patient's health and treatment. In the evolving world of value-based care it is increasingly important to paint the full picture so that outcomes and quality measures are not adversely affected by coding practices.

For more information, contact us or visit to view all our medical coding courses.

Since 1992, HIA has been the leading provider of compliance audits, coding support services and clinical documentation audit services for hospitals, ambulatory surgery centers, physician groups and other healthcare entities. HIA offers PRN support as well as total outsource support.


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The information contained in this coding advice is valid at the time of posting. Viewers are encouraged to research subsequent official guidance in the areas associated with the topic as they can change rapidly.

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