Have you ever wondered about the default codes guideline for ICD-10-CM? Guideline I.A.18. states:

A code listed next to a main term in the ICD-10-CM Alphabetic Index is referred to as a default code. The default code represents that condition that is most commonly associated with the main term or is the unspecified code for the condition. If a condition is documented in a medical record (for example, appendicitis) without any additional information, such as acute or chronic, the default code should be assigned.

In my work as an auditor and coding educator, I have found that this guideline is often overlooked. I often hear coders defending their code assignment and questioning the index because they don’t want to “diagnose the patient.” Quite contrary, you are diagnosing the patient by failing to adhere to this coding convention. The International Classification of Diseases (ICD) assigns codes based on the condition most commonly associated with the documented condition.

How about a little backstory?

I started my journey into the HIM field in the early 90s at a local community college, enrolled in an RHIT program. Correction, at that time it was an ART (Accredited Record Technician) program. The “bible” for that program was Medical Record Management by Edna K Huffman. When I brought that book home, it got a giggle out of my mother, who used the same book decades earlier when she went through the same program at the same community college. I still have this book. It’s horribly out of date (remember terminal digit and calculated filing inches?!) with faded highlights from my days studying to take the RHIT (ART) exam, but somehow, I haven’t been able to part with it yet. And today I realized why: it defines nomenclature and classification systems and the difference between the two.

What does this have to do with default codes in ICD-10-CM?  Everything.

Although the two terms are used interchangeably, there is a noticeable difference between a classification system and a nomenclature.

The word “nomenclature” comes from the Latin term nomen, meaning name, and clature, meaning calling. In coding terms, nomenclatures are recognized systems of preferred terminology for naming disease processes or procedures. That means that the code must match the disease or procedure exactly and there is no room for “other specified” and “unspecified” codes. The Current Procedural Terminology (CPT®) is an example of a nomenclature. If you think about it, there are no other specified or unspecified CPT codes. We have unlisted codes, which are used when there is not a specific code for what was done, but those codes aren’t helpful in analyzing data. I suppose ICD-10-PCS is also considered a nomenclature because there are no unspecified codes. Huffman didn’t mention that in her book, but then again, we didn’t know about ICD-10-PCS in the early 90s when the 9th revision was published!

By contrast, classification systems focus on grouping similar conditions to produce statistical information. ICD-10-CM is a close cousin to its predecessor, ICD-9-CM, which groups similar diagnostic conditions under a single code. To capture anything without a specific code, ICD-10-CM has “other specified” and “unspecified” codes at nearly every category/subcategory level.

DRGs were implemented in the early 80s and since then, prospective payment systems (PPS) have been implemented across various healthcare settings.  At times, I feel we’ve focused so much on the coding rules as they relate to payment and compliance that we overlook the origin of the classification system. ICD-10-CM is not just a book of diagnoses, it’s a book of diagnoses in tabular and index form with a terrific amount of thought applied to how to properly classify conditions.

A common example I see from coders who don’t want to “diagnose the patient” is the coding of multiple valvular disorders. ICD-10-CM classifies mitral valve insufficiency to category I34, Nonrheumatic mitral (valve) insufficiency. But when a patient has mitral valve insufficiency with aortic or tricuspid valve disease, the conditions are suddenly assigned to category I08, which has codes specifically indicating rheumatic heart disorders. How did we make the flip from nonrheumatic to rheumatic? The answer lies in the most common cause of multiple valvular heart disease: rheumatic fever. In this case, we aren’t diagnosing the patient by assigning an I08 code, the classification system is.

Many of the “whys” coders ask when assigning codes can be answered by realizing that more thought goes into the creation of new codes and the indexing than we could imagine. Many whys are often answered by becoming familiar with the general coding conventions of ICD-10-CM as found in section I.A. of the ICD-10-CM Official Guidelines for Coding and Reporting.

If you want to learn more about valvular disorders and procedures, check out June’s webinar release “The Doors of the Heart:  ICD-10-CM/PCS Coding for Heart Valve Procedures."

Kristi Pollard, RHIT, CCS, CPC, CIRCC, AHIMA-Approved ICD-10-CM/PCS Trainer

Kristi Pollard, RHIT, CCS, CPC, CIRCC, AHIMA-Approved ICD-10-CM/PCS Trainer

Director of Coding Quality & Education

Kristi is a senior consultant with more than 20 years of industry experience. She develops and delivers training on ICD-10-CM/PCS and CPT, both virtually and in classroom settings.
Kristi also performs DRG and APC audits and is known for her vast knowledge on coding vascular interventional radiology procedures. Kristi has an extensive background in coding education and consulting and is a national speakers on topics related to ICD-10 and CPT coding as well as code-based reimbursement.


  1. Debra Beisel

    Hi Kristi, I totally agree with you in this article. However, regarding OCRG, 1.A.18 and default codes. as a HCC Auditor and Educator myself for over 25 years I have seen a trend usually in the MAO Risk Adjustment world that may or may not be intentional regarding the use of default codes to capture HCC’s. Let me give you an example which can better explain this. This is from an EHR system that identifies an HCC in the Visit List or Problem List as Beta Thalassemia (HCC). The providers are instructed to pull into their office notes any condition identified as an HCC. They do not add if the Beta Thalassemia is Major or Minor. The default code is Major (severe) D56.1 which is an HCC, however this patient had the minor trait of the Beta Thalassemia (D56.3) which is not an HCC. It was noted under a different tab under Histories and previous lab results the patient had the trait only. There were no treatments or labs ordered for 2022, however the provider pulled in the Hgb levels from prior years. Since there was no information pulled into this particular date of service I was auditing and I could not query I was told I had to validate this HCC and use the default code due to the provider’s maybe intentional omission the patient had only the trait minor BT and it was coded as Major BT for the identified HCC. That is why I feel coders are cautious using default codes or unspecified codes when there is actually more information in the patient’s entire years worth of data but we are auditing only from one DOS. I hope this makes sense.

    • kkluglein

      Hi Debra,

      The example you’ve listed here is a great example of the importance of clinical documentation improvement (CDI) programs. Based on guideline I.A.18. for default codes and guidance published in Coding Clinic, 3rd Quarter 2013, page 27, it is inappropriate to refer to previous encounters for coding purposes. While HCC coding presents new challenges by tracking certain conditions over the course of a year and comparing those reported conditions to other years, it is still appropriate to assign codes based on the documentation within the current encounter and assign default codes according to the guidelines when it is not possible to query. It does, however, highlight the need for coding and CDI professionals to work with clinicians on their documentation practices to ensure consistent documentation of conditions over time.

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