Nothing exemplifies the expansion of medical knowledge and diagnosis quite like ICD codes. The International Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM) is not a static morbidity classification, but one that evolves to reflect changes in medical knowledge.

The amount of changes in 2017 in ICD-10-CM is significant: the printed edition has grown around 20%, from 1593 pages in 2016 to 1910 pages in 2017. It is easy to get lost in these changes, and even to understand what these changes represent. Therefore, if you are working with a particular specialty, we highly encourage you to check in detail how these changes might affect that. For example, the American Congress of Obstetricians and Gynecologists has published an informative list of changes for their specialty in 2017 [2].

What is a change? What are headers and codes?

Changes that occur between releases of ICD-10-CM are published in an Addendum (for humans) and in tabular format (machine readable). One would think that these changes would be easy to identify, when in fact, there is more to an ICD-10-CM code than meets the eye.

Even though every entry in ICD-10-CM has a code attached, not everything is considered a code. ICD-10-CM entries are divided into headers and codes, and only entries present in the icd10cm_codes_2017.txt file or flagged as a code in icd10cm_order_2017.txt are valid for HIPAA-covered transactions, the rest are considered headers.

So…. How has ICD-10-CM changed in 2017?

Let’s look at how all the different changes (deletions, revisions, and additions, of both headers and codes) are distributed in each chapter of ICD-10-CM. We can quickly surmise that addition constitutes a significant portion of changes made, and that the majority of all changes made to ICD-10-CM, was in Chapter 19.

Graph of changes in ICD-10-CM for 2017

However, to understand the rationale behind those changes, it is important to observe how they are distributed within the hierarchy. Changes were made from the top level of the hierarchy (level 1) all the way down (level 5), with the frequency of changes increasing the lower in the hierarchy you get.

At the top we see changes like the revision of “N10: Acute tubulo-interstitial nephritis” to “N10: Acute pyelonephritis”. Toward the bottom, we see the addition of very specific codes, such as “S99.102P: Unspecified physeal fracture of left metatarsal, subsequent encounter for fracture with malunion”.

You can easily see the distribution of these changes in this figure, which shows the granularity of changes in 2017 for all chapters as a cluster.

Clustering of changes in ICD-10-CM for 2017

The figure also shows that “Chapter 2 Neoplasms” had the majority of its revisions at a high level in header sections, which then propagate to the codes beneath these header sections. On the other side, changes in “Chapter 3, Endocrine, nutritional and metabolic diseases …” focus on lower levels of the hierarchy with the bulk of the changes being additions.