Science Simplified: What Are ICD Codes and How ICD Codes Are Used in Healthcare and Beyond

Want to learn about scientific topics without needing a PhD? Check out the Science Simplified blog from TESS Research Foundation! Dr. Tanya Brown, PhD, works with researchers to make science accessible and empower rare disease community members with scientific knowledge. Dr. Brown has over a decade of experience in neurodevelopmental research and is currently the Scientific Director for TESS Research Foundation. Please reach out to her at [email protected] if you have questions or comments.

This article was written by Tanya Brown, PhD. Tanya is the Scientific Director at TESS Research Foundation.

There are so many different diseases and it’s hard to keep track of them all. Tracking diseases is particularly important for healthcare providers, patients, and even those trying to study each disease. One of the ways that diseases are tracked in the healthcare system is through the International Classification of Disease (ICD) codes. Why are we taking time to explain these codes? ICD codes are used every time someone visits a doctor and are used afterwards behind the scenes: they are used by doctors to identify which disease is affecting their patient, they are used by insurance providers to determine reimbursements for medications, and they are used by researchers to study diseases. Most rare diseases, including SLC13A5 Epilepsy, do not have an ICD code. Let’s walk through why this is particularly important for the rare disease community.

How are ICD codes used by doctors?

ICD codes are used to track medication information when someone visits the doctor. When you visit the doctor, the doctor can see what previous ICD codes are in your file and this means that a doctor could quickly learn:

  • What disease(s) you have
  • What kinds of medications can help
  • What kind of medications to avoid
  • What referrals need to be made to other healthcare providers

Since many rare diseases do not have a unique ICD code, this can make it harder for a doctor to figure out what disease a patient has and what medications to provide. This is particularly challenging for those in the rare disease community since many doctors haven’t heard about a rare disease until they see their first patient!

How are ICD codes used to learn about a disease?

Since ICD codes are used every time a person visits a healthcare provider, this means that there is a lot of information in electronic healthcare records that can be used to understand the epidemiology of a disease—how common a disease is, how many people it affects, and who is affected. By using ICD codes, researchers can learn:

  • How many people are diagnosed with this disease each year?
  • What kinds of other symptoms/diseases affect this disease group?
  • What kinds of doctors treat this disease?
  • What are the top medications used to treat this disease?

This kind of information can provide important insight into each disease. This type of information can be particularly helpful for rare diseases. Without an ICD code, this kind of information is much harder to collect.

How do you get an ICD code? 

Globally, ICD codes are maintained by the World Health Organization (WHO). In the United States, the National Center for Health Statistics, a division of the Centers for Disease Control (CDC), maintains the ICD system. To get an ICD code, an organization submits a written application to the CDC and may then be invited to present their application at an ICD meeting in either March or September. After that, there is an open comment period where individuals and organizations can submit written comments in support or against each application. Then a committee decides whether to include the disease in an update to the ICD codebook or not. This process often takes many years to complete and involves gathering support from many key opinion leaders to demonstrate support for each application. TESS Research Foundation submitted an application for an ICD code for SLC13A5 in June of 2021 and was invited to present in March of 2023.

Currently, the US uses the 10th revision of the ICD codebook. This is called ICD-10. Even though the WHO adopted the 11th version of the ICD system, called ICD-11, the United States is still using ICD-10. Historically, the US has taken about a decade to adopt the newest version of the ICD codebook. For example, the WHO endorsed ICD-10 in 1990 but the clinical use of ICD-10 was adopted in the US in 2015! This suggests that the US may use the ICD-10 codebook for many more years.


To review, International Classification of Disease or ICD codes are important for many different reasons. They provide significant information to doctors that help determine the best care for patients. ICD codes can also act as an essential research tool to learn about individual diseases. In rare diseases, we need both the best care possible for our community and to learn as much as possible about each rare disease. Although less than 20% of rare diseases have their own ICD code, organizations such as TESS Research Foundation are working to change that by applying for unique ICD codes.

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