Better ICD-10 codes for primary immunodeficiency could improve patient care

This article was first published in the Fall 2026 edition of the ADVOCATE newsletter.

From doctor’s offices to insurance companies to biomedical research, ICD-10 codes are a critical part of healthcare. ICD-10, which stands for the International Statistical Classification of Diseases and Related Health Problems, 10th Edition, is a system of more than 150,000 structured codes for classifying health problems. Despite the large number of codes overall, they barely scratch the surface of primary immunodeficiencies (PIs), also known as inborn errors of immunity (IEI). There are currently only 48 ICD-10 codes to describe more than 550 different PIs.

The goal of the ICD-10 system is to allow everyone involved in a patient’s care, and beyond, to communicate about their condition accurately and consistently using a shared, standardized, and machine-readable vocabulary. Thanks to the Health Insurance Portability and Accountability Act (HIPAA) of 1996, ICD-10 codes are also the U.S. standard for all electronic records containing health information. Ideally, every distinct diagnosis would have its own code.

In reality, the ICD-10 system is inconsistent, particularly for rare and genetic diseases. Some PI codes are very specific, like D81.31 for “severe combined immune deficiency (SCID) caused by ADA deficiency.” Others can be used for literally hundreds of different conditions, like D81.89 for “other combined immunodeficiencies.”

The problem is that the limited set of codes for PI forces healthcare providers to shoehorn people into whichever one best fits their condition or their treatment needs. This muddies the water in both directions. The specific diagnosis a person actually has loses visibility within their records, and the code now includes data from someone who may have a very different condition than other people they're grouped with.

“Every day in clinic, I see how much a single diagnostic code can shape a patient’s path,” said Maria Paula Henao, M.D., the Immune Deficiency Foundation’s medical director. “Better codes ultimately offer better patient care.”

Having a code for every PI may not be realistic, but there are clear ways to improve the current system. Several PIs have specific, proven treatments and could benefit from having their own ICD-10 code. For example, CTLA4 haploinsufficiency is a primary immune regulatory disorder that includes autoimmune problems as well as infection risks. Immunologists commonly use abatacept, a medication approved to treat several forms of arthritis, ‘off label’ to treat those with CTLA4 haploinsufficiency. However, this off-label use requires both awareness on the part of clinicians and documentation to justify insurance approval. A dedicated ICD-10 code for CTLA4 haploinsufficiency could smooth both processes, alerting clinicians to abatacept as the recommended treatment and allowing for insurance approval with less paperwork.

The current codes for PI could also be improved by breaking up overly broad ones, like D84.1 for “complement deficiencies.” D84.1 currently covers more than 35 different diagnoses. Some of them are linked to severe infections from specific bacteria (e.g., meningitis in C5 deficiency), others involve unpredictable swelling (e.g., hereditary angioedema), and still others involve too much complement activity, leading to kidney damage (e.g., membrane attack complex inhibitor deficiency). The range of symptoms and treatments for these disorders means the D84.1 code doesn’t really communicate much. Dividing complement deficiencies among even a handful of codes that correspond to a narrower range of symptoms would make the coding for these conditions much more useful. Hereditary angioedema, in particular, has multiple approved treatments, making it a good candidate for its own code.

The argument for more specific ICD-10 codes for PI is also broader than management or insurance coverage for individual patients.

“ICD-10 codes are essential to how rare diseases are addressed across the healthcare system. Refining existing codes and adding new ones for PIs will improve data on disease prevalence and incidence, help shorten the time to diagnosis, and strengthen research and innovation,” said Eric Feigen, the foundation’s regulatory affairs manager.

The joint committee run by the Centers for Disease Control and Prevention (CDC) and Centers for Medicare and Medicaid Services (CMS) that oversees ICD-10 codes in the U.S. recognizes that there are problems with the current system. They are revamping how new codes for rare and genetic diagnoses get added, with an eye toward helping the system keep pace with the explosion of distinct diagnoses. The launch of the new process in spring of 2027 represents an opportunity to improve codes for PI—and patient care and research along with them.