ICD-10: International Classification of Diseases, 10th Revision
ICD-10 is the WHO's international standard for coding diseases and health conditions, with 70,000+ codes used universally in US healthcare for clinical documentation, claims billing, epidemiological surveillance, and quality measure reporting.
What is ICD-10?
The International Classification of Diseases, 10th Revision (ICD-10) is a globally standardised system of alphanumeric codes for classifying diseases, health conditions, injuries, and causes of death. Maintained by the World Health Organization (WHO) and adapted by the US Centers for Disease Control and Prevention (CDC) for American clinical use, ICD-10 is mandatory for all HIPAA-covered entities submitting healthcare claims in the United States as of October 1, 2015.
ICD-10-CM vs ICD-10-PCS
The US uses two distinct ICD-10 systems:
- ICD-10-CM (Clinical Modification): Used by all healthcare providers for diagnoses on claims and in medical records. Contains approximately 72,000 codes organised into 21 chapters. Updated annually each October 1st.
- ICD-10-PCS (Procedure Coding System): Used exclusively by hospitals for inpatient procedure coding. Contains over 87,000 codes structured as 7-character alphanumeric values where each character represents a specific dimension of the procedure.
Code Structure
ICD-10-CM codes consist of 3–7 alphanumeric characters. The first character is always a letter; characters 2–3 are numbers; characters 4–7 provide additional specificity. For example, E11.65 indicates Type 2 diabetes mellitus with hyperglycemia. The added specificity over ICD-9 (which had ~14,000 codes) allows for precise clinical documentation that supports HCC risk adjustment, DRG assignment, and quality measure denominator identification.
ICD-10 in Quality Reporting and Risk Adjustment
ICD-10 code specificity directly affects HCC (Hierarchical Condition Category) risk adjustment scores, DRG assignment for hospital payment, and inclusion in quality measure denominators and numerators. A diagnosis of "diabetes" (E11.9 — unspecified) generates a lower RAF score than a fully specified code like E11.65 or E11.40, which may map to a higher-value HCC. Coding specificity is therefore a financial and quality analytics imperative, not merely a documentation exercise.