In the United States, the system for reporting medical diagnoses is the International Classification of Diseases Ninth Revision—Clinical Modification, or ICD-9-CM. The World Health Organization (WHO) developed the ICD system. The National Center for Health Statistics (NCHS) and the Centers for Medicare and Medicaid Services (CMS) maintain the ICD-9-CM, which has been modified from the WHO’s ICD-9 to address specific requirements of the U.S. health system. ICD-9-CM is “the official system of assigning codes to diagnosis and procedures in the United States.” Agencies that categorize mortality data from death certificates use the unmodified ICD-9 system. The WHO first published ICD-9 in 1979, and the U.S. government published ICD-9-CM at about the same time. As we will discuss when considering procedural coding, the diagnosis and procedural coding systems will see major changes when the next version of ICD, the tenth revision, becomes the standard in just a few years.

ICD-9-CM has three volumes. Volume 1 lists all disease code numbers and descriptors in the form of a very long table. Volume 2 has an alphabetical index to the disease entries. And Volume 3 contains an alphabetical index and
a tabular list of the procedural coding system used by hospitals for describing surgical, diagnostic, and therapeutic interventions. In this section, we will discuss the codes found in Volumes 1 and 2, and address Volume 3 in the chapter on procedural coding.

The ICD-9-CM divides diseases into 17 broad categories. Examples include “diseases of the nervous system and sense organs,” “diseases of the digestive system,” and “congenital anomalies.” In addition, a supplementary classification system, known colloquially as “V-codes,” describes situations when conditions other than current disease or injury are applicable. One would report a past history of a now-inactive condition or a family history of a disease state with a V-code rather than a primary diagnosis code. For example, a family history of a malignant neoplasm in the gastrointestinal tract (V16.0) could support a screening colonoscopy at a younger age than is currently recommended for the general population. Think V-codes for risk factors and diagnosis codes for known patient conditions.