What is the ICD and how does it work?

You have already read an article here on this blog about the latest version of the International Classification of Diseases (ICD), namely ICD-11. But is it really clear how this classification came about and how it is structured?

After all, if you have ever needed a medical record, a test authorization or a hospital admission, you have very likely come across a code like “ICD-10: J06.9” or something similar. But what does it actually mean? These codes come from the International Classification of Diseases, better known as the ICD — a system created to organize and standardize the recording of diseases, symptoms and causes of death worldwide.

What is the ICD?

The ICD is a project led by the World Health Organization (WHO). Its origins go back to the 19th century, when physicians and statisticians realized that each country used different names for the same diseases, which made any kind of international comparison difficult.

The first official version was published in 1893, still under the name “International List of Causes of Death”. Since then, the document has been constantly revised and updated — and today it is already in its 11th edition (ICD‑11).

What the ICD is for

The main function of the ICD is to standardize medical language. In other words, it allows healthcare professionals, managers, researchers and even hospital billing systems to speak the same language.

When a physician includes an ICD code in a medical record, they are stating the diagnosis precisely, without relying on subjective descriptions. This makes everything easier, from the exchange of information between medical teams to the analysis of public health data — and it is also essential to the hospital revenue cycle , in which each diagnosis must be classified correctly for billing and auditing purposes.

How the ICD is organized

Each disease or health condition has an alphanumeric code (made up of letters and numbers). In ICD-10, for example, the codes range from A00 to Z99 and are divided into 21 chapters. Each initial letter indicates a large group of diseases:

• Chapter I (A00 to B99): Certain infectious and parasitic diseases

• Chapter II (C00 to D48): Neoplasms [tumors]

• Chapter III (D50 to D89): Diseases of the blood and blood-forming organs and certain immune disorders

• Chapter IV (E00 to E90): Endocrine, nutritional and metabolic diseases

• Chapter V (F00 to F99): Mental and behavioral disorders

• Chapter VI (G00 to G99): Diseases of the nervous system

• Chapter VII (H00 to H59): Diseases of the eye and adnexa

• Chapter VIII (H60 to H95): Diseases of the ear and mastoid process

• Chapter IX (I00 to I99): Diseases of the circulatory system

• Chapter X (J00 to J99): Diseases of the respiratory system

• Chapter XI (K00 to K93): Diseases of the digestive system

• Chapter XII (L00 to L99): Diseases of the skin and subcutaneous tissue

• Chapter XIII (M00 to M99): Diseases of the musculoskeletal system and connective tissue

• Chapter XIV (N00 to N99): Diseases of the genitourinary system

• Chapter XV (O00 to O99): Pregnancy, childbirth and the puerperium

• Chapter XVI (P00 to P96): Certain conditions originating in the perinatal period

• Chapter XVII (Q00 to Q99): Congenital malformations, deformations and chromosomal abnormalities

• Chapter XVIII (R00 to R99): Symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified

• Chapter XIX (S00 to T98): Injury, poisoning and certain other consequences of external causes

• Chapter XX (V01 to Y98): External causes of morbidity and mortality

• Chapter XXI (Z00 to Z99): Factors influencing health status and contact with health services

A curiosity: Chapter XXII (U04 to U99) covers codes reserved for special WHO purposes.

Here are some examples of disease coding in ICD‑10:

• A09: Intestinal infections of unspecified origin

• B20: Human immunodeficiency virus (HIV) disease resulting in infectious and parasitic diseases

• C34: Malignant neoplasm of the bronchi and lungs

• D50: Iron deficiency anemia

• E11: Non-insulin-dependent diabetes mellitus

In addition, each group is subdivided into more specific categories. For example, J06.9 corresponds to “Acute upper respiratory infection, unspecified”. In short, the code is like a “ZIP code” for diseases: it shows where each one fits on the big map of health.

The move to ICD‑11

The most recent version, ICD-11 , was launched by the WHO in 2022 with a focus on digital modernization. It brings more detailed codes, compatibility with electronic health systems and new chapters, such as the one on conditions related to traditional medicine. This update makes the work of hospitals and health plans easier, making recording and billing more accurate — something essential in times of integration between technology and hospital management .

As we have seen, the ICD is much more than a list of diseases. It is a global communication tool that helps physicians, managers and authorities better understand the health landscape of the planet. And in the information age, when every piece of data counts, using the correct classification is synonymous with efficiency — not only in diagnosis, but also in hospital management and public policy planning.