CTV3 is a comprehensive computerised coding dictionary used by clinicians to record key clinical information about patients and also associated tests, diagnosis, and medicines.

CTV3 codes are used in the OpenSAFELY-TPP implementation, and fully align with the GP subset of SNOMED CT, the NHS standard.⁹

There are almost 300 000 codes in total, each five characters long.

CTV3 codes can be organised into hierarchies much like a book with chapters. CTV3 has a tree data structure with ‘parent’ concepts describing broader clinical areas and ‘child’ codes of increasing specificity as you move down the hierarchy.

For example, a concept such as ‘laboratory procedures’ will have ‘child’ codes that are more specific such as ‘haematology’, which will in turn be broken down further into increasingly detailed concepts.

Most child codes can only have a single parent, although 3% have multiple parents.