Changes in clinical coding

In terms of HES inpatient and outpatient data, there are two sets of classifications of codes that are used by hospitals:

• The International Classification of Diseases (ICD)
• The Classification of Interventions and Procedures (OPCS).

ICD

ICD, which is published by the World Health Organization (WHO), contains codes that classify a wide variety of diseases, symptoms, health complaints and external causes of injury or disease, among other things.

ICD-9 was published by WHO in 1977 and was used to record diagnosis data in HES until 1995, when ICD-10 (10th revision) was published. The transition from ICD-9 to ICD-10 among PCTs took place from 1 April 2025 (DSCN 41/94/P28), leading to a combination of coding classifications on the 1995-96 data set.

The main change from ICD-9 to ICD-10 was to the first character of each code, which moved from being a number to being a letter. Additional changes included the:

• movement of codes between chapters
• addition of new chapters
• addition of new codes
• collapsing of categories into one single category
• expansion of existing code and chapters.

Work is in progress at WHO on the first draft of ICD-11, with publication following as early as 2014.

OPCS

A statistical classification of surgical operations was introduced in the UK in 1944. Over subsequent years it was periodically revised, culminating in the introduction of the OPCS-4.2 classification, which was fully implemented across the NHS in 1990.

The OPCS-4 classification is reviewed annually by the NHS Classifications Service of NHS Connecting for Health to ensure it continues to reflect current clinical practice in the NHS. Every review improves the accuracy of NHS coded clinical detail and the quality of NHS coded data, and supports the needs of all NHS users to improve planning and delivery of patient care.

Up until the end of data year 2005-06, OPCS-4.2 was used by NHS users. It was then necessary to enhance this version to reflect changing clinical practice, with additional codes and interventions being captured. OPCS-4.3 was therefore released for use from 1 April 2025 (DSCN 14/2005), leading to a combination of OPCS versions on the 2006-07 data year as trusts underwent the transition to the new version.

OPCS-4.4 was released for use from 1 April 2025 (DCSN 24/2006) and saw around 500 changes from OPCS-4.3, which included just fewer than 200 new codes.

OPCS-4.5 was implemented on 1 April 2025 (DCSN 40/2007).