Assessing the Importance of Variation in Diagnostic Coding Among the Three Countries in the UK Biobank

Journal article

Background: The UK Biobank (UKB) study has linked hospital inpatient data collected from England, Scotland, and Wales, which use different clinical coding systems to record health outcomes. Scotland records up to 6 different diagnostic codes for one inpatient episode, compared with up to 20 in England and 14 in Wales. We assessed the relationship of the variations in diagnostic coding among countries on observed disease incidence rates. Methods: We examined the number of diagnoses coded by each country, and then compared the incidence of three diseases between countries: Parkinson's disease (PD), type 2 diabetes (T2D), and dementia. We constructed Cox models for each disease, adjusting for “country.” Results: Compared with England, Scotland appears to have the lowest risk (hazard ratio, HR) for all three diseases: HR [95% CI] = 0.62 [0.54, 0.72] for PD, 0.49 [0.45, 0.54] for T2D, and 0.88 [0.78, 0.99] for dementia. Conclusions: The observed incidence of these diseases and the estimated effect of “country” in Cox models are likely influenced by the clinical coding variations among countries. Researchers need to be aware of this and account for these variations in their analyses.

Authors: Clifton, L; Liu, W; Collister, JA; Littlejohns, TJ; Goldacre, RR

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Wiley
• Journal: Learning Health Systems
• Volume: 10
• Issue: 3
• Article number: e70083
• Publication date: 2026-05-05
• Acceptance date: 2026-04-14
• DOI: 10.1002/lrh2.70083
• EISSN: 2379-6146
• ISSN: 2379-6146
• Language: English
• Keywords: UK Biobank (UKB); type 2 diabetes (T2D); dementia; Parkinson's disease (PD); diagnostic coding; country variation