Clinical code usage in UK general practice: a cohort study exploring 18 conditions over 14 years

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To assess the diagnostic Read code usage for 18 conditions by examining their frequency and diversity in UK primary care between 2000 and 2013.
Population-based cohort study
684 UK general practices contributing data to the Clinical Practice Research Datalink (CPRD) GOLD.
Patients with clinical codes for at least one of: asthma, chronic obstructive pulmonary disease (COPD), diabetes, hypertension, coronary heart disease, atrial fibrillation, heart failure, stroke, hypothyroidism, chronic kidney disease, learning disability (LD), depression, dementia, epilepsy, severe mental illness (SMI), osteoarthritis, osteoporosis, and cancer.
Primary and secondary outcome measures
For the frequency ranking of clinical codes, canonical correlation analysis was applied to 1-,3-, and 5-year correlations of clinical code usage. Three measures of diversity (Shannon entropy index of diversity, richness, and evenness) were used to quantify changes in incident and total clinical codes.
Overall, all examined conditions except LD, showed positive monotonic correlation. Hypertension, hypothyroidism, osteoarthritis, and SMI codes’ usage had high 5-year correlation. The codes’ usage diversity remained stable overall throughout the study period. Cancer, diabetes, and SMI had the highest richness (code lists need time to define) unlike atrial fibrillation, hypothyroidism, and LD. SMI (high richness) and hypothyroidism (low richness) can last for 5 years, whereas, cancer and diabetes (high richness) and LD (low richness) only last for 2 years.
This is an underreported research area and the findings suggest the codes’ usage diversity for most conditions remained overall stable throughout the study period. Generated mental health code lists can last for a long time unlike cardiometabolic conditions and cancer. Adopting more consistent and less diverse coding would help improve data quality in primary care. Future research is needed following the transfer to the Systematized Nomenclature of Medicine Clinical Terms (SNOMED CT) coding.
Original languageEnglish
JournalBMJ Open
Publication statusPublished - 25 Jul 2022


  • Primary Care
  • Clinical Codes
  • Electronic Health Records
  • QOF
  • Quality and Outcomes Framework (QOF)


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