Coding of covariates
Socio-economic status in THIN is
coded by the patient postcode-based Townsend Deprivation Index which is
constructed from 4 census variables: households without a car,
overcrowded households, households not owner-occupied and persons
unemployed. It is scaled from 1 to 5, where the first quintile
represents the least deprived and the fifth represents the most deprived
group.31 In the final analysis, patients within
quintiles 1 and 2 were re-coded as low, 3 as medium, 4 and 5 as high
level of deprivation. BMI was categorized as healthy weight and
overweight compared to obese. To classify hypertension, measurements of
systolic and diastolic blood pressure
(SBP≥140 mmHg and DBP≥90 mmHg)
were used in conjunction with Read codes, as previous research showed
that using only Read codes to select hypertensive patients in THIN
underestimates the actual prevalence of hypertension in the
UK.32 Depending on the use of anti-hypertensive drugs
at baseline, hypertensive patients were categorized as treated or
untreated. Uterine and ovarian status was grouped as: intact (no history
of removal of uterus and ovaries), hysterectomy with oophorectomy
(hysterectomy and at least one ovary removed), and oophorectomy only
(one or both ovaries removed). A very small group of women with
hysterectomy without oophorectomy was not included in the model. Birth
year was grouped into four decade-long cohorts.