21 Age was examined by five age bands (16–24, 25–44, 45–64, 65–74, 75 and above). Deprivation was examined using quintiles of Townsend score from ‘one’ (least deprived) to ‘five’ (most deprived). The Townsend score22 is a combined measure of owner-occupation, car ownership, overcrowding and unemployment based on a patient’s postcode and linkage to population census data for 2001 for approximately 150 households in that postal area.
Cases were defined using the Read codes for depression diagnosis and depressive symptoms. Annual incidence rates were calculated by dividing the annual number of incident cases by the total person-years at risk (PYAR) for each year. Single variable analysis was undertaken to examine the association between year of diagnosis (1996 to 2006), gender, age group and quintiles of deprivation score on the one hand and incidence of recorded depression diagnosis or depressive symptoms on the other. Poisson regression was undertaken to investigate the adjusted associations between incidences of recorded depression and depressive symptoms and year of diagnosis, gender, age group and deprivation. The significance of variables in the Poisson regression modelling was assessed using Wald tests. Robust standard errors for the estimated incidence rate ratios (IRRs) were utilised to account for clustering within practices. We examined whether there were interactions between age, gender, deprivation and calendar year. Analysis was conducted using Stata version 10 for Windows.
There were 2 982 024 registered patients aged 16 years and over from January 1996 to March 2006. The number of incident recorded episodes of diagnosed depression was 255 667, and of depressive symptoms 156 907. The median follow-up time was 4.9 years (10th centile 1.6, 90th centile 11.8) for all patients, 6.2 years (10th centile 2.1, 90th centile 14.2) for those with a recorded depression diagnosis and 6.3 (10th centile 2.1, 90th centile 13.2) for those with recorded depressive symptoms.
The most frequently used codes for diagnosis were non-specific codes: ‘ depressive disorder NEC’ (not elsewhere classified) (n = 90 928 episodes, 35.6%) and ‘depression NOS’ (not otherwise specified) (n = 46 681 episodes, 18.2%). The most common symptom codes were ‘depressed’ (n = 46 071, 29.4%) and ‘ complaining of feeling depressed’ (n = 45 203, 28.8%).
Owing to the large sample size (events and PYAR) all associations were found to be statistically highly significant (P<0.0001). The magnitude of the IRRs and associated 95% confidence intervals (CIs) were therefore of primary interest in interpreting any associations.
Incidence of recorded depression diagnoses
The incidence of recorded diagnoses fell from 22.5/1000 PYAR in 1996 to 14.0/1000 PYAR in 2006 (IRR = 0.64, 95% CI 0.57–0.71; online Table DS1). Females were more than twice as likely to have a diagnosis recorded than males. People in the 25 to 44 age group had the highest rate of diagnosed depression, their incidence being more than 50% higher than those in the 16 to 24 age group. People in the most deprived group had nearly twice the rate of depression diagnosis compared with the least deprived group. The effect of gender and deprivation on incidence of depression did not change substantially with time (online Table DS1).
There was an interaction of age with calendar year, and age with gender. Therefore the IRRs for year, compared with the baseline of 1996, were stratified by both age and gender (Tables 1 and 2). The incidence of recorded depression over time fell more markedly in older (aged 65 years or more) compared with younger age groups. For example, in 1996 the incidence of diagnosed depression in those aged 75 and over was 32% greater than that for 16- to 24-year-olds. By 2006 this was reversed with the incidence in those aged 75 and over being 23% lower. In females the overall incidence of depression in those aged 75 and over was 16% less than in the 16- to 24-year-olds, whereas in males there was a 43% increase.
Incidence of recorded depressive symptoms
The incidence of recorded depressiv