Abstract
Objective To estimate the conversion rate from unipolar depression (ICD10 codes F32–F33) to bipolar disorder (BP) (ICD10 codes F31) in an English national cohort. It was hypothesised that early-onset BP (age <18 years) is a more severe form of the disorder, with a more rapid, and higher rate of conversion from depression to BP. Method This record linkage study used English national Hospital Episode Statistics (HES) covering all NHS inpatient and day case admissions between 1999 and 2011. Results The overall rate of conversion from depression to BP for all ages was 5.65% (95% CI: 5.48–5.83) over a minimum 4-year follow-up period. The conversion rate from depression to BP increased in a linear manner with age from 10–14 years – 2.21% (95% C: 1.16–4.22) to 30–34 years – 7.06% (95% CI: 6.44–7.55) (F1,23=77.6, p=0.001, R2=0.77). The time to conversion was constant across the age range. The rate of conversion was higher in females (6.77%; 95% CI: 6.53–7.02) compared to males, (4.17%; 95% CI: 3.95–4.40) (χ2=194, p<0.0001), and in those with psychotic depression 8.12% (95% CI: 7.65–8.62) compared to non-psychotic depression 5.65% (95% CI: 5.48–5.83) (χ2=97.0, p<0.0001). Limitations The study was limited to hospital discharges and diagnoses were not standardised. Conclusions Increasing conversion rate from depression to bipolar disorder with age, and constant time for conversion across the age range does not support the notion that early-onset BP is a more severe form of the disorder.