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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, R(2)=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) (χ(2)=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) (χ(2)=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.

Original publication




Journal article


J Affect Disord

Publication Date





123 - 128


Bipolar disorder, Conversion, Depression, Adolescent, Adult, Age Distribution, Bipolar Disorder, Child, Cohort Studies, Depressive Disorder, England, Female, Follow-Up Studies, Humans, Male, Sex Distribution, Young Adult