Research reportConversion from depression to bipolar disorder in a cohort of young people in England, 1999-2011: A national record linkage study

- English national hospital study of conversion from depression to bipolar disorder.
- The rate of conversion for all ages was 5.65% (95% CI: 5.48-5.83).
- The conversion rate increased in a linear manner with age (F=23.1, p=0.001).
- The time to conversion was constant across the age range.
- The conversion rate was greater in females and for psychotic depression.

ObjectiveTo 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.MethodThis record linkage study used English national Hospital Episode Statistics (HES) covering all NHS inpatient and day case admissions between 1999 and 2011.ResultsThe 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).LimitationsThe study was limited to hospital discharges and diagnoses were not standardised.ConclusionsIncreasing 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.