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

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 BR 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 5.650% (95% Cl 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: 116-4.22) to 30-34 years - 7.06% (95% Cl 6.44-7.55) (F-1,F-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% Cl: 6.53-7.02) compared to males, (4.17%; 95% Cl: 3.95-4.40) (chi(2)=194, p <00001), and in those with psychotic depression 8.12% (95% Cl: 7.65-8.62) compared to non-psychotic depression 565%(95% Cl: 5.48-5.83) (chi(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. (C) 2015 Elsevier B.V. All rights reserved.