Serum level of nerve growth factor is a potential biomarker of conversion to bipolar disorder in women with major depressive disorder.

Aim: The aim of this study was to identify biomarkers associated with major depressive disorder (MDD) and conversion from MDD to bipolar disorder (BD) in an outpatient sample of women.

Methods: This was a longitudinal study including women diagnosed with MDD and aged 18 to 60 years. The follow-up was 3 years. The diagnosis was performed using the Mini International Neuropsychiatric Interview Plus. Blood collection was just performed in the first phase. Serum interleukin-6, tumor necrosis factor-α, brain-derived neurotrophic factor, glial cell-derived neurotrophic factor, and nerve growth factor (NGF) levels were measured using a commercial immunoassay kit.

Results: We included 156 women. The conversion rate from MDD to BD was 15.4% (n = 24). NGF serum levels were increased in patients who converted to BD compared to the remitted MDD group and current MDD group (P = 0.013). The Bonferroni post-hoc test for multiple comparisons revealed significant differences for higher NGF levels in patients who converted to BD compared to patients with current MDD (P = 0.037). Interleukin-6, tumor necrosis factor-α, brain-derived neurotrophic factor, and glial cell-derived neurotrophic factor serum levels did not differ among the groups.

Conclusion: Our results suggest that NGF might be a useful biomarker associated with early detection of conversion to BD, helping clinicians in the clinical diagnosis.