BDNF and depressive symptom measurement: In a study by Koch et al. [45] among major depressive disorder patients (N = 30) undergoing 12 sessions of interpersonal psychotherapy, there was neither a difference in plasma BDNF levels between baseline and the 21st day of treatment, nor a difference between nonresponder and responder groups, where a response was defined as a reduction of at least 50% of the baseline Hamilton Depression Rating Scale.