BDNF and treatment resistant depression: The antidepressant effects of ECT with ketamine anaesthesia were not correlated with changes in serum BDNF levels (all <i>p</i> > 0.05).<h4>Conclusion</h4>This preliminary study indicated that serum BDNF levels do not appear to be a reliable biomarker to determine the antidepressant effects of ketamine as an anaesthesia in ECT for patients with TRD.