Some (eight) case reports and case series including a total of seventeen patients reported data regarding the efficacy of ketamine in patients with TRD.
Szymkowicz et al.  reported that three patients responded successfully to the ketamine infusions and no significant side-effects have been found after ketamine administration.
After 24 hours following the first dose of ketamine, Murrough et al.  showed a significant antidepressant activity (89% change in MADRS scores) of ketamine in a subjects treated with ketamine. A sustained (three months) remission from depression has also been reported.
Segmiller et al.  found that 50% of the six TRD patients which were enrolled showed an improvement in depressive symptoms in both the short and longer term. In two patients (33.3%) remission has been reached whereas two other patients experienced dissociative symptoms.
Robust changes in depression symptoms measured by the BDI scores in response to ketamine have also been suggested by Messer et al. . No memory or concentration impairments have been associated with ketamine infusions.
Similarly, Paslakis et al.  showed that a significant improvement was obtained with the use of ketamine in two patients whereas no significant side effects were reported.
Subjects experienced a significant improvement of depressive symptoms on the second day post ketamine infusion based on Hamilton Depression Rating Scale (HDRS), and Beck Depression Inventory (BDI) scores . The first improvement has been reported 25 minutes after ketamine infusion but a persistent antidepressant effect has also been observed throughout the subsequent 7 days. Specifically, after the first ketamine infusion, a profound improvement of depressive symptoms was reported the second day post infusion whereas the second infusion was less effective.
Finally, Paul et al.  reported that one patient did not respond to both intravenous administration of ketamine and S-ketamine whereas another patient showed a rapid antidepressant effect as assessed by a relevant decrease in HAMD21 and BDI at days 1 and day 3 but not until day 6. Both patients experienced psychomimetic side effects during ketamine infusion which were absent when S-ketamine has been administered. Table 11 summarizes the most relevant case reports reporting the antidepressant efficacy of ketamine in patients with Treatment Resistant Depression.