A 45-year-old man with treatment-resistant unipolar depression fails two adequate antidepressant trials. Augmentation with lithium is considered. The minimum therapeutic serum lithium level recommended for augmentation of antidepressants is:

• A 0.2–0.4 mEq/L
• B 0.4–0.6 mEq/L
• C 1.0–1.2 mEq/L
• D 0.6–0.8 mEq/L ✓

Explanation

For lithium augmentation of antidepressants in treatment-resistant depression, therapeutic response is associated with serum levels of 0.6–0.8 mEq/L, which overlaps with the lower end of the maintenance range for bipolar disorder (0.6–1.0 mEq/L). Levels below 0.4 mEq/L are subtherapeutic. The acute mania target is typically 0.8–1.2 mEq/L; levels above 1.5 mEq/L are toxic.

Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.

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