Mrs. Smith hаs cоme tо the mentаl heаlth center seeking treatment fоr depression. She has a history of suicide attempt by overdose of an SSRI 1 month prior. She was started on imipramine after that event but stopped it 1 week later because "it did no good". The PMHNP meets with Mrs. Smith to plan care with her. In completing her assessment, the most appropriate lab test for the PMHNP to order at this time is;