Final answer:
The VDRL test is the least important baseline lab value to obtain before starting lithium therapy for bipolar disorder since it assesses syphilis status, which is generally unrelated to the management of the disorder.
Step-by-step explanation:
The least important baseline lab value to obtain on a patient starting lithium therapy is the venereal Disease Research Laboratory (VDRL) test. Lithium is most commonly used for the treatment of bipolar disorder and can impact thyroid and renal function, which is why thyroid function tests and renal function tests are important to obtain before starting therapy. An electrocardiogram (ECG) may be relevant for patients with cardiovascular risk, and since lithium can affect the leukocyte count, a WBC (white blood cell) count is also important. However, the VDRL test, which screens for syphilis, is generally not relevant to the management of bipolar disorder unless there is a particular clinical indication for this test.