What blood tests are necessary before starting l-methylfolate? B12, Folate, CBC?

I believe the answer to your question is yes. I doubt a doctor will prescribe methylfolate unless a folate deficiency is documented.

I found the following info about the brand of methylfolate called Deplin (there are two brands of methylfolate, Deplin is the one used for depression and folate deficiency):

The Deplin brand of l-methylfolate is used in the following situations:

in people with major depressive disorder who have folate deficiency. Deplin is used together with anti-depressant medications in people with major depressive disorder and low folate levels.

in people with schizophrenia who have hyperhomocysteinemia related to folate deficiency.

Deplin is not an antidepressant. However, it may enhance the effects of antidepressant medications.

A doctor would test for folic acid (folate) levels in the blood before prescribing methylfolate.

Hope this helps,

Laurie