Hi Wendy: There can be many reasons that your doctor would recommend it, but yes, there is a blood test that can determine what kind of antidepressant you would need. The problem is that the tests are currently only available in Canada and are very expensive. You would have many tests done on your blood, at about $40 a test, and would need about 50 tests. These are not yet available in the United States just now. That being said, your doctor may be checking many other issues before he wants to prescribe the right medicine for you. You need to ask specifically what he is looking for with the blood test. For instance, he may want to check your thyroid, other endocrine levels, and get general information about how you are doing physically before he tries a medication. Ask your doctor what he is looking for with your blood. If you trust him, then do what he recommends. If not, then find a doctor you trust.   I hope this helps, Dr. Diana Walcutt   *Dr. Walcutt is a Psychologist. Dr. Walcutt's answers are not intended to diagnose or treat any medical or mental disorder. Any information given in a post about medication is for educational purposes only, and primarily to aid you in having an informed discussion with your own Psychiatrist/Physician.