Vitamin level tests measure the amount of specific vitamins in your blood. These tests can help diagnose vitamin deficiencies or excesses, which can cause various health problems. Common Vitamin Level Tests: Vitamin D: Measures 25-hydroxyvitamin D, the main storage form of vitamin D in the body. Vitamin B12: Measures the level of vitamin B12 in the blood. Folate (Vitamin B9): Measures the level of folate in the blood. Vitamin A: Measures the level of retinol, a form of vitamin A, in the blood. Vitamin C: Measures the level of ascorbic acid, the active form of vitamin C, in the blood. Vitamin E: Measures the level of alpha-tocopherol, the most active form of vitamin E, in the blood. Vitamin B6: Measures the level of pyridoxine in the blood.