The folate blood test is extremely useful in tracing vitamin deficiencies by measuring the liquid portion of the blood (plasma or serum). The test can also tell you how much folate is in your red blood cells. The folate and vitamin B12 blood test can easily determine the levels of the two major B complex vitamins that the human body requires for many key functions such as making standard red blood cells, white blood cells, heal cells, and tissues, in addition to synthesizing DNA.