Folate (also called folic acid or vitamin B9) is a B vitamin the body needs to make and repair DNA and to produce red blood cells. This blood test measures the amount of folate in the blood, usually as serum folate. It is sometimes ordered together with vitamin B12 because the two vitamins work closely together. This test is used to help detect a folate deficiency, which can contribute to certain types of anemia. It may be ordered when a person has symptoms such as fatigue or when blood count results suggest larger-than-normal red blood cells. It can also be used to monitor people at higher risk of low folate, such as those with poor nutrition or certain digestive conditions.