A vitamin B12 and folate test checks the blood serum levels for two B vitamins important to proper nervous system function. It is most likely to be recommended by a doctor when a standard complete blood count (CBC) test comes back showing enlargement of the red blood cells or unusual white blood cells. It may also be appropriate for people who feel tired, dizzy or weak and for patients who are suffering from memory loss, depression or tinging or itching feelings. 

Folate and vitamin B12 tests can help a patient evaluate his or her general health and nutritional status. It can also help detect serious nutrition or metabolic diseases, such as malnutrition caused by alcoholism or liver disease, cancer of the digestive tract or conditions that prevent the body from absorbing fats and vitamins. These diseases can cause a person with an otherwise healthy diet to feel unwell and show vitamin deficiencies that may not be easy to detect without a test.

To get an accurate result on a vitamin B12 and folate test, patients should FAST for 10 hours before they have their blood drawn. Some seizure medications, such as phenytoin can decrease blood folate levels, while omeprazol and metformin cause problems with vitamin B12. If the test results are low, taking supplements or improving the diet may help the problem. It's important to take both types of supplements since taking only folates can mask signs of a vitamin B12 deficiency.