Vitamin B12 and folate blood tests explained — what they measure, why GPs order them, and how they are reported in Australia.
Vitamin B12 and folate (vitamin B9) are two vitamins the body needs to make healthy red blood cells and to keep the nervous system working normally. They are often tested together because a shortage of either can cause similar changes in the blood.
The usual test measures the total amount of each vitamin in the blood. Some laboratories also offer an "active B12" test (holotranscobalamin), and folate can be measured in serum or inside red blood cells (red-cell folate).
In Australia, B12 is usually reported in pmol/L and folate in nmol/L.
B12 and folate may be checked when there are reasons such as:
Your GP decides which tests fit your situation.
Both are standard blood tests and generally do not require fasting. If they are ordered alongside other tests that do need fasting, your GP or the laboratory will let you know. Results are usually available within a day or two.
B12 is reported in pmol/L and folate in nmol/L. As a commonly used guide, B12 results in the low range (around the bottom of the laboratory's reference interval) and serum folate at the lower end are sometimes flagged for follow-up. Where a B12 result is borderline, some labs add the "active B12" test to help clarify the picture.
Reference intervals differ between laboratories and assays. Always read the reference range printed on your own report, and only the clinician who ordered the test can interpret what your result means for you.
They are two different vitamins that both help make red blood cells. B12 also has an important role in nerve function and comes mainly from animal foods; folate comes mainly from leafy greens, legumes, and fortified foods. They are often tested together.
Long-term metformin use is recognised as one factor that can be associated with lower B12 over time, which is one reason a GP may check it. What this means for you is a matter for your GP.
Active B12 (holotranscobalamin) measures the portion of B12 available to cells. Some laboratories use it as a second-line test when a standard B12 result is borderline.