Which is the best test for diagnosing folate deficiency?

Folate deficiency is best assessed using the red blood cell (RBC) folate level because it reflects the body's folate stores over a longer duration compared to serum folate. Serum folate levels can fluctuate based on recent dietary intake and may not accurately represent an individual's chronic folate status. In contrast, RBC folate incorporates the folate that has been taken up by red blood cells during their formation in the bone marrow, providing a more stable indication of folate availability.

Measuring homocysteine levels can provide supportive information since elevated levels can indicate a deficiency in folate, vitamin B12, or vitamin B6; however, it is not specific enough to diagnose folate deficiency alone. Similarly, while methylmalonic acid is a marker for vitamin B12 deficiency, it does not directly indicate folate status and is not useful in diagnosing folate deficiency specifically. Therefore, RBC folate is the most reliable test for diagnosing folate deficiency due to its reflection of long-term folate levels in the body.