L.B., I have not had any folic acid in supplements or foods, nor any supplemental folate/folinic acid/etc. for 8 weeks, and my recent blood test for serum folate showed that I was at 9.7 ng/mL with a reference range that being above 3.0 is normal.
I do try to eat some veggies every day (I should eat more), and I don't often have the ones that are highest in folate, although I should.
And - I am compound heterozygous for the 2 MTHFR genes that have been studied the most, which is apparently like a 70% reduction in the ability to process folate ( or something like this -- I'm not clear about that area yet, so don't take my word for it).
So I was surprised my blood test for folate came back so "normal".
I have read tonight, before seeing your question, that there are two different types of test for folate, one is serum folate and one is red blood cell folate. There seems to be disagreement about which is best. I'll put the references below.
I was aware that there were two, and I thought that I was getting the better one when I ordered my recent blood tests, but perhaps I did not get the right one.
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Wikipedia talks about the 2 tests in several entries, I won't link to them here
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Somewhere on labtestsonline.org I saw where they said there was disagreement about which test was best, but I can't find that quote now.
The best I can do is:
http://labtestsonline.org/understanding/conditions/vitaminb12/start/3
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"The measurement of
folate in red blood cells (RBCs) is preferred since it reflects long-term folate status in the body compared to plasma/serum folate which may be influenced by recent dietary intake."
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2716806/
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"Evidence from the literature indicates that
serum folate measurements provide equivalent information to red cell folate measurements when attempting to determine whether folate deficiency is present. There seems to be no basis for the routine testing of all samples for serum/plasma folate and a red cell folate."
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1770141/
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"Tests commonly used for the detection of these vitamin deficiencies are serum folate, red cell folate and serum B12 concentrations. Serum folate becomes subnormal in the early stages of negative folate balance, before reduction of folate stores. Red cell folate is a direct measure of tissue folate stores. Measurement of both serum and red cell folate yields the maximum information, but in practice, red cell folate needs to be assayed only when there is macrocytosis and the serum folate is low. Serum B12 is a sensitive index of deficiency, but a low level does not necessarily indicate deficiency."
"the
serum folate level depends on recent dietary intake and
is not strictly a diagnostic test for body folate depletion"
"Measurement of serum folate
only will not differentiate between negative folate balance and tissue folate depletion. Measurement of red cell folate
only may miss the early stage of negative folate balance. Serum and red cell folate yield complementary data and together the maximum information. However,
in practice, it is usual to `screen' with the serum folate assay and to proceed to red cell folate only if the serum folate is subnormal. In a folate deficient patient recently given folic acid, only red cell folate will detect deficiency."
http://www.australianprescriber.com/magazine/22/1/16/8/