Developing red blood cells take up folate, but once red blood cells are released from the bone marrow there is no further uptake of folate.
In theory, red cell folate is not affected by recent folate intake and could be a better marker of folate status than serum folate.
However, in reality this is not the case, because red cell levels are influenced by numerous other factors, including:
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vitamin B12 status (vitamin B12 is required for uptake of folate into developing red cells. For this reason, red cell folate may be falsely low in B12 deficient patients)
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haemoglobin oxygen saturation (lower O2 saturation is associated with more stable folate forms and a higher red cell folate result)
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haematocrit (this needs separate measurement and introduces an additional variable)
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preanalytical preparation steps also render red cell folate measurements less precise
A large increase is seen in serum folate levels 1-3 hours after folate supplementation. However, a "normal" diet contains far less folate and the increase in serum folate after food intake is much smaller. Serum folate levels take 8 - 9 weeks to stabilise after changing dietary intake, thereby reflecting long term folate status.
Serum folate shows closer correlation with markers of folate deficiency (such as homocysteine), and a more direct relationship with haemoglobin response to supplementation.
Red cell folate is also a more expensive test than serum folate due to the additional preanalytical preparation steps required.
For these reasons red cell folate is no longer offered by LabPlus from 20th October 2016.
Ref: Farrell C-J L, Hirsch SH and Herrmann M. Red cell or serum folate: what to do in clinical practice?
Clin. Chem. Lab. Med. 2013; 51(3): 555-69