This chapter presents microbiological assay for serum, plasma, and red cell folate, using cryopreserved, microtiter plate method. In the 500-ml beaker a solution of 0.5% (w/v) sodium ascorbate is prepared by dissolving 2.5 g of sodium ascorbate in 500 ml of water. This solution is used for preparing the working standard and for all assay dilutions. The working standard is prepared in the following way—a universal tube containing stock standard solution is brought to room temperature and a 50-μl aliquot is taken and diluted to 100 ml with 0.5% (w/v) sodium ascorbate in a volumetric flask. This solution is mixed thoroughly, then 5 ml is taken and diluted to 100 ml with 0.5% (w/v) sodium ascorbate in a second volumetric flask to give a final working standard concentration of 500 ng/liter. Duplicate 50- μl aliquots of serum or plasma or duplicate 25- μl aliquots of whole-blood lysate are pipetted into labeled 4-ml polypropylene tubes. Using the adjustable repetitive sampling pipette the aliquots are diluted to a total of 1 ml with 0.5% (w/v) sodium ascorbate.

Microbiological assay for serum, plasma, and red cell folate using cryopreserved, microtiter plate method