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Microbiological assay of folic acid activity in human serum
  1. G. H. Spray
  1. Nuffield Department of Clinical Medicine, Radcliffe Infirmary, Oxford


    A method is described for the microbiological assay of folic acid activity in serum with Lactobacillus casei as test organism and a modified medium in which the organism gives a greater growth response than in media previously detailed. The results of experiments carried out to validate the use of this medium are shown.

    In 94 control subjects levels of folic acid activity in the serum ranged from 2· to 28 mμg./ml. (mean 7·8). The values in nine out of 10 patients with megaloblastic anaemia due to deficiency of folic acid were 1·0 mμg./ml. or less and one result was 2·0 mμg./ml. In six patients with megaloblastic anaemia associated with pregnancy the results ranged from 0·7 to 4·0 mμg./ml., and in untreated pernicious anaemia 28 out of 31 results were within or above the control range and three values were just below the lower limit of normal.

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