A method for the measurement of serum folic acid activity is described, which is a modification of previous methods.
The material in serum with activity for L. casei is made up of a stable and a labile component. The amount of stable component in normal subjects and patients with megaloblastic anaemia is similar. The amount of labile component varies. In patients with folic acid deficiency none is present; in normal subjects it constitutes between 65 and 94% of the total serum L. casei activity. The labile component appears to be an index of folic acid metabolism, and the assay of total serum L. casei activity is therefore a valuable method for differentiating patients requiring treatment with folic acid from normal subjects and patients with primary vitamin B12 deficiency. Normal subjects had serum folic acid levels from 5·9 to 21·0 mμg./ml. (mean 9·9 mμg./ml. ± 0·3 mμg./ml. S.E.). In patients with megaloblastic anaemia requiring treatment with folic acid, other than megaloblastic anaemia of pregnancy, the levels were less than 4·0 mμg./ml. Patients with uncomplicated pernicious anaemia had levels from 6·0 to 27·0 mμg./ml. (mean 16·6 mμg./ml. ± 1·1 mμg./ml. S.E.). The mean level in this group was higher than in normal subjects, and the highest levels of all were found in patients with subacute combined degeneration of the cord with minimal anaemia (range 14·4 to 36·8 mμg./ml.; mean 24·8 mμg./ml. ± 2·4 mμg./ml. S.E.).
The L. casei activity of the labile component is lost during autoclaving or storage at −20°C. This loss can be prevented during autoclaving by using adequate amounts of ascorbic acid in the phosphate buffer used to dilute the serum for assay and by adding ascorbic acid to serum that is to be stored. Moreover, the activity lost during the storage of serum not protected by ascorbic acid could be restored, for periods up to three months, by adding ascorbic acid to this serum before assay.
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