The reproducibility and use of the tritiated folic acid urinary excretion test as a measure of folate absorption in clinical practice: effect of methotrexate on absorption of folic acid.

Investigations into the standardization and reproducibility of the urinary excretion method for determining the absorption of tritiated folic acid were carried out. By proper timing of tissue-loading doses of folic acid a clear difference between the percentage excretion ranges of normal and coeliac subjects was obtained. In addition conditions were found whereby the procedure could be repeated on an individual up to four times with reproducible results. Methotrexate in pharmacological amounts was found to have no inhibitory effect on the human intestinal absorption of a small oral dose (300 mug) of folic acid as determined by this method. This indicates that reduction of folic acid is not necessary for its absorption in man.