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To the Editor.—
Doctors Huffman et al are quite correct that the fluorometric assay method is relatively insensitive, that is, its lower limit of sensitivity is 0.1μg/ml. As a matter of fact, on all of the graphs in the article referred to, the ordinate which shows the serum methotrexate concentration uses 0.1μg/ml as its lower limit instead of (0), since the test did not provide sensitivity below this level.Doctors Huffman et al do not say whether they gave the methotrexate intravenously, intramuscularly, or orally. By the "distribution" phase, I presume they mean the time it takes for the drug not only to distribute within the bloodstream, but also to disseminate out to the tissues and to equilibrate in the extracellular and, perhaps, intracellular space. This kind of measurement cannot be made if the drug is given intramuscularly or orally, since absorption is going on at the same time as
Half-Life of Methotrexate: New Evidence-Reply. Arch Dermatol. 1972;105(5):762. doi:10.1001/archderm.1972.01620080084031