21 % of 228 tests for vitamin B12 absorption were unsatisfactory because of a discrepancy between the result obtained by the urinary excretion method and by the measurement of plasma radioactivity. The usual error was recording a too low value for the urinary excretion because of incomplete urine collection or impaired renal function. In all these cases the plasma radioactivity was within the normal range. In 4 patients plasma radioactivity was too low but urinary excretion normal.
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Chanarin et al. (1974) studied this question.
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