The results of the measurement of vitamin B12 absorption by counting the radioactivity of 5 ml. serum obtained eight to 10 hours after the ingestion of an oral dose of 0·5 μg. vitamin B12 labelled with 0·5 μc. 57Co are compared with those obtained with the urinary excretion (Schilling) test. Inadequate urine collection and impaired renal function were responsible for low results in the Schilling test in four of the 12 control subjects, and an incomplete urine collection in four patients with pernicious anaemia could have led to doubt about the validity of the low result.
The measurement of serum radioactivity for 1,000 seconds gave conclusive results, the range in the patients with malabsorption of vitamin B12 being between 0 and 24 counts per minute, and in the control subjects and other patients with megaloblastic anaemia between 28 and 64 counts per minute. The highest serum radioactivity level in a patient with pernicious anaemia was 19 counts per minute.
Serum counting is simpler than the Schilling test and may be done alone when the patient's renal function is known to be poor, when urine collection is expected to be unreliable, or when the flushing dose of vitamin B12 should be avoided. Otherwise there is an advantage in doing both tests together for confirmation.
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