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Schilling's test assesses the absorption of vitamin B12.
Following an overnight fast, the patient is first injected intramuscularly with non-labelled vitamin B12, sufficient to saturate the body's stores. This is followed by oral doses of vitamin B12 labelled with cobalt-58, and a second dose labelled with cobalt-57 bound to intrinsic factor.
Prior saturation body stores ensures that the radio-labelled vitamin B12 is absorbed and rapidly excreted in the urine. Malabsorption is indicated if less than 10% is excreted in 24 hours.
Malabsorption of vitamin B12 due to pernicious anaemia is indicated by an elevated ratio of Co-57 to Co-58; mean 3.5, range, 1.2-9.5. The ratio is near unity in patients with ileal disease.