First, you should be aware of the reference range for the serum B12 level, because it varies among laboratories. In addition, this level changes with age, and a slightly lower level is acceptable in elderly patients without representing true deficiency. Usually, proof of a defect in absorption of vitamin B12 is necessary to confirm the diagnosis of pernicious anemia. If these antibodies cannot be detected in a patient with a low serum B12 level, the Schilling test should be performed. Obviously, a complete 24-hour urine collection is vital for a correct test outcome.
Incomplete collection is the most common reason for falsely abnormal results on the Schilling test.
The second phase duplicates the first, with the addition of externally supplied intrinsic factor. Results of this phase are typically abnormal in cases of malabsorption, tapeworm, and bacterial overgrowth. To investigate suspected bacterial overgrowth further, you may administer a broad-spectrum antibiotic, which should normalize absorption, and then repeat the second phase of the test. A challenge is often posed by elderly patients who have a low B12 level, no anemia, minimal macrocytosis, no antibodies to intrinsic factor, and normal results on the Schilling test and bone marrow examination. However, the most accurate way of diagnosing such functional cobalamin deficiency is measurement of serum methylmalonic acid and total homocysteine levels.