nutritional disorders
Vitamin B12 Deficiency
Diagnosis of Vitamin B12 Deficiency
Complete blood count (CBC) and vitamin B12 and folate levels Sometimes methylmalonic acid levels or Schilling test It is important to remember that severe neurologic disease may occur without anemia or macrocytosis. 145 pmol/L) indicates vitamin B12 deficiency. The folate level is measured because vitamin B12 deficiency must be differentiated from Folate Deficiency as a cause of megaloblastic anemia; folate supplementation can mask vitamin B12 deficiency and may alleviate megaloblastic anemia but allow the neurologic deficits to progress or even accelerate. When clinical judgment suggests vitamin B12 deficiency but the vitamin B12 level is low-normal (200 to 350 pg/mL [145 to 260 pmol/L]) or hematologic indexes are normal, other tests can be done. They include measuring the following: Serum methylmalonic acid (MMA) levels: An elevated MMA level supports vitamin B12 deficiency but may be due to renal failure. MMA levels can also be used to monitor the response to treatment. MMA levels remain normal in folate deficiency. Homocysteine levels: Levels may be elevated with either vitamin B12 or folate deficiency. Less commonly, holotranscobalamin II (transcobalamin II–B12 complex) content: When holotranscobalamin II is<40 pg/mL (<30 pmol/L), vitamin B12 is deficient. After vitamin B12 deficiency is diagnosed, additional tests (eg, Schilling test) may be indicated for younger adults but usually not for older people. Unless dietary vitamin B12 is obviously inadequate, serum gastrin levels or autoantibodies to intrinsic factor may be measured; sensitivity and specificity of these tests may be poor. The Schilling test is useful only if diagnosing intrinsic factor deficiency is important, as in classic pernicious anemia. This test is not necessary for most older patients. The Schilling test measures absorption of free radiolabeled vitamin B12. Radiolabeled vitamin B12 is given orally, followed in 1 to 6 hours by 1000 mcg (1 mg) of parenteral vitamin B12, which reduces uptake of radiolabeled vitamin B12 by the liver. Absorbed radiolabeled vitamin B12 is excreted in urine, which is collected for 24 hours. The amount excreted is measured, and the percentage of total radiolabeled vitamin B12 is determined. If absorption is normal, ≥ 9% of the dose given appears in the urine. Reduced urinary excretion (<5% if kidney function is normal) indicates inadequate vitamin B12 absorption. Improved absorption with the subsequent addition of intrinsic factor to radiolabeled vitamin B12 confirms the diagnosis of pernicious anemia. The test is often difficult to do or interpret because of incomplete urine collection or renal insufficiency. In addition, because the Schilling test does not measure absorption of protein-bound vitamin B12, the test does not detect defective liberation of vitamin B12 from foods, which is common among older people. The Schilling test repletes vitamin B12 and can mask deficiency, so it should be done only after all other diagnostic tests and therapeutic trials. If Overview of Malabsorption is identified, the Schilling test can be repeated after a 2-week trial of an oral antibiotic. If antibiotic therapy corrects malabsorption, the likely cause is 