Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1290 page)

   Late pregnancy

   PA

   Limitations

   Serum samples should be protected from light at room temperature (15–30°C) for no longer than 1 hour. If the assay will not be completed within 2 hours, samples should be frozen and be protected from light exposure.

   Drugs such as chloral hydrate increase vitamin B
₁₂
levels. On the other hand, alcohol, aminosalicylic acid, anticonvulsants, ascorbic acid, cholestyramine, cimetidine, colchicine, metformin, neomycin, oral contraceptives, ranitidine, and triamterene decrease vitamin B
₁₂
levels.

   Many other conditions are known to cause an increase (vitamin C, vitamin A, estrogens, hepatocellular injury, myeloproliferative disorders, uremia) or decrease (pregnancy, smoking, hemodialysis, multiple myeloma) serum B
₁₂
levels.

   The evaluation of macrocytic anemia requires measurement of both vitamin B
₁₂
and folate levels; ideally, they should be measured simultaneously.

   Specimen collection soon after blood transfusion can falsely increase vitamin B
₁₂
levels.

   Patients taking vitamin B
₁₂
supplementation may have misleading results.

   A normal serum concentration of B
₁₂
does not rule out tissue deficiency of vitamin B
₁₂
. The most sensitive test for B
₁₂
deficiency at the cellular level is the assay for MMA. If clinical symptoms suggest deficiency, measurement of MMA and homocysteine should be considered, even if serum B
₁₂
concentrations are normal.

VITAMIN B
₂
(RIBOFLAVIN)

   Definition

   Vitamin B
₂
, or riboflavin, is one of the water-soluble vitamins. It is synthesized in plants and microorganisms and occurs naturally in three forms: the physiologically inactive riboflavin and the physiologically active coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). The latter accounts for about 90% of the total riboflavin in whole blood. Because of their capacity to transfer electrons, FAD and FMN are essential for proton transfer in the respiratory chain, for the dehydration of fatty acids, the oxidative deamination of amino acids, and for other redox processes.