Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1287 page)

   Hartnup disease

   Pellagra

   Limitations

   Whole blood is the preferred specimen for thiamine assessment. Approximately 80% of thiamine present in whole blood is found in RBCs.

   Drugs that may decrease vitamin B
₁
levels include glibenclamide, isoniazid, and valproic acid.

   Diets high in freshwater fish and tea, which are thiamine antagonists, may cause decreased vitamin B
₁
levels.

   Thiamine deficiency can be assessed by measuring the blood thiamine concentration, erythrocyte thiamine transketolase (ETKA), or transketolase urinary thiamine excretion (with or without a 5-mg thiamine load). Most laboratories now measure blood thiamine concentration directly, in preference to the ETKA method. The ETKA method is a functional test, and results are influenced by the hemoglobin concentration.

VITAMIN B
₁₂
(CYANOCOBALAMIN, COBALAMIN)

   Definition

   Vitamin B
₁₂
is essential in DNA synthesis, hematopoiesis, and CNS integrity. Its absorption depends on the presence of intrinsic factor (IF) and may be due to lack of IF secretion by gastric mucosa (e.g., gastrectomy, gastric atrophy) or intestinal malabsorption (e.g., ileal resection, small intestinal diseases). Vitamin B
₁₂
deficiency frequently causes macrocytic anemia, glossitis, peripheral neuropathy, weakness, hyperreflexia, ataxia, loss of proprioception, poor coordination, and affective behavioral changes. These manifestations may occur in any combination; many patients have the neurologic defects without macrocytic anemia. PA is a macrocytic anemia caused by B
₁₂
deficiency that is due to a lack of IF secretion by gastric mucosa. Serum methylmalonic acid (MMA) and homocysteine levels are also elevated in vitamin B
₁₂
deficiency states. A significant increase in RBC MCV may be an important indicator of vitamin B
₁₂
deficiency.

   
Normal range:
180–914 pg/mL.

   Indeterminate range: 145–180 pg/mL

   Deficient range: <145 pg/mL