Volume 35 Number 4 | August 2021

Minh Kosfeld, PhD, MLT(ASCP)CM

Patient Physical Examination on Admission

Our patient is a previously healthy 56-year-old white man who presents with progressive neuropathy and declining mental status over several months. The patient initially developed numbness of his fingertips and the balls of his feet and began to lose motor control of his hands, which manifested as dropping objects or flinging them as he tried to pick them up. As symptoms progressed, he had visual tracking problems that were severe enough to interfere with driving a car, and he developed short-term memory loss and slowing of cognitive function.

Laboratory Results
Hemoglobin (Hgb) 15 g/dL (13.5 to 17.5 g/dL)
Hematocrit (Hct) 45% (41-50%)
Mean corpuscular volume (MCV) 93 fL (80–100 fL)
Serum Vitamin B12 (cobalamin) 175 pg/mL (200-910 pg/mL)
Serum Methylmalonic acid (MMA) 6434 nmol/L (0-400 nmol/L)
Plasma Homocysteine 11 μmol/L (5–15 μmol/L)
Anti-intrinsic factor (anti-IF) antibody 1.0 (0-1.1 Unit)
Acetylcholine Receptor (AChR) binding antibodies negative (≤ 0.02 nmol/L)
Head computed tomography (CT) No cerebral atrophy or white matter changes consistent with demyelination


Diagnosis and Treatment

Based on the clinical presentation and laboratory findings, a diagnosis of psychomotor regression due to B12 deficiency was made. The patient was then treated with a series of B12 intramuscular injections, which resulted in rapid remission of associated neurological symptoms. In follow-up laboratory examinations, his B12 level was normalized in two months (606 pg/mL), and MMA in eight months (197 nmol/L).


B12 is the largest and most complex of the water-soluble B vitamins. Since it contains cobalt, compounds with B12 activity are collectively called “cobalamins.” For humans, the only natural dietary sources of B12 are animal products (meat, dairy), in which it is bound to protein. The B12 absorption mechanism is complex, requiring several transporter proteins. First, it must be freed from the food matrix by gastric HCl and pepsin. Once liberated, it is transported to the duodenum by haptocorrin (transcobalamin I), a cobalamin-binding protein produced in the saliva. In the duodenum, pancreatic digestive enzymes free B12 from haptocorrin, allowing it to bind intrinsic factor (IF), a transporter protein synthesized by the parietal cells of the gastric mucosa. This B12-IF complex travels to the terminal ileum where it is absorbed and the B12 is separated from IF. B12 then is delivered to peripheral tissues and the liver by transcobalamin II and haptocorrin, respectively.1,2

B12 is essential for DNA synthesis, hematopoiesis, and myelination. In different metabolically active forms, it functions as a cofactor for two different enzymes. As methylcobalamin, B12 activates cytoplasmic methionine synthase to convert homocysteine to the essential amino acid methionine. Methionine is required for the formation of S-adenosylmethionine, a universal methyl donor for almost 100 different substrates, including DNA, RNA, proteins, and lipids. As adenosylcobalamin, B12 activates the mitochondrial L-methylmalonyl-CoA mutase to convert L-methylmalonyl-CoA to succinyl-CoA in the metabolism of propionate, a short-chain fatty acid.3,4 In B12 deficiency, these cofactors are unavailable, causing homocysteine and MMA to accumulate.

Despite our understanding of the metabolic disturbances resulting from B12 deficiency, its pathogenesis is not as well understood. Deficiency of B12 can lead to two major clinical syndromes, megaloblastic anemia and neuropathy.1,3,4 It is thought that severe B12 deficiency interferes with the synthesis of DNA needed for hematopoiesis, leading to megaloblastic anemia, the appearance of hypersegmented neutrophils, and possible pancytopenia.5 Regarding neuropathy, it is thought that long-term B12 deficiency may lead to impaired synthesis of ethanolamine, phospholipids, and sphingomyelin, resulting in altered myelin integrity.6 An enigmatic feature of B12 deficiency is that its clinical presentations vary and may entail only hematologic or neurological abnormalities, or both.1

Common diagnostic lab tests for B12 deficiency typically begin with serum B12 measurement and a complete blood count (CBC). Low B12 levels and evidence of megaloblastic anemia (decreased RBC, Hgb, Hct, WBC, platelet count, increased MCV, large oval RBCs and hypersegmented neutrophils) indicate B12 deficiency. For ambiguous results, MMA and homocysteine levels should also be measured to confirm.2,4 Serum MMA is the most sensitive marker of B12 status where its increase indicates decreased tissue B12. However, it also rises with renal insufficiency and tends to be higher in older adults. Plasma homocysteine is a sensitive indicator for early B12 deficiency where it rises quickly as B12 status declines. However, it also rises in folate or B6 deficiency and especially with renal insufficiency, reducing its specificity.3,4 Additional tests include serum or RBC folate to differentiate causes of macrocytic anemia, and Anti-IF or Anti-parietal cell antibodies to confirm pernicious anemia.1,2

Caregivers should be aware of the different clinical presentations of B12 deficiency and screen for it in high-risk populations. Risk factors include insufficient dietary B12 intake (vegetarians), lack of intrinsic factor (autoimmune pernicious anemia), food-bound malabsorption (atrophic gastritis of aging or chronic H. pylori infection), gastrointestinal surgery (post gastrectomy or ileal resection), pancreatic or intestinal disorders (chronic pancreatitis, Crohn’s, or Celiac disease), genetic disorders (Transcobalamin II deficiency), and use of certain drugs (long-term use of metformin, H2 receptor antagonists or Proton-pump inhibitors).1,2,3

Diagnosis of B12 deficiency can be complicated since symptoms may be vague and lab test results can be equivocal. Despite the low B12, high MMA, and significant neurologic symptoms, this patient’s homocysteine level was normal, and he was not anemic. With no known risk factor for B12 deficiency, assays were done for anti-IF antibodies to evaluate for pernicious anemia and AChR binding antibodies for Myasthenia Gravis. However, this patient’s positive symptomatic and serologic responses to B12 supplementation suggest that simple vitamin B12 deficiency was the etiology. Fortunately, B12 therapy reversed his most severe neurological symptoms, although that may not always be the case. Since the reason for the deficiency is unknown, he will continue to receive B12 supplementation indefinitely.

  1. Michael J Shipton and Jecko Thachil, Vitamin B12 deficiency–A 21st century perspective. Clin Med (Lond). 2015 Apr; 15(2): 145–150. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4953733/
  2. https://labtestsonline.org/conditions/vitamin-b12-and-folate-deficiencies
  3. Vitamin B12. https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
  4. Bishnu Prasad Devkota. Methylmalonic Acid. https://emedicine.medscape.com/article/2108967-overview#showall
  5. Mark J Koury, Prem Ponka. New insights into erythropoiesis: the roles of folate, vitamin B12, and iron. Annu Rev Nutr. 2004;24:105-31. https://pubmed.ncbi.nlm.nih.gov/15189115/
  6. Brahim El Hasbaoui, Nadia Mebrouk, Salahiddine Saghir, Abdelhkim El Yajouri, Rachid Abilkassem, and Aomar Agadr. Vitamin B12 deficiency: case report and review of literature. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8140678/

Minh Kosfeld is Director/Assistant Professor, Investigative and Medical Sciences Program, in the Department of Clinical Health Sciences at Saint Louis University in Missouri.