Vitamin B12 deficiency is a well-described disorder with a spectrum of manifestations ranging from macrocytic anemia to neuropsychiatric disorders including depression, dementia, and subacute combined degeneration of the spinal cord . Lack of vitamin B12 arises from insufficient intake or malabsorption . At-risk populations include the elderly, alcoholics, strict vegetarians, as well as patients with intestinal inflammatory diseases, autoimmune conditions, post-bariatric surgery, and users of certain medications including proton pump inhibitors, histamine receptor antagonists, and biguanides [2, 3]. There is also an association between primary hypothyroidism and B12 deficiency . In clinical practice, diagnosis is typically established by measurement of serum cobalamin (Cbl) levels. Vitamin B12 deficiency can be reflected in elevated methylmalonic acid (MMA) and homocysteine (Hcy) levels, but these tests are not routinely obtained unless the initial Cbl levels are equivocal .
A 75-year-old male presented to the hospital with generalized weakness, dizziness, and mild confusion for three weeks. His medical history significant for colon cancer treated by resection 13 years ago, hypertension, and benign prostatic hyperplasia (BPH). He denied fevers, chills, dyspnea, chest pain, bleeding, bruising, melanotic stools, diarrhea, vomiting, or dysuria. Three weeks prior to presentation, silodosin was started for treatment of BPH. The patient had been on ramipril and atenolol for hypertension which he continued to take as prescribed. He lived an active life prior to symptom onset (Table 1) (Figure 1).
On admission, vital signs were notable for orthostatic hypotension with a blood pressure of 106/82 supine and 80/52 standing. Physical exam was unremarkable and rectal exam revealed heme-negative stool. See Table 1 and Figure 1 for admission labs and peripheral smear findings.
The patient’s anti-hypertensive and BPH medications were held and he was transfused one unit of packed red cells. Deficiency in folate and/or vitamin B12 levels was suspected, however, serum folate level returned at 12.3 ng/ml [normal, 3.1 – 17.5 ng/ml] and vitamin B12 level at 489 pg/ml [normal, 254 – 1320 pg/ml], both sent pre-transfusion. Repeat vitamin B12 level was 321 pg/ml. A bone marrow biopsy showed erythroid hyperplasia with a left shift, megaloblastic changes, hypersegmented neutrophils, increased iron stores, and normal flow cytometry/cytogenetics (Figures 2 and 3).
Despite a normal serum cobalamin level, the clinical picture was suspicious for vitamin B12 deficiency, and empiric therapy with cyanocobalamin (1 mg intramuscularly daily) was initiated. Soon after, his labs revealed the following: methylmalonic acid, 28 nmol/ml [normal, <0.4 nmol/ml]; homocysteine, 72.4 µmol/L [normal, 3.2 - 10.7 µmol/L], and presence of both anti-parietal cell and intrinsic factor antibody titers, leading to the diagnosis of pernicious anemia. He was continued on cyanocobalamin injections, administered weekly for four weeks and then monthly injections thereafter, eventually achieving a hemoglobin of 12.8 g/dL, MCV of 97.2 fL, and platelets of 130,000/µL, and a durable resolution of his dizziness and confusion at his two month follow-up.
Serum cobalamin levels can be misleading in the workup of vitamin B12 deficiency. In this case, compelling evidence to suggest clinical B12 deficiency was apparent on presentation by a marked macrocytic anemia with evidence of hemolysis. The peripheral smear demonstrated oval macrocytes and hypersegmented neutrophils, commonly seen in folate/B12 deficiency . When the anemia is more severe, there may be marked poikilocytosis with teardrop cells and red-cell fragments, also observed in this case . Folate/B12 deficiency can further lead to thrombocytopenia and morphologic aberrations resembling a variety of hematologic disorders such as myelodysplastic syndrome and acute leukemia . The decision to perform a bone marrow biopsy was based on the discrepancy between the clinical presentation and normal folate/Cbl levels as well as presence of thrombocytopenia, with biopsy results further supporting B12 deficiency as the etiology of his anemia. The diagnosis confirmed by elevated MMA/Hcy levels and increased anti-parietal cell/intrinsic factor antibody titers, leading to pernicious anemia.
In clinical practice, serum total Cbl levels is the initial test of choice for detecting B12 deficiency as it is widely available and cost-effective. This test is limited in specificity and sensitivity, often missing many patients within the laboratory “gray zone” of deficiency [1, 2, 3]. Such patients usually have Cbl levels ranging from 125 pg/ml to 250 pg/ml, although these cut-offs vary based on the assay type . In plasma, vitamin B12 is bound to two proteins - haptocorrin and transcobalamin. The Cbl assay predominantly reflects B12 bound to haptocorrin (metabolically inactive), and transcobalamin-bound B12 (metabolically active) which contributes much less to the total serum Cbl level . Conditions such as chronic myeloid leukemia can cause an increase in serum haptocorrin levels leading to a false-negative result in true vitamin B12 deficiency . Other factors causing false-normal results include intrinsic assay error, assay variability among manufacturers, and intra-patient variability [8, 9, 10].
This case highlights the difficulty in establishing a diagnosis of B12 deficiency with a Cbl assay within normal limits on repeated studies. Measurements of serum MMA and Hcy levels, which accumulate in B12 deficiency, become useful when Cbl levels are equivocal but clinical suspicion remains high [1, 2]. Although generally considered more sensitive indicators of B12 deficiency than total Cbl levels (i.e. “confirmatory tests”) [2, 10], the utility of these metabolites may be limited. Renal insufficiency may cause elevation of both MMA and Hcy, while increased Hcy may also represent lack of folate or vitamin B6 [2, 11]. Metabolite assay cost and slow turnaround time are further limitations to these tests . Accumulating evidence indicates that serum holotranscobalamin (holoTC), an earlier marker of B12 deficiency that decreases before total Cbl, may be superior to other assays and has been proposed as a first-line diagnostic test, but is not yet routinely available in the clinical setting [2, 12]. Clinicians should rely on metabolite assays combined with clinical judgment in establishing the diagnosis of B12 deficiency when Cbl assay results are equivocal.
Early vitamin B12 replacement is important in preventing potentially irreversible neurologic damage. Treatment should be started expeditiously for severe neurologic symptoms with their risk of irreversibility . Treatment should be continued until clinical improvement is evident or a definitive diagnosis can be made [1, 2]. In patients with pernicious anemia, vitamin B12 supplementation may begin with daily injections of cyanocobalamin at a dose of 1000 µg for the first week, followed by weekly injections for the first month, and maintenance monthly injections thereafter [2, 13]. Oral dosing is acceptable in patients with intact gastrointestinal absorption and absence of severe neurologic manifestations . Iron levels should be monitored and corrected as replacement therapy may cause increased demand on iron stores. Folate deficiency may be unmasked with repletion of vitamin B12 . Treatment response should occur with reticulocytosis within one week as well as eventual reduction in MCV and resolution in the anemia within 1-2 months .
Given its high prevalence, vitamin B12 deficiency must be considered in the differential diagnosis of patients presenting with progressive neuropsychiatric findings and/or hematologic derangements, as rapid diagnosis and supplementation may prevent permanent complications.
CONFLICT OF INTEREST
The authors have no conflicts of interest or financial disclosures to report.
2012 Ross Science Publishers