From the Guidelines
Vitamin B12 deficiency anemia is typically characterized by macrocytic anemia with elevated mean corpuscular volume (MCV), decreased hemoglobin and hematocrit levels, and reduced red blood cell count, along with decreased serum B12 levels, elevated methylmalonic acid and homocysteine levels, and other abnormalities such as hypersegmented neutrophils and macro-ovalocytes on peripheral blood smear. The diagnosis of vitamin B12 deficiency anemia relies on a combination of direct and functional markers, including serum cobalamin, holo-transcobalamin (holo-TC), and methylmalonic acid (MMA) levels, as well as total homocysteine (tHcy) levels 1. Some key laboratory findings in patients with vitamin B12 deficiency anemia include:
- Decreased serum B12 levels (below 200 pg/mL)
- Elevated methylmalonic acid and homocysteine levels, which serve as functional markers of B12 deficiency
- Macrocytic anemia with elevated mean corpuscular volume (MCV) typically above 100 fL
- Decreased hemoglobin and hematocrit levels
- Reduced red blood cell count
- Hypersegmented neutrophils (with five or more lobes) and macro-ovalocytes on peripheral blood smear
- Elevated lactate dehydrogenase (LDH) due to ineffective erythropoiesis and hemolysis
- Pancytopenia with reduced white blood cell and platelet counts in severe cases
- Megaloblastic changes with large, immature precursor cells on bone marrow examination. These laboratory abnormalities occur because vitamin B12 is essential for DNA synthesis, and its deficiency leads to impaired cell division particularly affecting rapidly dividing cells like blood cell precursors, resulting in the production of large, dysfunctional cells and ineffective hematopoiesis 1.
From the FDA Drug Label
Laboratory Tests During the initial treatment of patients with pernicious anemia, serum potassium must be observed closely the first 48 hours and potassium replaced if necessary Hematocrit, reticulocyte count, vitamin B12, folate and iron levels should be obtained prior to treatment. Hematocrit and reticulocyte counts should be repeated daily from the fifth to seventh days of therapy and then frequently until the hematocrit is normal. If folate levels are low, folic acid should also be administered If reticulocytes have not increased after treatment or if reticulocyte counts do not continue at least twice normal as long as the hematocrit is less than 35%, diagnosis or treatment should be reevaluated. Repeat determinations of iron and folic acid may reveal a complicating illness that might inhibit the response of the marrow
The typical lab findings in patients with vitamin B12 deficiency anemia include:
- Low vitamin B12 levels
- Low hematocrit
- Low reticulocyte count
- Low folate levels (in some cases)
- Low iron levels (in some cases)
- Elevated serum potassium (in some cases) 2 3
From the Research
Typical Lab Findings
The typical lab findings in patients with vitamin B12 deficiency anemia include:
- Low serum vitamin B12 level 4, 5
- Elevated mean corpuscular volume (MCV) 6
- Abnormally increased levels of homocysteine and methylmalonic acid 7, 5
- Decreased levels of circulating total B12 and transcobalamin-bound B12 5
- Megaloblastic anemia resulting from a defect in thymidine and DNA synthesis in rapidly dividing cells 7
Laboratory Assessment
Initial laboratory assessment for vitamin B12 deficiency anemia should include:
- Complete blood count (CBC) 4
- Serum vitamin B12 level 4, 5
- Measurement of serum methylmalonic acid to confirm deficiency in asymptomatic high-risk patients with low-normal levels of vitamin B12 4
- Measurement of serum homocysteine levels 5
Diagnostic Value of MCV
The diagnostic value of the mean corpuscular volume (MCV) in detecting vitamin B12 deficiency is limited, with a sensitivity of 17% to 77% depending on the population studied 6
- The MCV can be used to make the diagnosis of B12 deficiency more or less probable, but it should not be used as the only parameter to rule out the diagnosis of B12 deficiency 6