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Discuss the laboratory diagnosis of vitamin B12 deficiency
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Essay of 2 pages for the course laboratory investigation of disease at LJMU (essay prep for exam)
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Discuss the laboratory diagnosis of vitamin B12 deficiency.Vitamin B12, or more commonly known as cobalamin, is an essential cofactor that is integral
in the methylation, synthesis and repair of DNA. Deficiency of this vitamin results in a
decreased production of the base thymidine. B12 is a coenzyme for methionine synthase
that catalyses the reaction of homocysteine to methionine which allows the further usage of
folate in the form of 5-methyltetrahydrofolate. The continuation of the folate cycle is
therefore, dependent on 5-methyltetrahydrofoalte and the availability of B12. A number of
different factors causes deficiency of B12. In the developed world, the main causes of
deficiency is due to an inadequate dietary intake. Those who choose a vegan diet are at risk.
Malabsorption is another leading cause of deficiency and this impaired absorption can occur
at the gastric level due to pernicious anaemia or partial/total gastrectomy. Malabsorption
also occurs at the intestinal level due to ileal resection (this can happen in Crohn’s disease).
Symptoms of B12 deficiency include weakness, tiredness, heart palpitations and shortness
of breath, glossitis, pallor, loss of appetite and nerve problems i.e. numbness/tingling of the
hands and feet. The first stage of diagnosis is a full blood count and blood film. If the patient
is deficient, the presence of oval macrocytic red cells should be present on the film alongside
hypersegmented neutrophils. The FBC should show an increased MCV greater than 110 fl
and an increased haemoglobin level. The MCV value however, is not a specific diagnostic
indicator for B12 deficiency. Excess alcohol consumption, drug use and myelodysplastic
syndrome may cause elevations of the MCV.
Serum cobalamin (vitamin B12) measurements are the standard assays currently used to
run initial diagnostic tests. This assay quantifies both the inactive forms i.e. transcobalamin I
and bound transcobalamin III and the active form i.e. bound transcobalamin II of B12 in the
serum. The assay used is widely available, low cost and has the ability for usage in
automation based on intrinsic factor binding of cobalamin and immune-chemiluminescence
based assays. A serum cobalamin result less than 200 ng/L is said to have a 97% sensitivity
to diagnose deficiency.
The total plasma homocysteine is another laboratory test that can be performed. Vitamin
B12 deficiency results in an elevation of the plasma homocysteine levels and this
measurement is an ideal sensitive biomarker for this deficiency. Plasma homocysteine levels
increase in the early stages of the deficiency. The results are not 100% specific for B12
deficiency as levels are also elevated in folate deficiency. The utilisation of the total
homocysteine measurements depends on the correct collection and processing of the
sample. The plasma must be kept cool, centrifuged and removed from the red cells within
two hours of collection.
Another laboratory investigation is the quantification of plasma methylmalonic acid. This is
elevated in B12 deficiency but not elevated in folate deficiency. Exceptionally high levels of
methylmalonic acid, greater than 0.75 µmol/L, is almost invariable indicative of B12
deficiency. It is quantified using gas-chromatography-mass spectrometry. This is a high cost
test which has prevented its widespread usage as a diagnostic indicator.
In the UK, parenteral treatment for B12 deficiency is preferred over oral therapy.
Intramuscular injections of 1 mg hydroxocobalamin should be given 3 times a week for 2
weeks. At this stage, response to treatment should be assessed via a simple blood test. If
treatments shows improvement, 1mg intramuscular injection should be given once every 3
months. High dose oral cyanocobalamin is licenced in several countries outside the UK and
is readily available on the internet. Some reviews suggest the use of oral cobalamin is as
effective as intramuscular B12 with added benefits i.e. fewer visits to health centres and the
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