FBC - macrocytic anaemia; a thrombocytopaenia may occur secondary to ineffective megakaryopoiesis
blood film - macrocytosis, hypersegmented neutrophils
bone marrow - megaloblastic, erythropoiesis, giant metamyelocyte
Measurement of B12 / folate levels - in B12 deficiency red cell folate levels are low (B12 required in synthesis) but serum folate is generally normal or high (1).
Measurement of B12 absorption - Schilling test - very rarely undertaken now (3).
Anti-intrinsic factor (anti-IF) antibodies in serum:
positive in about 50% of patients with pernicious anaemia (1)
the presence of intrinsic factor (IF) is diagnostic of pernicious anaemia but negative intrinsic factor antibodies does not exclude pernicious anaemia (due to the test’s low sensitivity (50-60%)
highly specific but not very sensitive (2)
Anti-gastric parietal cell (GPC) antibodies in serum:
seen in 95% of cases of pernicious anaemia and, although there is an overlap with other autoimmune diseases and with normal individuals, a negative result makes pernicious anaemia unlikely (1)
reasonably sensitive but not as specific as anti-IF antibodies
note a review stated testing for anti-gastric parietal cell antibodies is not recommended because of the variable specificity of 50-100% (3)
A positive anti-GPC and/or anti-IF antibody test does need repeating (2).
Other investigations to help define the cause of the vitamin B12 deficiency:
thyroid function tests and anti-thyroid antibodies
test for coeliac disease
tissue transglutaminase (tTG)
tests for generalised malabsorption (if symptoms are suggestive) - faecal tests are generally only requested by a gastroenterologist/after gastroenterological advice
calcium and vitamin D
Urinary methylmalonyl CoA urinary excretion is increased in B12 deficiency - B12 is the co-enzyme in the conversion reaction of methylmalonyl CoA to succinyl CoA.
Clinical picture is the most important factor in assessing the results of the serum vitamin B12. Definitive cut off points for clinical and subclinical deficiency are not possible (1).
the test measures total, not metabolically active vitamin B12
levels are not easily correlated with clinical symptoms, although patients with vitamin B12 levels <100ng/L almost always have clinical or metabolic evidence of vitamin B12 deficiency, and <150ng/l usually do
in most patients with clinically significant vitamin B12 deficiency, the serum level is below 200ng/L but clinically significant vitamin B12 deficiency may be present even when levels are in the normal range, especially in elderly patients (1)
about a third of patients with B12 deficiency may not have macrocytosis (3)