FBC - macrocytic anaemia, may show neutropaenia and thrombocytopenia
peripheral blood film shows anisocytosis, poikilocytosis
low serum vitamin B12; normal folate
serum bilirubin may be increased because of haemolysis
bone marrow shows features of megaloblastic erythropoiesis
detection of autoantibodies:
parietal cells - serum (95%), gastric juice (70%)
intrinsic factor - serum (50-60%), gastric juice (85%)
detection of serum antibodies is usually performed for convenience; only those to IF are diagnostic
presence of intrinsic factor antibodies in serum
positive in 50-60% 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%)) - gastric parietal cell (GPC) antibodies are 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)
a positive anti-GPC and/or anti-IF antibody test does need repeating (3)
Other investigations include absorption tests - Schilling test (very rarely undertaken now) - and gastrointestinal investigations.
Pulse 2004; 64(35):88.
NHS Wiltshire CCG. Investigation and treatment of Vitamin B12 (cobalamin) deficiency in primary care
Royal United Hospital Bath NHS Trust. Guidelines for the Investigation & Management of vitamin B12 deficiency (accessed 25/4/2020).
Mohamed M et al. Pernicious anaemia. BMJ 2020;369:m1319.