B12 & folate deficiency
What it means
Both → macrocytic (MCV >100) megaloblastic anaemia with hypersegmented neutrophils. B12 deficiency additionally causes neurology — subacute combined degeneration (dorsal columns + corticospinal). Causes: pernicious anaemia (anti-IF/parietal), terminal-ileal disease, vegan diet, metformin. Replace B12 BEFORE folate in combined deficiency (folate alone can precipitate SACD).
Worked example
MCV 112, low B12, glove-and-stocking paraesthesiae with proprioceptive loss → B12 deficiency with SACD.
In the exam
A vegan has a macrocytic anaemia with paraesthesiae and impaired proprioception; the team must replace one vitamin before the other to avoid precipitating cord degeneration.
What settles it
B12 deficiency gives NEUROLOGY (SACD); folate deficiency does not. Replace B12 first.
Classically confused with
Iron studies interpretation
Source: BSH; NICE CKS