Vitamin B12 deficiency vs Folate deficiency
How to tell Vitamin B12 deficiency and Folate deficiency apart when a board question makes them look alike.
B12 has neurologic findings and high MMA; folate has normal MMA and no neurologic findings.
Side by side
Megaloblastic anemia with neuropathy, gait impairment, or posterior column findings.
Megaloblastic anemia without neurologic deficits, often from poor intake, pregnancy, or drugs.
Impaired DNA synthesis plus methylmalonic acid accumulation injures myelin.
Impaired DNA synthesis from reduced folate availability.
High methylmalonic acid and high homocysteine support B12 deficiency.
Normal methylmalonic acid with high homocysteine supports folate deficiency.
The trap
Folate alone can correct the megaloblastic anemia and normalize the smear while B12 subacute combined degeneration progresses silently. Never give folate empirically for macrocytosis without first checking B12 and methylmalonic acid. The cost of guessing wrong is irreversible posterior column and corticospinal tract damage.
Watch it explained
The repair move
For any macrocytic anemia, do four things in order before treating: examine vibration and proprioception, check serum B12, send methylmalonic acid (MMA), and send homocysteine. High MMA and high homocysteine fix the answer at B12 even when serum folate is also low. Normal MMA with isolated high homocysteine fits folate alone.
Try a free question
CoreA vegan patient has macrocytic anemia, paresthesias, impaired vibration sense, and elevated methylmalonic acid. Which deficiency is most likely?
Show answer and explanation
Answer: A. Vitamin B12 deficiency. Neurologic findings plus elevated methylmalonic acid point to vitamin B12 deficiency.
Why the other choices are tempting
- B. Folate deficiency causes megaloblastic anemia without neurologic findings or elevated MMA.
- C. Iron deficiency causes microcytic anemia.
- D. B6 deficiency can cause sideroblastic anemia and neuropathy but not high MMA megaloblastosis.
- E. Copper deficiency can cause neurologic findings but is not the classic high-MMA macrocytic pattern.
Flashcards
Tap a card to flip it.
What lab separates B12 deficiency from folate deficiency?
Methylmalonic acid rises in B12 deficiency but stays normal in folate deficiency.
Why does B12 deficiency cause neurologic findings?
B12 is needed for methylmalonyl-CoA metabolism, and methylmalonic acid buildup is linked to myelin injury.
Quick answers
What is the fastest way to tell Vitamin B12 deficiency from Folate deficiency?
B12 has neurologic findings and high MMA; folate has normal MMA and no neurologic findings.
What is the key clue for Vitamin B12 deficiency?
Megaloblastic anemia with neuropathy, gait impairment, or posterior column findings.
What is the key clue for Folate deficiency?
Megaloblastic anemia without neurologic deficits, often from poor intake, pregnancy, or drugs.
What is the common trap with Vitamin B12 deficiency vs Folate deficiency?
Folate alone can correct the megaloblastic anemia and normalize the smear while B12 subacute combined degeneration progresses silently. Never give folate empirically for macrocytosis without first checking B12 and methylmalonic acid. The cost of guessing wrong is irreversible posterior column and corticospinal tract damage.