B12 deficiency vs Folate deficiency
How to tell B12 deficiency and Folate deficiency apart when a board question makes them look alike.
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Elevated methylmalonic acid distinguishes B12 deficiency from folate deficiency.
Side by side
Macrocytic anemia with paresthesias, gait instability, or loss of vibration sense suggests vitamin B12 deficiency.
Macrocytic anemia without neurologic deficits and with nutritional or medication risk factors suggests folate deficiency.
B12 deficiency impairs DNA synthesis and methylmalonyl-CoA metabolism, causing megaloblastosis and neurologic demyelination.
Folate deficiency impairs thymidylate synthesis and DNA replication, causing megaloblastic anemia without methylmalonic acid accumulation.
MCV is elevated with hypersegmented neutrophils, methylmalonic acid and homocysteine are both elevated, and anti-intrinsic factor antibody may indicate pernicious anemia.
MCV is elevated with hypersegmented neutrophils, homocysteine is elevated, methylmalonic acid remains normal, and serum folate is low.
The trap
Treating undifferentiated macrocytosis with folate alone can improve anemia while allowing irreversible neurologic injury from unrecognized B12 deficiency. Check B12 and methylmalonic acid before isolated folate replacement when etiology is unclear.
The repair move
Next best step: confirm deficiency pattern with B12, folate, methylmalonic acid, and homocysteine, then replace B12 promptly when neurologic signs are present, for example cyanocobalamin 1000 mcg IM weekly for 4 to 8 weeks then monthly. Treat folate deficiency with folic acid 1 mg orally daily after excluding concurrent B12 deficiency.
Try a free question
CoreA 55-year-old man has macrocytic anemia with MCV 110 fL and numbness in both feet. BP 122/76 mm Hg, HR 88/min, temp 36.7 C. Homocysteine and methylmalonic acid are both elevated. What is the most likely diagnosis?
Show answer and explanation
Answer: B. Vitamin B12 deficiency. Neurologic symptoms plus elevated methylmalonic acid identify vitamin B12 deficiency.
Why the other choices are tempting
- A. Folate deficiency usually does not elevate methylmalonic acid or cause classic neurologic deficits.
- C. Iron deficiency causes microcytosis, not megaloblastic macrocytosis.
- D. Anemia of chronic disease is typically normocytic or mildly microcytic, not this pattern.
- E. Thalassemia trait causes microcytosis with normal or high RBC count, not macrocytosis with neurologic signs.
Flashcards
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Which metabolite is elevated in B12 deficiency but usually normal in folate deficiency?
Methylmalonic acid is elevated in B12 deficiency and usually normal in folate deficiency.
What neurologic findings point toward B12 deficiency?
Paresthesias, impaired vibration sense, and gait instability point toward B12 deficiency.
Quick answers
What is the fastest way to tell B12 deficiency from Folate deficiency?
Elevated methylmalonic acid distinguishes B12 deficiency from folate deficiency.
What is the key clue for B12 deficiency?
Macrocytic anemia with paresthesias, gait instability, or loss of vibration sense suggests vitamin B12 deficiency.
What is the key clue for Folate deficiency?
Macrocytic anemia without neurologic deficits and with nutritional or medication risk factors suggests folate deficiency.
What is the common trap with B12 deficiency vs Folate deficiency?
Treating undifferentiated macrocytosis with folate alone can improve anemia while allowing irreversible neurologic injury from unrecognized B12 deficiency. Check B12 and methylmalonic acid before isolated folate replacement when etiology is unclear.