Elevated MCV — The First Clue
An MCV over 100 fL on a routine CBC opens the macrocytic differential.
- >100 fL: MCV threshold (placeholder reference range)
- Oval RBCs: Common finding (placeholder smear clue)
- CBC + smear: Initial workup (placeholder first step)
- B12/folate: Next step (placeholder next labs)
Why MCV rises
Placeholder: impaired DNA synthesis enlarges developing red cells.
Megaloblastic Split — B12 or Folate
Serum B12, folate, and methylmalonic acid separate the two megaloblastic paths.
- MMA↑: B12 marker (placeholder specific marker)
- Homocysteine↑: Folate marker (placeholder shared marker)
- B12 only: Neuro signs (placeholder distinguishing clue)
- Folate: Diet clue (placeholder dietary link)
Distinguishing tests
Placeholder: MMA elevated only in B12 deficiency, not folate.
Hypothyroidism as a Non-Megaloblastic Cause
Elevated TSH points toward thyroid-driven macrocytosis without megaloblastic change.
- TSH: Key test (placeholder screening test)
- Slowed turnover: Mechanism (placeholder short mechanism)
- Round macrocytes: Smear finding (placeholder morphology clue)
- Levothyroxine: Treatment (placeholder management)
Thyroid link
Placeholder: hypothyroidism slows red cell maturation, raising MCV.
Alcohol and Liver-Mediated Macrocytosis
Chronic alcohol use and liver dysfunction independently raise MCV.
- GGT↑: Key marker (placeholder liver marker)
- Round macrocytes: Smear finding (placeholder morphology clue)
- Alcohol use: History clue (placeholder history clue)
- Membrane lipid change: Mechanism (placeholder short mechanism)
Alcohol mechanism
Placeholder: alcohol directly toxic to marrow and alters membrane lipids.
Selecting the Confirmatory Test
Final labs confirm the leading candidate before starting treatment.
- MMA + B12: B12 path (placeholder confirm test)
- RBC folate: Folate path (placeholder confirm test)
- Free T4: Thyroid path (placeholder confirm test)
- LFTs + GGT: Liver path (placeholder confirm test)
Final selection
Placeholder: confirmatory test chosen from the highest-likelihood branch.
Placeholder: combine history, labs, and smear to finalize diagnosis.