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MTX Macrocytosis Interpretation — SCE Rheumatology MCQ

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EasyAutoimmune SerologyMTX Macrocytosis InterpretationSCE Rheumatology

A 40-year-old woman with RA on Methotrexate develops macrocytosis (MCV 108 fL) on routine monitoring. Her Hb is 125 g/L (normal). She is on folic acid 5 mg weekly. What does isolated macrocytosis on Methotrexate indicate?

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Correct answer: E — Mild macrocytosis is an expected pharmacological effect of Methotrexate (anti-folate) and does not require treatment change if Hb is stable and there is no cytopenias; however rising MCV should prompt checking B12 folate thyroid function and alcohol intake

Methotrexate causes macrocytosis through its anti-folate mechanism which impairs DNA synthesis in rapidly dividing cells. Mild macrocytosis (MCV 100-110 fL) is common and expected during Methotrexate therapy and does not necessarily indicate toxicity. BSR 2025 csDMARD guidelines recommend investigating macrocytosis if: MCV >105 fL or rising trend (check B12 folate thyroid function alcohol intake) or if accompanied by any cytopenia. Increasing folic acid dose may attenuate macrocytosis. The key safety concern is if macrocytosis accompanies falling cell counts (suggesting megaloblastic marrow suppression). Isolated stable macrocytosis with normal cell counts and adequate folic acid does not require Methotrexate adjustment.

Reference: BSR 2025 csDMARD guidelines