Microcytic Anemia with Inflammation: Iron Deficiency or Anemia of Chronic Disease?
A 71-year-old woman with long-standing rheumatoid arthritis has fatigue and Hb 92 g/L, MCV 76 fL, RBC 4.9 × 10^12/L, RDW 16.2%, and platelets 410 × 10^9/L. Reticulocytes are 1.1% (absolute 54 × 10^9/L). Serum ferritin is 180 µg/L, CRP 62 mg/L, serum iron 5 µmol/L, TIBC 62 µmol/L, and transferrin saturation is calculated by the laboratory. The peripheral smear shows microcytes and pencil cells; no blasts. She has been on methotrexate and prednisone; no overt bleeding, but heavy menstrual periods ended 20 years ago. Stool occult blood is negative. The clinician asks whether she has iron deficiency and whether oral iron is worthwhile. The laboratory scientist must interpret the iron studies in the setting of inflammation, calculate the Mentzer index and transferrin saturation, and recommend the next diagnostic step.
1. Calculate the Mentzer index and transferrin saturation. How do you interpret them alongside the ferritin and CRP?
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Mentzer index = MCV/RBC = 76/4.9 = 15.5. A value greater than 13 traditionally favors iron deficiency, but it is not specific, particularly with inflammation. Transferrin saturation = serum iron/TIBC × 100 = 5/62 × 100 = 8.1%. A TSAT below 20% indicates iron-deficient erythropoiesis. However, ferritin 180 µg/L is not low; ferritin is an acute-phase reactant and rises with rheumatoid arthritis (CRP 62 mg/L), so a normal ferritin does not exclude iron deficiency. Low TSAT, microcytosis, thrombocytosis, and a high RDW together strongly suggest iron deficiency coexisting with anemia of chronic disease. The Mentzer index alone cannot separate the two, and the ferritin must be interpreted against the inflammatory state.
2. Why is ferritin unreliable here, and what test or trial would best clarify the diagnosis?
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In inflammation, hepcidin blocks duodenal iron absorption and macrophage iron release, lowering TSAT while ferritin is retained. A ferritin below 100 µg/L is usually diagnostic of iron deficiency; 100–300 µg/L is indeterminate when CRP is elevated, and further testing or a treatment trial is needed. Soluble transferrin receptor is less affected by inflammation; a high sTfR/log ferritin ratio supports iron deficiency. Zinc protoporphyrin is also elevated. The most practical next step is a supervised oral iron trial with a 4-week hemoglobin response, expected to be about 10–20 g/L if the patient is deficient and adherent, after excluding blood loss. If sTfR is available it can be added, but it should not delay treatment.
3. Would you recommend oral iron? How would you monitor it, and what if there is no response?
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Yes, a trial is reasonable because the TSAT is 8% and the ferritin is indeterminate. Use alternate-day dosing, for example 60–120 mg elemental iron, to improve absorption and reduce hepcidin stimulation; take it on an empty stomach if tolerated, with vitamin C optional. Counsel about constipation and dark stools. Recheck CBC and ferritin at 4 weeks; a rise in Hb confirms deficiency. If there is no response, assess adherence, ongoing inflammation, and occult blood loss, and consider IV iron and gastrointestinal investigation. Do not attribute the anemia solely to rheumatoid arthritis while the TSAT is 8%, because untreated iron deficiency will persist and may mask a gastrointestinal lesion.